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Originally published In Press as doi:10.1074/jbc.M411019200 on January 22, 2005

J. Biol. Chem., Vol. 280, Issue 13, 12279-12291, April 1, 2005
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Kinetic Analysis of Oxidation of Coumarins by Human Cytochrome P450 2A6*{boxs}

Chul-Ho Yun{ddagger}, Keon-Hee Kim{ddagger}, M. Wade Calcutt§, and F. Peter Guengerich§

From the {ddagger}Hormone Research Center, School of Biological Sciences and Technology, Chonnam National University, Gwangju 500-757, Republic of Korea and the §Department of Biochemistry and Center in Molecular Toxicology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-0146

Received for publication, September 24, 2004 , and in revised form, January 18, 2005.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
 REFERENCES
 
Human cytochrome P450 (P450) 2A6 catalyzes 7-hydroxylation of coumarin, and the reaction rate is enhanced by cytochrome b5 (b5). 7-Alkoxycoumarins were O-dealkylated and also hydroxylated at the 3-position. Binding of coumarin and 7-hydroxycoumarin to ferric and ferrous P450 2A6 are fast reactions (kon ~ 106 M–1 s–1), and the koff rates range from 5.7 to 36 s–1 (at 23 °C). Reduction of ferric P450 2A6 is rapid (7.5 s–1) but only in the presence of coumarin. The reaction of the ferrous P450 2A6 substrate complex with O2 is rapid (k ≥ 106 M–1 s–1), and the putative Fe2+ ·O2 complex decayed at a rate of ~0.3 s–1 at 23 °C. Some 7-hydroxycoumarin was formed during the oxidation of the ferrous enzyme under these conditions, and the yield was enhanced by b5. Kinetic analyses showed that ~1/3 of the reduced b5 was rapidly oxidized in the presence of the Fe2+·O2 complex, implying some electron transfer. High intrinsic and competitive and non-competitive intermolecular kinetic deuterium isotope effects (values 6–10) were measured for O-dealkylation of 7-alkoxycoumarins, indicating the effect of C–H bond strength on rates of product formation. These results support a scheme with many rapid reaction steps, including electron transfers, substrate binding and release at multiple stages, and rapid product release even though the substrate is tightly bound in a small active site. The inherent difficulty of chemistry of substrate oxidation and the lack of proclivity toward a linear pathway leading to product formation explain the inefficiency of the enzyme relative to highly efficient bacterial P450s.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
 REFERENCES
 
P4501 enzymes are involved in the oxygenation of a variety of natural products and xenobiotic chemicals in microbial systems (3, 4). Much is known about the structure, function, and catalytic features of some of the P450s, particularly the more extensively studied of the bacterial P450s (4, 5). In mammalian systems P450s oxidize many drugs, steroids, carcinogens, fatty acids and eicosanoids, fat-soluble vitamins, and other endobiotic and xenobiotic chemicals (6). Less information is available about the biochemical details of most of the 57 human P450s (7). In particular, the basis of the inherently lower catalytic activities of these and other mammalian P450s relative to some of the microbial forms is not clear.

P450 2A6 is a low-to-medium abundance P450 in human liver (7–9) and is also expressed in some extrahepatic tissues (10). The history of this gene/protein goes back to Phillips et al. (11), who identified a human P450 cDNA as a relative of rat P450 2B1. The 7-hydroxylation of coumarin has long been used as an assay of P450 activity in animal and human liver microsomes (12, 13), and Yamano et al. (14) isolated a P450 2A6 cDNA (then termed 2A3) and first showed that the protein derived from heterologous expression had coumarin 7-hydroxylation activity. Miles et al. (15) also provided similar evidence for this particular sequence being associated with coumarin 7-hydroxylation. Our group purified a protein from human liver microsomes, identified it as P450 2A6, and showed it to be the major coumarin 7-hydroxylase in human liver (8). Subsequently P450 2A6 has been studied extensively, in large part because of its role in the metabolism of nicotine and carcinogenic N-alkylnitrosamines (16, 17). Genetic polymorphisms have been identified (18, 19) and may be of relevance to cancer risk; (i) impaired metabolism of nicotine has been proposed to reduce cigarette smoking in P450 2A6-deficient individuals (20); (ii) impaired metabolism can yield reduced levels of activation of the N-nitrosamines found in tobacco (21). Some drugs are oxidized by P450 2A6 (7, 22). P450 2A6 also catalyzes the oxidation of indoles (23, 24), and we have used P450 2A6 mutants to synthesize new indirubins with activity as protein kinase inhibitors (25).

Recently x-ray crystal structures have been reported for P450 2A6, including forms with the substrates coumarin and nicotine bound (26). These structures more than any of the other mammalian P450s solved to date have a small binding site akin to that of bacterial P450 101A1 (27). The space for the substrate coumarin is very restricted, and the coumarin-bound structure has the C-7 atom located near the heme iron (26). A major conformational change is required to open and close the enzyme and allow the substrate (coumarin) to enter and leave (26).

We have been studying aspects of catalysis of mammalian P450s, including the rate-limiting steps in reactions (28–31). P450 2A6 was of interest because of the recently reported structure, the useful fluorescence properties and common use of coumarins as P450 substrates, and our inherent interest in the catalytic properties of P450 2A6 (8, 23–25, 32). Rates of several steps were measured (Scheme 1). Studies of O-dealkylation of 7-OR coumarins showed high kinetic hydrogen isotope effects and demonstrate the kinetic difficulty of C–H bond breaking. The 7-OR coumarins showed extensive formation of 3-OH products as well as 7-OH coumarin. Together the results provide a picture of a very dynamic catalytic cycle with considerably more flexibility than apparent with the more efficient bacterial P450s, providing some potential insight into rate differences.



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  		SCHEME 1.
Catalytic cycle of P450. Adapted from previous reviews (30, 33, 34). Step 5 can be divided into two parts, (a) Fe2+·O2–+ H+ {rightleftarrows} FeII-OOH, and (b) FeOOH -> FeO3+, although the dioxygen species have also been postulated to catalyze certain oxidations directly (35). Step 6 can also be divided into two parts, (a) FeO3+ + RH -> [FeOH3+ R·], and (b) [FeOH3+ R·] -> Fe3+ + ROH.

 

   EXPERIMENTAL PROCEDURES
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
RESULTS
 DISCUSSION
 REFERENCES
 
Chemicals—Coumarin, 7-OH coumarin, 7-OMe coumarin, and 7-OEt coumarin were purchased from Aldrich and recrystallized from C2H5OH/H2OorCH3OH/H2O before use. 5-Deazaflavin was a gift of the late V. Massey (University of Michigan, Ann Arbor, MI) (36).

Deuterated 7-OMe coumarins were prepared by reaction of 7-OH coumarin with deuterated methyl iodides (Cambridge Isotopes, Cambridge, MA) in the usual manner (29, 37, 38) and recrystallized from CH3OH/H2O. [1-Ethyl-d2]-7-OEt coumarin was prepared in the same way from [1-d2]-ethyl iodide (Cambridge). In the synthesis of [1-ethyl-d1]-7-OEt coumarin, CH3CHO was reduced with LiAlD4 in diethylene glycol diethyl ether to prepare [1-d1]-C2H5OH (39), and the distilled product was reacted with tosyl chloride in dry pyridine to form the tosylate (72% yield, m.p. 27–30 °C (literature (40), 21 °C). The tosylate was then reacted with 7-OH coumarin in acetone/K2CO3 (under reflux) in the same manner as used with the alkyl iodides to yield [1-ethyl-d1]-7-OEt coumarin, which was recrystallized from CH3OH, H2O (39% yield, m.p. 85–87 °C, literature (41), 88–90 °C); UV (CH3OH) {epsilon}323 1.23 x 104 M–1 cm–1; electrospray MS, m/z 191.1 (MH+); NMR (CDCl3) {delta} 1.37 (dd, 3H, CH3), 4.03 (m, 1H, CHD), 6.22 (d, 1H, H-3), 6.69–6.82 (m, 2H, H-6, H-8), 7.33 (d, 1H, H-5), 7.60 (d, 1H, H-4). All deuterated coumarin derivatives were >98% isotopically enriched at the site of modification as judged by MS and NMR spectroscopy.

