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Originally published In Press as doi:10.1074/jbc.M308167200 on August 20, 2003

J. Biol. Chem., Vol. 278, Issue 43, 42098-42105, October 24, 2003
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A Novel Lipid Hydroperoxide-derived Cyclic Covalent Modification to Histone H4*

Tomoyuki Oe, Jasbir S. Arora, Seon Hwa Lee, and Ian A. Blair{ddagger}

From the Center for Cancer Pharmacology, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6160

Received for publication, July 25, 2003 , and in revised form, August 18, 2003.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
 REFERENCES
 
Previous studies have established that 4-hydroxy-2-nonenal is a lipid hydroperoxide-derived aldehydic bifunctional electrophile that reacts with DNA and proteins. However, it has now been recognized that 4-oxo-2-nonenal is also a major product of lipid hydroperoxide decomposition. Furthermore, 4-oxo-2-nonenal is more reactive than 4-hydroxy-2-nonenal toward the DNA-bases 2'-deoxyguanosine, 2'-deoxyadenosine, and 2'-deoxycytidine and proteins. The formation of 4-oxo-2-nonenal can be induced through vitamin C-mediated or transition metal ion-mediated homolytic decomposition of polyunsaturated {omega}-3 lipid hydroperoxides such as 13(S)-hydroperoxyoctadecadienoic acid. We have discovered that synthetic 4-oxo-nonenal or 4-oxo-2-nonenal-generated from 13(S)-hydroperoxyoctadecadienoic acid recognizes the specific amino acid motifs of His75, Ala76, and Lys77 in bovine histone H4. Reaction of the histidine and lysine residues with 4-oxo-2-nonenal results in the formation of a novel cyclic structure within the protein. The cyclic structure incorporates the histidine imidazole ring and a newly formed pyrrole derived from the lysine. The cyclic imidazole-pyrrole derivative that is formed from the small N{alpha}-acetyl-His-Ala-Lys peptide exists as a mixture of two atropisomers that inter-convert upon heating. Such lipid hydroperoxide-derived modifications could potentially modulate transcriptional activation in vivo. Furthermore, the ability to synthesize cyclic peptides using 4-oxo-2-nonenal will facilitate the preparation of novel structural analogs with potential biological activity.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
 REFERENCES
 
Over-production of reactive oxygen species during oxidative stress leads to the formation of PUFA1 lipid hydroperoxides through hydroxyl radical-mediated abstraction of a bis-allylic hydrogen atom of the PUFA followed by attachment of molecular oxygen (1). Linoleic acid, the major {omega}-6 PUFA present in plasma lipids, is converted to a complex mixture of 9- and 13-HPODE isomers. There is increasing awareness that oxidative stress can also lead to the production of PUFA lipid hydroperoxides by enzymatic pathways (2, 3). LOXs (4) and COXs (5) can convert linoleic acid into HPODEs but with much greater stereoselectivity than is observed in free radical reactions. Human 15-LOX produces mainly 13-HPODE (6). COX-1 and COX-2 produce mainly 13-HPODE and 9-HPODE by a mechanism similar to that described for dihomo-{gamma}-linolenic acid (7). The HPODEs are subsequently reduced to the corresponding 13(S)- and 9(R)-hydroxyoctadecadienoic acids through the peroxidase activity of the COXs (5, 8). The other C-18 PUFAs, linolenic acid ({omega}-3) and dihomo-{gamma}-linolenic acid ({omega}-6), which are minor constituents of plasma lipids, are also metabolized by 15-LOX. Linolenic acid is a poor substrate for COX-1 and COX-2 (9), but dihomo-{gamma}-linolenic acid has not been studied in detail. C-20 PUFAs all undergo 15-LOX-mediated conversion to hydroperoxides. The major 5-LOX- and COX-derived products from C-20 PUFAs (apart from eicosadienoic acid) are prostaglandins, thromboxanes, and leukotrienes rather than lipid hydroperoxides.

PUFA lipid hydroperoxides undergo transition metal ionand vitamin C-dependent (10) decomposition to the {alpha},{beta}-unsaturated aldehyde genotoxins, 4-oxo-2-nonenal, 4,5-epoxy-2(E)decenal, and 4-hydroxy-2-nonenal (11). 4-Oxo-2-nonenal is a particularly potent lipid hydroperoxide-derived genotoxin (12), which reacts with DNA-bases such as dGuo to form heptanoneetheno-dGuo adducts (Fig. 1) (13). We speculated that the guanidine moiety present in the amino acid arginine would react with 4-oxo-2-nonenal in a manner analogous to that observed with dGuo (Fig. 1A, box). Indeed, arginine residues have been found to react with 4-oxo-2-noneal (14). This finding stimulated an examination of how lipid hydroperoxide-derived 4-oxo-2-nonenal would modify arginine-containing proteins. Histones represent a class of proteins that are extremely arginine rich. Almost 14% of the histone H4 amino acids are arginine residues (Fig. 1B). Furthermore, it is known that alkylation of histone arginine residues results in transcriptional repression (15). Alkylation of arginine by lipid hydroperoxide-derived bifunctional electrophiles could potentially exert a similar epigenetic effect. Thus, up-regulation of COXs, LOXs, or other biochemical pathways involved in free radical generation could be involved in modulation of transcription through pathways that have not been considered previously.