The synthesis of 3-OH coumarins was done using the general procedure of Neubauer and Flatow (42), which involves condensation of salicylaldehyde or a 4-substituted salicylaldehyde with hippuric acid (N-benzoylglycine) to form the N-benzoyl enamine, which was hydrolyzed in 10 N NaOH (100 °C, 45 min) to give the desired coumarin. 4-OEt salicylaldehyde was prepared by BCl3 treatment of 2,4-(OEt)2 salicylaldehyde (Aldrich) in CH2Cl2 (80% yield) (43). The identities of the 3-OH coumarins were confirmed by their m.p. values and spectroscopy: 3-OH coumarin, m.p. 151–154 °C (literature (44), 154 °C), UV (CH3OH) {epsilon}307 1.22 x 104 M–1 cm–1, {epsilon}294 1.19 x 104 M–1 cm–1, {epsilon}235 4.91 x 103 M–1 cm–1, fluorescence (CH3OH) {lambda}excitation 310 nm, {lambda}emission 395 nm, electrospray MS, m/z 163.1 (MH+), NMR (CDCl3) {delta} 6.48 (bs, 1H, H-4), 7.46–7.67 (m, 4H, H-5,6,7,9); 3-OH,7-OMe coumarin, m.p. 179–182 °C (literature (45), 175.5–177.5 °C), UV (CH3OH) {epsilon}322 1.4 x 104 M–1 cm–1,{epsilon}235 5.6 x 103 M–1, {epsilon}215 8.4 x 10 M–1 cm–1, electrospray MS, m/z 193.1 (MH+), NMR (CDCl3) {delta} 3.79 (s, 3H, CH3), 6.88 (dd, 1H, H-6), 6.94 (d, 1H, H-8), 7.08 (s, 1H, H-4), 7.43 (d, 1H, H-5); 3-OH,7-OEt coumarin, m.p. 155–157 °C, UV (CH3OH) {epsilon}322 1.34 x 104 M–1 cm–1, {epsilon}235 5.6 x 103 M–1 cm–1, {epsilon}215 8.4 x 10 M–1 cm–1, electrospray MS, m/z 206.9 (MH+), NMR (CDCl3) {delta} 1.32 (t, 3H, CH3), 4.05 (q, 2H, CH2), 6.87 (dd, 1H, H-6), 6.89 (d, 1H, H-8), 7.08 (s, 1H, H-4), 7.42 (d, 1H, H-5).

Enzymes—P450 2A6 was expressed from a plasmid (originally obtained from P. Soucek, National Institute of Public Health, Prague) in Escherichia coli, except that a His5 tag was attached to the C terminus (24, 46). Rat NADPH-P450 reductase was expressed in E. coli and purified as described (47). Recombinant human b5 was expressed in E. coli JM109 cells from a plasmid (pSE420 (Amp)) kindly provided by Satoru Asahi (Takeda Pharmaceutical, Osaka, Japan). The protein was solubilized and purified to electrophoretic homogeneity using modifications of the DEAE-cellulose and hydroxylapatite chromatography methods described elsewhere (48).

Spectroscopy—NMR spectra were recorded using Bruker 300 and 400 MHz instruments in the Vanderbilt facility. UV-visible spectra were generally acquired using an OLIS/Cary 14 or a OLIS/Aminco DW2a instrument (OLIS, Bogart, GA). Mass spectra were recorded using HPLC-MS methods (octadecylsilane columns, positive ion-electrospray, or atmospheric pressure chemical ionization) in the Vanderbilt facility using a Thermo-Finnigan TSQ-7000 instrument (Thermo-Finnigan, Sunnyvale, CA). Fluorescence measurements were made using either an SPEX Fluoromax-3 instrument (SPEC/Jobin Yvon, Edison, NJ) or an OLIS RSM-1000 instrument (OLIS), operating in the stopped-flow mode.

Stopped-flow kinetic UV-visible measurements were made using an OLIS RSM-1000 instrument (slit width 1.24–3.16 nm for absorbance beam). Some kinetic traces were obtained in the single wavelength mode; the rapid scanning mode was used with a 16 x 1-mm scanning disk to obtain spectra (16–1000 scans s–1). In some cases the acquired spectra were used to derive kinetic traces at individual wavelengths, and fitting was done with the manufacturer's software.

Assays—Typical steady-state coumarin oxidation reactions included 50 pmol P450 2A6, 100 pmol of NADPH-P450 reductase, 50 pmol of b5 (when indicated), and 30 µg of di-12:0 GPC in 0.50 ml of 50 mM potassium phosphate buffer (pH 7.4) along with a specified amount of the coumarin substrate. An aliquot of an NADPH-generating system was used to start reactions (final concentrations, 10 mM glucose 6-phosphate, 0.5 mM NADP+, and 1 international unit of yeast glucose 6-phosphate ml–1 (49)). Stock coumarin solutions (5 mM) were made in H2O. 7-OMe and 7-OEt coumarin stocks (50 mM) were made in CH3CN and diluted into enzyme reactions, with final organic solvent concentrations <1% (v/v).

Incubations were generally done for 10 min at 37 °C, terminated with 0.10 ml of 17% HClO4, and centrifuged (103x g, 10 min). CH2Cl2 (1.0 ml) was added to the supernatant to extract the products followed by centrifugation at 103x g (process repeated one more time). The organic layers were combined, and the CH2Cl2 was removed under a N2 stream. The products were analyzed by HPLC using a Toso ODS-80TM octadecylsilane (C18) column (4.6 x 150 mm, 5 µm) with the mobile phase H2O:CH3CN (55:45, v/v) containing 10 mM HClO4, a flow rate of 1.0 ml min–1, and monitoring at A330. Kinetic parameters (Km and kcat) were determined using nonlinear regression analysis with Graph-Pad Prism software (Graph-Pad, San Diego, CA). In some cases (stopped-flow), 7-OH coumarin was monitored directly (F390/460).

Assays involving competitive deuterium isotope effects were done by HPLC-MS analysis of formaldehyde or acetaldehyde derived from [methyl-d3]-7-OMe coumarin or [1-ethyl-d2]-7-OEt coumarin, respectively, using the general approach described elsewhere (51, 52). With [1-ethyl-d1]-7-OEt and [1-ethyl-d2]-7-OEt coumarin as substrates, the mass spectra were complicated due to the presence of contaminants in some of the reagents, particularly the solvents and glycerol. To minimize complications arising from residual aldehyde contamination in reagents and solvents, the following changes were made to the procedure. Reconstituted enzyme solutions (P450 2A6, NADPH-P450 reductase, and b5) were dialyzed against glycerol-free 50 mM potassium phosphate buffer (pH 7.4) containing 0.2 mM EDTA and 0.1 mM dithiothreitol (two changes over 12 h at 4 °C) before the addition of di-12:0 GPC. Dissolution of 7-OMe and 7-OEt coumarin in H2O was achieved by sonication (Branson sonicator, microtip probe, 70% full power) (Branson, Danbury, CT) to avoid the introduction of organic solvents. Aqueous stock solutions (~0.5 mM) were then filtered and quantitated spectrophotometrically (see above). Hexanes and CH3CN were heated with and distilled from 2,4-dinitrophenylhydrazine. 2,4-Dinitrophenylhydrazine (for use as a derivatization reagent) was recrystallized twice from CH3OH/H2O, dried in vacuo, dissolved in 6 N HCl (0.1%, w/v), and washed multiple times with a hexane, CH2Cl2 mixture (7:3, v/v) to remove hydrazone impurities before use for derivatization. Deuterium incorporation was determined using HPLC/negative ion atmospheric pressure chemical ionization MS of the resulting 2,4-dinitrophenylhydrazone derivatives (source temperature 550 °C; heated capillary voltage 20 V; heated capillary temperature 180 °C; ionization current 5 µA; sheath gas (N2) pressure 70 p.s.i.; auxiliary gas (N2) pressure 10 p.s.i.) (53).

Anaerobic experiments involved the use of glass anaerobic cuvettes and tonometers with a gas train connected to a manifold, alternating between vacuum and Ar. The basic systems are described elsewhere (54–56), and recent further modifications have been described (31). The OLIS RSM-1000 stopped-flow spectrophotometer has stainless steel lines leading up to the observation cell instead of Teflon, reducing the diffusion of O2. As described earlier (31), the drive syringes were filled with anaerobic 0.10 M Na2S2O4 (in 0.2 M potassium phosphate buffer, pH 7.4) overnight before use to scrub O2 (31, 56). The drive syringes were then loaded with the contents of a tonometer containing 0.25 mM safranin T and 0.5 mM methyl viologen (photo-reduced) in anaerobic 0.10 M Tris·HCl buffer (pH 7.7) containing 10 mM EDTA. The lack of O2 in the system is indicated by blue color (methyl viologen radical cation), as opposed to red (oxidized safranin). Thus, the final displacement of the dye by the enzyme solution provides a reasonable check on the anaerobic nature of the system.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
DISCUSSION
 REFERENCES
 
Oxidations Catalyzed by P450 2A6—Earlier work led to the characterization of P450 2A6 as the major coumarin 7-hydroxylase (8, 14, 15). Recently the structure of a P450 2A6 crystal has been reported (26), with coumarin bound and positioned with the C-7 atom near the iron atom.

Assays of P450 2A6-catalyzed coumarin oxidation commonly utilize a sensitive fluorescence assay that reports 7-hydroxylation (57, 58). HPLC-UV assays indicated the formation of the single product 7-OH coumarin by chromatographic and spectral comparison to standard material (Table I, also see Supplemental Fig. 1). 7-OMe and 7-OEt coumarins were O-dealkylated to form 7-OH coumarin, but both of these substrates also formed the 3-hydroxy products, as judged by comparisons with synthetic materials. The identification of 3-OH, 7-OEt coumarin as a product of oxidation of 7-OEt coumarin has been reported previously with human liver microsomes (59, 60). The 3-hydroxylation of coumarin has been reported with human liver microsomes (61, 62). Neither we nor Born et al. (62) detected conversion of coumarin to 3-OH coumarin by P450 2A6 systems.2


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  		TABLE I
Steady-state parameters for coumarin oxidation catalyzed by P450 2A6

 

Subsequent analysis of steady-state kinetic parameters indicated that 3-hydroxylation was observed to a greater extent for 7-OEt coumarin than 7-OMe coumarin (Table I).3 The addition of b5 stimulated the formation of 7-hydroxylation of coumarin 2-fold, as reported by others (46, 63). However, the activities with 7-OR coumarins were not stimulated except for the decreased Km for the 3-hydroxylation of 7-OEt coumarin.

Spectral Properties of P450 2A6—The spectra of the ferric, ferric-coumarin, and ferrous-coumarin forms of P450 2A6 are shown in Fig. 1. The spectral properties were utilized in several subsequent experiments to measure rates of changes within the catalytic cycle (Scheme 1). Second-derivative analysis (64, 65) of the spectra (Fig. 1) yielded estimates of 98% low spin P450 iron in substrate-free ferric P450 2A6 and 88% high-spin iron with coumarin bound.



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  		FIG. 1.
Spectra of P450 2A6 complexes. A, spectra were recorded in 50 mM potassium phosphate buffer (pH 7.4) with 5.2 µM P450 2A6 either with or without 50 µM coumarin. The ferrous form was produced by the addition of a few grains of Na2S2O4. The inset (B) shows an expansion of the {alpha},{beta} region. Solid lines, ferric P450 2A6 (Fe3+); dots and dashes, ferric P450 2A6·coumarin complex (Fe3+·S); dashes, ferrous P450 2A6·coumarin complex (Fe2+·S).

 
Substrate Binding (Step 1 of Scheme 1)—A large spectral difference is observed upon binding of the substrate coumarin (Fig. 2). Titration of P450 2A6 with coumarin yielded classic "Type I" difference spectra, with Ks = 0.38 (±0.03) µM (Fig. 2, A and B).



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  		FIG. 2.
Binding of coumarin to ferric (Fe3+) P450 2A6. A, difference spectra obtained by titration of a 2 µM P450 solution (100 mM potassium phosphate, pH 7.4) with increasing concentrations (spectra shown with arrows) of coumarin (to 7.5 µM). B, plot of data from A, fit to a quadratic expression Kd = ([Et–E·S][St–E·S])/ES, where Et = total P450 concentration, St = total coumarin concentration, and ES = P450-coumarin complex (66) yielded Kd = 0.38 (±0.03) µM. C, rate of {Delta}A390 for similar a binding experiment with 8 µM coumarin, fit to a single exponential of 17 s–1.D, kobs for traces obtained in C as a function of coumarin concentration. From kobs = kon [S] + koff (67), k = 2.7 x 106 M–1 s–1, and koff = 5.7 s–1 (k0ff/kon = 2.1 µM).

 
The rates of spectral changes could be monitored using stopped-flow spectroscopy (Fig. 2C). These traces were fit to single-exponential plots. Analysis of rates as a function of coumarin concentration (Fig. 2D) fit a relationship describing a two-state system (Fe3+ + S {rightleftarrows} Fe3+·S), i.e. kobs = kon[S] + koff (67) and yielding kon = 2.7 x 10 M–1 s–1 and koff = 5.7 s–1, with Kd = koff/kon = 2.1 µM, in reasonable agreement with the Kd values estimated by titration (Fig. 2B).

Product Release (Step 7 of Scheme 1)—Titration of ferric P450 2A6 with 7-OH coumarin yielded a "Type II" difference spectrum, which is probably indicative of ligation of the phenolic oxygen to the iron atom (Fig. 3A). The titration indicated binding as tight as for the substrate, Kd = 0.82 ± 0.05 µM (Fig. 3B). The rate of binding could be measured using stopped-flow spectroscopy (Fig. 3C). Analysis of the rate as a function of 7-OH coumarin concentration (Fig. 3D) yielded kon = 2.0 x 106 M–1 s–1 and koff = 6.8 s–1 (koff/kon ~ 3.4 µM).



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  		FIG. 3.
Binding of 7-OH coumarin to ferric (Fe3+) P450 2A6 (product release). A, difference spectra obtained by titration of a 2 µM P450 2A6 solution (100 mM potassium phosphate, pH 7.4) with increasing concentrations (spectra shown with arrows) of 7-OH coumarin (to 8 µM). B, plot of data from A, fit to a quadratic expression yielded Kd = 0.82 (±0.05) µM. C, rate of {Delta}A230 for a similar binding experiment with 16 µM 7-OH coumarin, fit to a single exponential of 18 s–1. D, kobs for traces obtained in C as a function of 7-OH coumarin concentration. From kobs = kon [S]+ koff (67), kon = 2.0 x 106 M–1 s–1 and koff = 6.8 s–1 (koff/kon ~ 3.4 µM).

 
P450 2A6-catalyzed oxidations of coumarin, 7-OMe coumarin, and 7-OEt coumarin were examined and did not show kinetic bursts (Fig. 4). On the basis of these results and the rates measured in the experiment of Fig. 4, steps after product formation are not rate-limiting in the formation of 7-OH coumarin from any of these substrates.



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  		FIG. 4.
Lack of kinetic bursts in oxidations of coumarins to form 7-OH coumarin. In each case a reaction was done at 22 °C in the stopped-flow spectrofluorimeter (OLIS RSM-1000) by adding 22 µM NADPH and an NADPH-generating system (49) from one syringe to a typical mixture (final concentrations indicated) of P450 2A6 (0.5 µM), NADPH-P450 reductase (1.0 µM), b5 (0.5 µM), di-12:0 GPC (45 µM), and the substrate. Fluorescence excitation was at 390 nm, and emission was monitored at 420 nm. The substrate concentrations were 50, 200, and 200 µM for coumarin, 7-OMe coumarin, and 7-OEt coumarin, respectively. The fluorescence corresponding to 0.5 µM 7-OH coumarin (indicative of one enzyme cycle) was determined by comparison with a conventional mixing experiment done with these preparations and coumarin in an OLIS DM45 spectrofluorimeter. The vertical line at t = 0 is the mixing artifact. The origins of the four traces are not intended to be identical due to variations in the background fluorescence (as a result of both the coumarins and the NADPH).

 
Reduction of Ferric P450 2A6 (Step 2 of Scheme 1)—Rates of reduction of ferric P450 2A6 were measured in an anaerobic CO environment, with ferrous P450 trapped as the CO complex (Fig. 5). The rate of binding of ferrous P450 and CO is much faster than reduction.



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  		FIG. 5.
Reduction of ferric (Fe3+) P450 2A6. Experiments were done anaerobically under a CO atmosphere in an OLIS RSM-1000 stopped-flow spectrophotometer. One syringe contained NADPH (300 µM) in 100 mM potassium phosphate buffer (pH 7.4) The other tonometer contained a mixture of P450 2A6 (1.0 µM), NADPH-P450 reductase (2.0 µM), and di-12:0 GPC (45 µM) in 100 mM potassium phosphate buffer (pH 7.4). Coumarin (40 µM) was also included when indicated. The inset shows an expansion of the plot with coumarin present. Rates were fit to exponential plots using the manufacturer's software; without coumarin, 0.13 s–1 (no correction for the apparent lag); with coumarin, bi-exponential fit of 7.5 s–1 and 0.13 s–1.

 
The rate of reduction of ferric P450 was slow in the absence of coumarin, with a fit to a single exponential of 0.13 s–1 (Fig. 5). In the presence of coumarin, a bi-exponential fit yielded k1 = 7.5 s–1 and k2 = 0.13 s–1 (Fig. 5). About 50% of the P450 was reduced in the fast phase.

Dissociation of Substrate from Ferrous P450 2A6 (Step 2a of Scheme 1)—Although the binding of a substrate (coumarin) to ferric P450 2A6 produces a major spectral change, the changes observed upon the addition of coumarin to ferrous P450 2A6 are much weaker. A difference spectrum was observed with a trough at 438 nm and a peak at 460 nm (Fig. 6A). The rate of binding to NADPH-P450 reductase-reduced P450 2A6 under anaerobic conditions yielded kon = 1.5 x 106 M–1 s–1 and koff = 36 s–1 (koff/kon = 24 µM) (Figs. 6, B and C).



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  		FIG. 6.
Binding of coumarin to ferrous (Fe2+) P450 2A6. A, difference spectrum obtained with 2.6 µM P450 2A6 with and without 50 µM coumarin (in 50 mM potassium phosphate buffer, pH 7.4). Both cuvettes were reduced with solid Na2S2O4. B, kinetics of {Delta}A438 observed upon the addition of 15 µM coumarin to ferrous P450 2A6. The experiment was done in a stopped-flow spectrophotometer. One syringe contained 100 µM coumarin (in 50 mM potassium phosphate buffer, pH 7.4), and the other contained P450 2A6 (4 µM), NADPH-P450 reductase (4 µM), di-12:0 GPC (45 µM), potassium phosphate buffer (50 mM potassium phosphate buffer, pH 7.4), and an NADPH-generating system composed of 10 mM glucose 6-phosphate, 0.5 mM NADPH, and 1 international unit of yeast glucose-6-phosphate dehydrogenase ml–1 (49). Both syringes were under an anaerobic environment (Ar). The data fit to a single exponential of 57 s–1. C, kobs for traces obtained in B as a function of coumarin concentration. Fitting to the expression kobs = kon [S] + koff yielded values of kon = 1.5 x 106 M–1 s–1 and koff= 36 s–1 (Kd = Koff/Kon = 24 µM).

 
Formation (Step 3 of Scheme 1) and Decomposition of Ferrous P450 2A6·O2 Complex—Ferrous P450 2A6 (in the presence of 50 µM coumarin) was introduced anaerobically into one syringe of the stopped-flow spectrophotometer and mixed with aerobic buffer (200 µM O2). A rapid spectral change was observed, and the early spectra are shown in Fig. 7A. The rate of this reaction, as estimated from {Delta}A390, was ~75 s–1. This complex was not very stable and decayed to yield ferric P450 2A6 (Fig. 7B), with an estimated rate of 0.3 s–1 (t1/2 = 2.3 s) at 23 °C (Fig. 7C).



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  		FIG. 7.
Reaction of ferrous P450 2A6 with O2. P450 2A6 (4.9 µM), in an anaerobic mixture of 50 mM potassium phosphate, 100 mM Tris-HCl, 10 mM EDTA buffer (pH 7.4) containing 50 µM coumarin, 75 µM di-12:0 GPC, and 2 µM 5-deazaflavin (36), was photo-reduced and loaded into one syringe of the stopped-flow spectrophotometer. The other syringe contained air-saturated 50 mM potassium phosphate buffer. A, traces (rapid scanning) of the reaction. The first trace shown was recorded 21 ms after mixing, and the subsequent spectra were obtained every 25 ms. A390 decreased, and the broad absorbance at higher wavelength increased and then decreased. B, further spectral changes after mixing with O2. The first trace was recorded 64 ms after mixing, with subsequent spectra recorded every 1.6 s, with the direction shown by the arrow. C, kinetics of reaction from B ({Delta}A390), fit to a single exponential of 0.30 s–1.

 
Formation of Product in Limited Turnover Experiments— The oxidation of coumarin was measured under conditions in which the electron input was limited to what should be a single cycle, e.g. as in Fig. 7. P450 2A6 was either photo-reduced with 5-deazaflavin or reduced (by NADPH-P450 reductase) with a limited amount of NADPH (enough to fully reduce NADPH-P450 reductase and P450 once).4

The product 7-OH coumarin could be measured in all cases (Fig. 8), although the yields were low (Table II). The formation of product could be detected with only P450 present (Fig. 8A), which apparently indicates that two FeO2+2 complexes can interact to provide the second electron for product formation (e.g. 2FeO2+2 -> FeO2+2 (->-> FeO3+) + Fe3+). Product formation was more efficient when electrons were delivered from the reductase (Table II), although we do not know exactly how many electrons are delivered, i.e. it is possible that multiple 1-electron transfers could occur that are not relevant during steady-state catalysis.



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  		FIG. 8.
Oxidation of coumarin to 7-OH coumarin in limited cycle experiments. A, P450 2A6 (5 nmol) was photo-reduced under an Ar atmosphere (in the presence of 50 µM coumarin, 1.0 µM 5-deazaflavin, and 1.0 µM safranin T in 3.0 ml of buffer as described under Fig. 7). Reduction was monitored (e.g. Fig. 2) in the OLIS/Cary 14 spectrophotometer and was complete after 3 min of irradiation (36). The sample was mixed with air and quenched by the addition of 0.3 ml of 43% H3PO4 after 3 min. Products were extracted 3 times with 3.0 ml of CH2Cl2, and the combined organic phase was concentrated to dryness under an N2 stream and analyzed by HPLC using the fluorescence method of Soucek (58). The scale used in A is expanded 8-fold compared with the other parts. B, the experiment was as in A, except that 5 nmol of b5 was included (and photo-reduced before oxygenation). C, the experiment was as in A, except that 5 nmol of NADPH-P450 reductase was included, and 12.5 nmol of NADPH was added anaerobically to achieve reduction (instead of photoreduction). D, the experiment was as in C except that 5 nmol of b5 were also present, and 15 nmol of NADPH were used for reduction.

 


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  		TABLE II
Yields of products formed from coumarin in limited cycle experiments h{nu}, photochemical reduction.

 
In the cases of both photo-reduced P450 2A6 and P450 2A6 that was reduced with NADPH/reductase, the presence of reduced b5 raised the yield of product (Fig. 8, Table II). This result suggests that ferrous b5 might be able to transfer electrons to the P450 2A6 FeO22+-substrate complex.

Kinetic Deuterium Isotope Effects (Step 6 of Scheme 1)—The experiments presented thus far have dealt with the binding of ligands and the activation of O2 by P450. The relative rate of the step in which chemical transformation of the substrate occurs had not been addressed. This is a difficult question in the case of an aromatic or olefinic hydroxylation, unless an enzyme intermediate can be isolated and reacted directly to yield product. Such is not the case with P450s, so an alternate approach was used, that of measurement of kinetic deuterium isotope effects for reactions involving C–H bond cleavage of simple analogs. The work described in Table I showed that 7-OMe and 7-OEt coumarin O-dealkylations were catalyzed by P450 2A6 with catalytic efficiencies approaching that for coumarin 7-hydroxylation.

Several types of kinetic isotope effect experiments are possible (see Scheme 3) and reveal different information. A so-called non-competitive intramolecular experiment approximates the intrinsic kinetic deuterium isotope effect, the isotope effect on the C–H bond breaking step itself (1, 2). In the case of a C–H bond breaking step with a methyl group, the rapid rotation does not provide a kinetic barrier to attenuate the reaction, and the only limitation to the interpretation of this experiment is the potential contribution of a smaller geminal secondary kinetic isotope effect. P450 2A6-catalyzed 7-OMe coumarin O-demethylation showed a non-competitive intramolecular isotope effect of 9.8 (Table III). A somewhat lower isotope effect was measured for the O-deethylation of 7-OEt coumarin (6.0) (Table III), although the point must be made that the O-deethylation involves a prochiral substrate, 7-OEt coumarin, which can preclude interpretation about intrinsic isotope effects.



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  		SCHEME 3.
Outline of kinetic deuterium isotope effect experiments. A, non-competitive intramolecular experiment for approximating the intrinsic isotope effect (with a methyl; pro-chirality is a potential confounder with a methylene). B, competitive intermolecular experiment. C, non-competitive intermolecular experiment (provides information about how rate-limiting the C–H bond breaking step is in the overall steady-state reaction. This experiment provides information about the relative rates of exchange of substrates, i.e. if attenuation of the intrinsic isotope effect is observed, then exchange rates are partly rate-limiting (1, 2).

 


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  		TABLE III
Kinetic deuterium isotope effects for 7-alkoxycoumarin O-dealkylation reactions catalyzed by P450 2A6

 
We also measured competitive intermolecular isotope effects (see Scheme 3B). Attenuation of the intrinsic isotope effect in such an experiment is suggestive of a relatively slow exchange of substrate. The values for 7-OMe and 7-OEt coumarin in this experiment were still high (9.0 and 7.0, respectively) (Table III). The results are consistent with the rapid koff rate for the substrate coumarin (Fig. 2D).

The third type of experiment used was a non-competitive intermolecular system (see Scheme 3C). This type of experiment can provide some information about the extent to which the C–H bond-breaking step is rate-limiting (in the steady state) (1, 2). The value can be compared with the intrinsic isotope effect (see Scheme 3A, Table III). High kinetic isotope effects (both DV and D(V/K)) were observed in these experiments (Fig. 9, Table IV) (regardless of whether b5 was added or not; results not presented). These experiments provide strong evidence that the oxidation of the substrate by the active hypervalent oxygen species is difficult and can limit the rate of formation of these products, at least in the case of the O-dealkylation reactions. In the case of 7-OMe coumarin considerable "switching" to 3-hydroxylation was observed (Table IV, Fig. 9B). This result implies that the rotation of the substrate can occur to generate this product (68). Alternatively, the FeO3+-deuterated substrate complex could decompose, and the cycle can begin again with a protiated substrate (31). The switching effect (to 3-hydroxylation) was not significant in the case of 7-OEt coumarin (Fig. 9D, Table IV).



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  		FIG. 9.
Effect of deuterium substitution on oxidations of 7-OMe and 7-OEt coumarin by P450 2A6. Typical steady-state reaction conditions were used with d0 (A) or [methyl-d3]-7-OMe-coumarin (B) or d0 (C), or [1-ethyl-d2]-7-OEt-coumarin (D), and the products were analyzed by HPLC (100 µM substrate in each case).

 


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  		TABLE IV
Intermolecular non-competitive kinetic deuterium isotope effects for 7-alkoxycoumarin O-dealkylation reactions catalyzed by P450 2A6

 
Stoichiometry of NADPH Utilization—Rates of NADPH oxidation, formation of products, and H2O2 formation were measured, and H2O formation was calculated by difference (69) (Table V). The results indicate that formation of coumarin products is a relatively inefficient process, with the bulk of the NADPH used to reduce O2. As in the report of Tan et al. (63) with a baculovirus-based system, the addition of b5 to P450 2A6 suppressed the formation of H2O2. The suppression of H2O2 formation was also noted in the presence of the 7-OR coumarin substrates, although formation of the coumarin product was not enhanced (except for 7-OEt coumarin 3-hydroxylation).


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  		TABLE V
NADPH oxidation, product formation, and H2O2 formation by P450 2A6

 
Steady-state Spectral Measurements—Steady-state spectra of the coumarin 7-hydroxylation reaction were recorded with the prospect of identifying an accumulating intermediate. An aerobic mixture of P450 2A6, NADPH-P450 reductase, b5, di-12:0 GPC, coumarin, and an NADPH-regenerating system was mixed rapidly with NADPH, and spectra were recorded. The first phase occurred within 1 s (Fig. 10A). The reduction of the flavins of the reductase was observed (e.g. 440–500 nm). Several steps occur during this time, and a relatively stable spectrum accumulated by 1 s. The {lambda}max was ~415 nm and probably represents some iron-oxygen complex (or a mixture of more than one). It is not the ferric nor ferrous P450 2A6 (Fig. 1) and probably not the FeO22+ complex (Fig. 7).



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  		FIG. 10.
Spectra of P450 2A6 coumarin-oxidizing reaction. One syringe of the stopped-flow instrument contained P450 2A6 (10 µM), NADPH-P450 reductase (20 µM), b5 (10 µM), di-12:0 GPC (50 µM), coumarin (100 µM), glucose 6-phosphate (10 mM), yeast glucose 6-phosphate (1 international unit ml–1), and potassium phosphate buffer (100 mM, pH 7.4). The other syringe contained 100 µM NADPH in 100 mM potassium phosphate buffer (pH 7.4). The contents of the syringes were mixed, and spectra were recorded (23 °C). A, the indicated spectrum was observed at 58 ms, and the subsequent traces were recorded every 100 ms, to an elapsed time of 858 ms. The decrease in A390 was fit to a single exponential of 20 s–1. B, the indicated spectra were recorded every 3 s after mixing and correspond to a slow reduction of the b5 in the mixture. The decrease in A409 was fit to a single exponential of 0.14 s–1, and the increase in A424 was fit to 0.17 s–1.

 
This spectrum persisted for several seconds, and then the reduction of b5 was observed (Fig. 10B). The reduction appeared to follow first-order kinetics (as judged by {Delta}A409 or {Delta}A424), with k = 0.15 s–1. In other experiments with no P450 present, we measured the reduction of b5 to be a bi-exponential process, with a first step (stoichiometric with 1 b5 per NADPH-P450 reductase) of 18 s–1 and a subsequent rate of 1.7 s–1 (results not shown).

Oxidation of Ferrous b5—One hypothesis for the enhancement of the catalytic activity of P450 2A6 by b5 is the transfer of an electron from ferrous b5 to the P450 FeO22+-substrate complex (70, 71). This hypothesis has some support in the limited turnover experiments (Table II), where yields were enhanced in the presence of b5. However, alternate hypotheses could be valid (e.g. improvement of P450 efficiency through protein-protein interactions).

b5 was photo-reduced and mixed with air-saturated buffer (Fig. 11A). The first-order rate of oxidation was 0.13 s–1 (Fig. 11B). The experiment was repeated in the presence of P450 2A6 and coumarin. Thus, ferrous (photo-reduced) P450 2A6 reacts rapidly with O2 to form an oxygenated complex (Fig. 7A); if reduced b5 is present, it might transfer electrons to this FeO22+ complex before the complex decomposes. In this experiment (Fig. 11C), reduction was faster than in the absence of P450 2A6. About 1/3 of the b5 was oxidized rapidly (as judged by {Delta}A410 or {Delta}A424 measurements, and the data were fit to a bi-exponential plot with k1 = 3.6 s–1 and k2 = 0.04 s–1 (Fig. 11D). We conclude that b5 can participate in transferring electrons to the P450 2A6 FeO22+-substrate complex, although this does not appear to be a well coupled process.



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  		FIG. 11.
Oxidation of reduced b5 by ferrous-oxy-P450 2A6 (FeO22+). b5 (3.0 µM), in 50 mM potassium phosphate buffer (pH 7.4) containing 75 µM di-12:0 GPC, 100 mM Tris ·HCl, 20 mM EDTA, 50 µM coumarin, and 1 µM 5-deazaflavin (36), was photo-reduced and introduced into one side of the stopped-flow spectrophotometer. The sample was mixed with air-saturated 50 mM potassium phosphate buffer (pH 7.4) with a nominal O2 concentration of 200 µM. A, spectral traces of the reaction as described, with the first spectrum shown at 160 ms. Spectra were recorded every 3 s and progressed in the directions shown with the arrows. B, the data from A were fit to a single exponential with a rate of 0.13 s–1. C, the experiment of A was repeated in the presence of 3.0 µM P450 2A6. The first spectrum shown was recorded at 800 ms, and the subsequent spectra were recorded every 3 s. D, data from C ({Delta}A410) were fit to a bi-exponential equation with k1 = 3.6 s–1 and k2 = 0.04 s–1.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
REFERENCES
 
The purpose of this work was to characterize individual steps in catalysis by a mammalian P450 enzyme, human P450 2A6. The enzyme has a small active site (260 Å3), which is filled by the substrate coumarin (26) and binds the substrate reasonably tightly (Kd ~ 1 µM, Fig. 2B). However, catalysis is still relatively slow and inefficient in the context of the use of electrons for substrate oxidation (Tables I and V). Our results are interpreted in the context of a model with rapid exchange of ligands at multiple steps (requiring major conformational changes inferred from the x-ray crystallography work (26)), an unstable FeO22+ complex, and a relatively difficult step for the chemistry of oxidation by the FeO3+ complex.

Most of the work with P450 2A6 and coumarin substrates has been focused on the 7-hydroxylation reaction, which can be readily observed because of the strong fluorescence of the product 7-OH coumarin at neutral or alkaline pH (57, 58). HPLC-UV analysis also indicated the oxidation of a second site with the 7-OR coumarins (but not coumarin), which was identified as the 3-position by chromatographic and spectral comparison (Scheme 2, Fig. 9). Coumarin 3,4-epoxide has been reported not to convert to 3-OH coumarin (62), and therefore, we presume that the chemical mechanism is one involving formation of a bond between (Fe)O and the C-3 atom of the coumarin (4CH+-3CH-O-Fe), which collapses to form the enol product (favored over the keto tautomer). O-Dealkylation (at C-7) is a favorable reaction for 7-OR coumarins, presumably because the positioning approximates that observed in the P450 2A6-coumarin complex (26). The "shift" to 3-hydroxylation with 7-OEt coumarin was more extensive than for 7-OMe coumarin (Tables I and IV). Apparently the steric restriction imposed by the larger alkyl group shifts the equilibrium to an alternate form with the lactone carbonyl and the C-3 atom near the heme iron.



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  		SCHEME 2.
Oxidations of coumarins catalyzed by P450 2A6.

 
The molecular basis for the difference between oxidation at the 7 and 3 positions awaits the availability of the coordinates of the P450 2A6-coumarin crystal structure (26). The balance would appear to be sensitive not only to the bulk at the 7-position (Table I) but also the ease of oxidation of the C-7 substituent (Table IV). Thus, the explanation for the specificity will probably be both steric and dynamic. How difficult the prediction of regioselectivity and rates of oxidation will be for new putative ligands of P450 2A6 is unknown. In a general sense, the relatively low rates of coumarin oxidations by P450 2A6 (~10 min–1, Table I) would not have been expected if only a tight fit of the substrate and H2O exclusion from the active site (as evidenced by a strong shift to high spin iron) (Figs. 1 and 2) are the major factors involved in predicting catalysis, i.e. the properties of the P450 2A6-coumarin complex resemble those of P450 101A1-camphor, a system that turns over ~100-fold faster (5, 26).

The binding and dissociation rates were estimated for ferric P450 2A6 and two ligands, coumarin and the 7-OH product (Figs. 2 and 3). The results can be fit reasonably well to a simple 2-state model, and a more complex system may not be justified, at least at this point. The kon rates are near those reported for bacterial P450 101A1 (5, 72). Whether or not these are really a diffusion-limited rates is unclear, in that the second-order rates are still lower than for many enzymes, and theoretical calculations predict rates of ~109 M–1 s–1 (73, 74). A rapid rate of binding could be obscured by a slower, reversible transformation of the low spin to the high spin iron, yielding an artificially low apparent Kd (67). In principle, such a phenomenon should yield a hyperbolic plot of the apparent rate of binding (versus substrate concentration) instead of a linear form (Figs. 2D and 3D), but more analysis is needed to address this possibility.

The koff rates were 5.7 and 6.8 s–1 for the substrate coumarin and the product 7-OH coumarin, respectively (for ferric P450 2A6). The equation koff/kon = Kd yields parameters close to those estimated by steady-state spectral analysis (Figs. 2 and 3). The rapid off-rates are consistent with other results presented here, including the lack of a burst of 7-OH coumarin formation (Fig. 4) and the lack of attenuation of the kinetic deuterium isotope effect in the competitive intermolecular experiment (Table III). The rapid koff for coumarin (5.7 s–1 at 23 °C) is realistic in the context of the other parameters and is also competitive with the rate of reduction (Fig. 5).

Rapid reduction of ferric P450 2A6 (Fig. 5) was highly dependent upon the presence of the substrate coumarin, although only approximately one-half of the P450 2A6 was reduced in the fast phase (7.5 s–1 at 23 °C) even with a 2-fold excess of NADPH-P450 reductase. This rate is certainly much faster than overall catalysis (Table I) and should not be rate-limiting, even if fast reduction is only partial (Fig. 5). Our previous experience with (purified) microsomal P450s has been that about one-half of them show rapid reduction in the absence of substrate, and the other half requires substrate (75). Other P450s often show biphasic kinetics (75), probably due to spatial issues with reductase in the complexes (76). Although the iron of coumarin-saturated P450 2A6 is high spin (Fig. 1), a conclusion that only high spin P450 is rapidly reduced is unwarranted (75). Second-derivative analysis of the ferric Soret spectrum (Fig. 1) indicated ~88% high spin iron, but only 50% of the P450 2A6 was reduced rapidly (Fig. 5). We have previously presented evidence against a general linkage of substrate binding, low to high spin iron conversion, rapid reduction, and more positive redox potentials (Em,7) in P450s (65, 75, 77), unlike the situation with bacterial P450 101A1 (78). Although we have not directly estimated the Em,7 of P450 2A6 with and without substrate, consideration of the estimated Kd values of the Fe3+ and Fe2+ enzymes (Figs. 2 and 6) and the Nernst equation would suggest that the Em,7 of the Fe3+/Fe2+ couple would become somewhat more negative in the presence of coumarin, not more positive, applying a "thermodynamic box" analysis of substrate binding and reduction (73) and considering that the substrate is bound more tightly to the oxidized form of P450 2A6 (74) (Figs. 2, 6). Very recently Ost et al. (79) reported that the ligand 4-cyanopyridine is bound 3 orders of magnitude more tightly to ferrous than ferric P450 102A1, and accordingly, the presence of this ligand lowers the Em,7.

As discussed above, the koff rate for coumarin from ferrous P450 2A6 is considerable (36 s–1) (Fig. 6C) and may be an issue in the functionality of the Fe2+-substrate complex. That is, some fraction might not be competent in that it could dissociate, bind O2, and then decompose. The possibility also exists that substrate might dissociate from the FeO22+-substrate complex (or Fe-O complexes further in the catalytic cycle (Scheme 1)), although we do not have any measurements. In the case of rabbit P450 1A2, we recently presented evidence that dissociation of substrate from the FeO3+ complex itself (due to prevention of C–H bond breaking by deuterium substitution) was only accompanied by decomposition of the complex (31).

Reaction of the Fe2+ P450 2A6-coumarin complex with O2 produced two sets of spectral changes (Fig. 7). Although we did not examine the effect of varying O2 concentration on the rates, our view is that the rapid initial changes (Fig. 7A, e.g. A390 decrease) represent the formation of an FeO22+ (-substrate) complex and the succeeding changes (Fig. 7B, e.g. A390 increase) represent the decay of the complex to regenerate Fe3+ P450 2A6. The estimated rate of the first reaction was ~75 s–1, and the decay was 0.3 s–1 (18 min–1) at 23 °C. This complex appears to be much less stable than the Fe2+-O2 complexes reported for bacterial P450 101A1 (72, 80) and P450 108A1 (81) but has a stability similar to rabbit P450 1A2 (31, 82) and the heme domain of P450 102A1 (81); it is probably more stable than the complexes of rabbit P450 2B4 (83) and bacterial P450 119A1 (84). Analysis of 7-OH coumarin indicated low (but finite) yields in "limited cycle" experiments, e.g. when the kinetic reaction described above was analyzed for product formation (Fig. 8, Table II). Product formation apparently involves dismutation of two Fe2+·O2 complexes to achieve the requisite 2-electron stoichiometry (31). The amount of product formation in these experiments was increased when reduced NADPH-P450 reductase was present, indicating that electron transfer from reduced NADPH-P450 reductase to the P450 2A6 FeO22+-substrate complex occurs under these conditions, although the efficiency was low (Table II).

As reported previously (46, 63, 85), the presence of b5 enhanced the steady-state kcat for coumarin 7-hydroxylation (Table I). One general hypothesis for the enhancement of P450 catalytic activities by b5 is transfer of an electron from ferrous b5 to the P450 FeO2+-substrate complex (70, 71). The enhancement of yields of products derived from FeO22+-coumarin by ferrous b5 (Fig. 8, Table II) is consistent with this hypothesis but does not necessarily prove electron transfer. Another set of experiments was done in which a mixture of Fe2+ P450 2A6-coumarin-ferrous b5 was mixed with O2 to form a FeO22+-substrate-ferrous b5 complex, and the rate of oxidation of the ferrous b5 was monitored (at 410 or 424 nm) (Fig. 11). About one-third of the b5 was oxidized rapidly (3.6 s–1), whereas only the second, slower phase was observed for b5 oxidation in the absence of P450 2A6 (Fig. 11B). This result is consistent with transfer of an electron from reduced b5 to the P450 2A6 FeO22+-substrate complex, although the process appears to be less than quantitative. The situation is further complicated by the results of the experiment presented in Fig. 10, where a mixture of oxidized P450 2A6, NADPH-P450 reductase, and b5 (plus coumarin and di-12:0 GPC) was mixed with NADPH. In the first part of the reaction, the b5 was not reduced (up to ~1 s) and then became reduced at a rate of ~0.15 s–1 (at 23 °C). The b5 then stayed reduced for at least several minutes. This apparent rate was similar to the oxidation rate of ferrous b5 (Fig. 11B). For comparison, an experiment with only NADPH-P450 reductase and ferric b5 yielded a bi-exponential fit for reduction, with k1 = 18 s–1 and k2 = 1.7 s–1. The b5 experiments are complex and difficult to interpret, but our overall conclusion is that some electron transfer from ferrous b5 to the P450 2A6 FeO22+-coumarin complex occurs but that this does not seem to be a particularly efficient process. In other work we have shown that P450 2A6-catalyzed coumarin 7-hydroxylation can be enhanced by apo-b5 devoid of heme and precluding electron transfer (85). We conclude that part of the enhancement may be attributable to the electron transfer (Figs. 8 and 11 and Tables I and II) but that b5 probably has another effect, probably some type of conformational effect on the P450.5

Although the focus of this investigation was coumarin hydroxylation, the observed reaction (7-hydroxylation) was not amenable to the application of kinetic deuterium isotope effect studies (Scheme 2). However, 7-OMe and 7-OEt coumarin appear to be reasonable surrogates in that the P450 2A6 heme iron atom is apparently positioned in sites that yield O-dealkylation (C-7) or 3-hydroxylation. The apparent intrinsic deuterium isotope effect for 7-OMe coumarin was high (9.8), and the 7-OEt coumarin value was lower (6.0), possibly perturbed by the issue of prochirality (Table III).6 The observed isotope effects high and similar in both competitive and non-competitive intermolecular experiments (Table IV).7

When oxidation of a C-7 alkyl group was inhibited by substitution with deuterium, much of the decrease was compensated for in terms of enhanced 3-hydroxylation (Table IV). The switch may be less obvious in the case of 7-OEt coumarin in that the DV and D(V/K) values are not as high because of the already high rate of 3-hydroxylation. However, ~1/2 of the decrease in kcat due to deuterium substitution is manifested in the kcat for 3-hydroxylation for both 7-OMe and 7-OEt coumarin (Table IV). The lack of complete switching can presumably be attributed to the decomposition of the putative FeO3+ complex in the absence of substrate rotation or release followed by rebinding in the alternate orientation.

Collectively, these results (see Scheme 3) are interpreted to mean that (i) substrate exchange is rapid and (ii) the rate of C–H bond-breaking is an important contributor to the kcat for formation of the hydroxycoumarin products.7 The former conclusion is consistent with the measured kon and koff rates for coumarin with Fe3+ and Fe2+ P450 2A6 (Figs. 2 and 6). The latter result is consistent with the rapid rates estimated for all the steps that could be measured. The spectra recorded in the steady state do not provide a clear description of the step most limiting in the reaction (Fig. 10). Two complications exist; (i) a definite spectrum of the putative hypervalent iron (perferryl) complex is unavailable (FeO3+ spectra have only been recorded for P450s 101A1 (97) and 119A1 (98) and show {lambda}max ~ 370 nm (a region somewhat obscured by NADPH and the substrate coumarin), and (ii) the low efficiency of NADPH utilization (Table V) argues that a large fraction of iron-oxygen complexes will be non-productive.

Together the results provide a picture of P450 2A6 with a very dynamic reaction cycle and limited commitment to a dedicated course of reaction steps. The rates of ligand binding and dissociation we report here are not unusually high in the context of measured binding rates for other enzymes (67) and accepted Kd values. The knowledge that the substrate coumarin is well enclosed in the protein in the crystal structure (26) argues that the protein is opening and closing as fast as the kon and koff rates. The koff rate will be competitive with reduction (Figs. 2D and 5). The substrate can dissociate rapidly from the Fe2+ enzyme as well as the Fe3+ form (Fig. 6). The FeO22+-substrate complex is unstable; it breaks down rapidly (Figs. 7, B and C) and is relatively inefficient in accepting electrons from either NADPH-P450 reductase or b5 (Table II, Figs. 11, C and D). Thus, several steps in the cycle (Scheme 1) are inefficient. The isotope effect studies argue that it is the chemistry of substrate activation, not oxygen activation, that limits the rate of catalysis; however, steps preceding this step can be inefficient without necessarily being reflected in the isotope effect parameters.

One possible reason for the low rates of catalytic activity of most of the mammalian P450s is the existence of unproductive binding modes, which might be formed in large active sites (99–102). However, the crystal structure of P450 2A6 shows a tight active site (26) and the P450 2A6-coumarin complex has a low Kd (~1 µM) (Fig. 2B). Although a favorable alternate oxidation site (C-3) was found for the 7-OR substituted analogs (Table I, Scheme 2), the crystal structure can be used as an argument against the preponderance of non-productive binding modes. Reduction (of ferric P450) is reasonably fast with P450 2A6 although not as fast as in the P450 101A1 and 102A1 systems (5, 72, 103). An interesting feature of the P450 101A1 and 102A1 systems is that the binding of substrate has been demonstrated to lower the Em,7 (78, 104, 105). As discussed (see above), this change indicates that the substrate binds more tightly to the ferrous enzyme (74, 79). With a 1-electron process, a change of +120 mV translates to a 102-fold decrease in Kd. Thus, the values for P450 101A1 are koff {cong} 1 s–1 for the Fe3+ enzyme (5) and should be ~0.01 s–1 for Fe2+ P450 101A1 or 102A1. Thus, the bacterial P450(s) are committed to bind O2, which is a very fast process (kon = 8 x 107 M–1 s–1) (5, 72). The P450 101A1 FeO22+-camphor complex is more stable (kdecomposition = 0.01 s–1 at 25 °C) (72) than the P450 2A6 FeO22+ complex (Fig. 7C). Apparently the rate of introduction of the second electron is rate-limiting in the P450 101A1 oxidation of camphor (5, 72). Deuterium isotope effects are low for camphor hydroxylation (106) but can rise to 11–12 for slowly oxidized substrates (107). Thus, P450 101A1 can be viewed as having a very well defined linear pathway through the catalytic cycle (e.g. Scheme 1) with clear intermediates (although all seem to change structures) (108). P450 2A6, even though binding the substrate tightly at the start of the cycle (Scheme 1), has a much "noisier" trajectory, with considerable ligand exchange, slower forward reaction rates, and more unstable oxygenated intermediates. It is not clear whether the inefficiency of these earlier oxygen activation steps contributes to the high kinetic isotope effects7 or whether this is largely influenced by inherently poor geometry of the FeO3+-substrate complex (or intrinsically weaker chemical reactivity of the FeO3+ complex of some P450s). Some wobble does exist in the enzyme despite the low active site volume (26), as evidenced by the fraction of 3-hydroxylation observed for the 7-OR coumarins.

In concluding, a classical paradigm such as that shown in Scheme 1 may be too simplistic for an enzyme such as P450 2A6, which is very dynamic and introduces extra dimensions throughout the catalytic cycle, disrupting the smooth track normally assumed. We are interested in applying similar approaches to other human (and other) P450 enzymes to develop a better understanding of the factors that influence the rates and efficiencies of P450-catalyzed reactions. The demonstrated high rates of ligand exchange are of particular interest regarding substrates (e.g. several steroids) that appear to be sequentially utilized by P450s in multiple steps (33, 109).


   FOOTNOTES
 
* This work was supported in part by Korea Research Foundation Grant KRF-2000-015-FS002 and United States Public Health Service Grants R01 CA90426, P30 ES00267, and F32 ES12123. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. Back

{boxs} The on-line version of this article (available at http://www.jbc.org) contains Supplemental Fig. 1. Back

To whom correspondence should be addressed: Dept. of Biochemistry, Vanderbilt University School of Medicine, 638 Robinson Research Bldg., 23rd and Pierce Ave., Nashville, TN 37232-0146. Tel.: 615-322-2261; Fax: 615-322-3141; E-mail: f.guengerich{at}vanderbilt.edu.

1 The abbreviations used are: P450, cytochrome P450; OH, hydroxy; OMe, methoxy; OEt, ethoxy; OR, alkoxy; di-12:0 GPC, L-{alpha}-1,2-dilauroyl-sn-glycero-3-phosphocholine; MS, mass spectrometry; b5, cytochrome b5; HPLC, high performance liquid chromatography; Em,7, standard oxidation-reduction potential at pH 7.0. Conventions used for kinetic hydrogen isotope effects: Dk, intrinsic kinetic deuterium isotope effect, DV = Hkcat/Dkcat, and D(V/K) = (Hkcat/HKm)/(Dkcat/DKm) (1, 2). Back

2 3-Hydroxycoumarin was prepared (see "Experimental Procedures") and used as a standard for HPLC assays. A baculovirus-infected insect cell microsomal system was used in the earlier study (62), but no positive control for 7-hydroxylation was involved. Back

3 Higher rates of coumarin hydroxylation were observed when organic solvents were omitted from the reaction. Although microsomal coumarin 7-hydroxyation has been reported not to be very sensitive to CH3OH or dimethylsulfoxide (50), we found inhibitory effects of CH3OH and C2H5OH in our work with the reconstituted enzyme system. Coumarin stocks can be prepared at 5 mM in H2O and stored at 4 °C without any difficulty. Back

4 When NADPH-P450 reductase and the limited amount of NADPH were mixed in the presence of a slight excess of ferric cytochrome c, the extent of reduction of the cytochrome c ({Delta}{epsilon}550 = 2.1 x 104 M–1 cm–1) corresponded to 97% of the amount expected based on two cytochrome c molecules being reduced/NADPH (i.e., 5.0 nmol of NADPH and 5.0 nmol of NADPH-P450 reductase yielded 9.7 nmol of cytochrome c reduced). Back

5 The above experiments were all done with coumarin, for which the 7-hydroxylation reaction is stimulated by b5. However, the reason for the general lack of enhancement of the oxidations of the 7-OR coumarins is yet unexplained, and the above spectral experiments have not been extended to P450 2A6 in the presence of the other substrates. Back

6 The observed values we report here are not partitioned into primary and secondary kinetic isotope effects. Geminal secondary isotope effects are much lower than primary isotope effects (≤1.4) (73, 86). Relatively few secondary deuterium isotope effects have been measured in P450 reactions (87–89), and some of these were measured with crude microsomal systems, not individual enzymes (87, 88). The values (≤1.3) cannot be transposed to the P450 2A6 reactions under consideration here, i.e. 7-OR coumarin O-dealkylation. However, a reviewer has pointed out that, for [methyl-d2] 7-OMe coumarin O-demethylation, Dkobs = Dk1°/Dk2° (where 1° and 2° denote the true primary and secondary isotope effects). If Dk2° were 1.1, then with Dkobs = 9.8 (Table III), Dk1° = 10.8. However, in the intermolecular non-competitive experiment with [methyl-d3]-7-OMe coumarin, the D(V/K)2° should be raised to the third power, or 1.13 = 1.3, and D(V/K)obs = D(V/K)1°·D(V/K)2°, or D(V/K)1°= D(V/K)obs/D(V/K)2°. With D(V/K)obs = 9.0 (Table III), then D(V/K)1° = 6.9. Thus, some attenuation of the intramolecular isotope effect might not have been noticed in the comparisons, and the effect would be more dramatic if the secondary isotope effect is higher. In the absence of precise estimates of the (low) secondary isotope effects, we have accordingly avoided concluding that no suppression of the primary intrinsic isotope effect occurs in the intermolecular experiments. Nevertheless, the observed intermolecular D(V/K) values are still relatively high and are used in conclusions about the behavior of the enzyme reactions. Back

7 We have tried to avoid expressing a conclusion that the C–H bond-breaking step in 7-OR coumarin O-dealkylation is "rate-limiting." The difficulties of reaching conclusions about rate-limiting steps in enzyme reactions have been addressed by Northrop (90, 91), demonstrating that similar perturbations of reaction steps occurring either before or after the C–H bond-breaking step can have varying and somewhat surprising effects (90). A further complication exists with P450-catalyzed oxidations in that multiple alternate pathways often follow the apparently irreversible step of O2 activation, as opposed to a model in which the conversion of an enzyme-substrate complex to product is a rather linear pathway (90). The point has been raised that the observation of a D(V/K) isotope effect requires the existence of P450 branching after the irreversible step; i.e. decomposition or reduction of activated Fe-O complexes or "metabolic switching" to alternate oxygenations of the substrate (92, 93). This view has some validity (90) but does not offer a complete explanation for the observed high values of D(V/K) (or DV) presented in this study. One issue is that most microsomal P450s have poor coupling efficiency (31, 33, 34), including P450 2A6 (Table V). However, in kinetic simulations with P450 1A2 reactions using a simple model that has an irreversible O activation step followed by steps involving Fe-O complex decomposition (to H2O2 and H2O) and production of a product derived from the substrate (C–H bond-breaking), DV and D(V/K) both shared an inverse hyperbolic relationship with the rate of the C–H bond-breaking step (31). Moreover, the observed kinetic isotope effects can very considerably with different P450s and different reactions even though all are of low efficiency (31, 34). The variability of observed kinetic isotope effects in various P450 reactions (28, 29, 31, 34, 94) is further extended in further work with human P450s 2E1 and 3A4 (95, 96). Thus, the branched nature of the P450 reactions can contribute to the higher expression of kinetic isotope effects (92, 93) but is not a sufficient explanation in the absence of further kinetic details about particular reactions. Back


   ACKNOWLEDGMENTS
 
We thank M. V. Martin and W. A. McCormick for preparing the enzymes used here and K. Trisler for assistance in preparation of the manuscript.



   REFERENCES
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 INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
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