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  		FIG. 1.
A, reaction of dGuo with 4-oxo-2-nonenal. The box shows the guanidine moiety that is also present in the amino acid arginine. B, amino acid sequence of bovine histone H4.

 


   EXPERIMENTAL PROCEDURES
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
RESULTS
 DISCUSSION
 REFERENCES
 
Materials and Reagents—Bovine histone H4 was purchased from Roche Applied Science. All synthetic peptides were purchased from Bio-Synthesis Inc. (Lewisville, TX). Trypsin, Staphylococcus aureus V8, ammonium bicarbonate, monosodium phosphate, disodium phosphate, CNBr, trifluoroacetic acid, and {alpha}-cyano-4-hydroxycinamic acid were purchased from Sigma. Formic acid (98%) was obtained from EM Sciences (Darmstadt, Germany). Ethanol was purchased from Pharmaco (Brookfield, CT). MOPS was obtained from Fluka BioChimika (Milwaukee, WI). Chelex-100 chelating ion-exchange resin (100-200 mesh size) was purchased from Bio-Rad. HPLC-grade water and acetonitrile were obtained from Fisher. Gases were supplied by BOC Gases (Lebanon, NJ). ZipTipC18 cartridges were obtained from Millipore Co. (Bedford, MA), and gel permeation cartridges (cutoff of 7000 Da) were obtained from Pierce. The Vydac C18 column (100 x 1.0 mm inside diameter; 300 Å, 5 µm) was obtained from Grace Vydac (Hesperia, CA), the YMC basic column (150 x 4.6 mm inside diameter; 120 Å, 5 µm) was supplied by Waters Co. (Milford, MA), the BDS Hypersil C18 column (100 x 1.0 mm inner diameter; 120 Å, 5 µm) was supplied by Keystone Scientific Inc. (Bellefonte, PA), and the Daicel Chiralcel OD-RH column (150 x 4.6 mm inner diameter) was supplied by Chiral Technologies Inc. (Exton, PA). NMR tubes (200 x 5 mm outside diameter) were obtained from Kontes (Vineland, NJ). The assignments of b and y product ions followed the suggested nomenclature of Roepstorff and Fohlman (16) as modified by Johnson et al. (17).

Syntheses of Deuterated 4-Oxo-2-nonenals and [13C18]13-HPODE— Authentic 2-[2H]-, 3-[2H]-, and 2,3-[2H2]-4-oxo-2-nonenals were prepared from 4-hydroxy-2-nonynal diethylacetal. The combination of LiAlH4/LiAl[2H4] and work-up with combinations of NH4Cl/H2O and N[2H4]Cl/[2H2]O allowed regio-selective deuterium incorporation. Oxidation of the products using MnO2 followed by de-protection resulted in formation of the appropriate deuterated 4-oxo-2-nonenal. The site of deuterium incorporation was established by the absence of an NMR signal from the appropriate hydrogen atoms that were present in the protium forms. The amount of protium analogs in the deuterated standards was established by ESI/MS as <0.1%. Authentic [13C18]13-HPODE was prepared from [13C18]linoleic acid using soybean lipoxygenase, as described previously (12). The amount of [12C18]linoleic acid was established by ESI/MS as <0.5%.

MALDI-TOF/MS—Analyses were performed using a Voyager-DE Pro biospectrometer (Applied Biosystems, Foster City, CA) in the reflector mode. External calibration was made by using three points from the following: matrix dimer (379.0930), des-Arg-bradykinin (904.4681), angiotensin I (1296.6853), Glu1-fibrinopeptide B (1570.6774), and neurotensin (1672.9175). Samples were purified with ZipTipC18 cartridges that were washed with 0.1% trifluoroacetic acid in H2O (2 x 10 µl) and eluted with 0.1% trifluoroacetic acid in 50% aqueous acetonitrile (10 µl). They were pre-mixed (1:1) with a matrix solution consisting of saturated {alpha}-cyano-4-hydroxycinamic acid in 0.1% trifluoroacetic acid in 50% aqueous acetonitrile and air-dried.

LC/ESI/MS—Analyses were performed on an LCQ Classic ion trap mass spectrometer (Thermo Finnigan, San Jose, CA). ESI was conducted using a needle voltage of 4.5 kV. Nitrogen was used as the sheath (60 psi), and auxiliary (5 units) gas was used with the heated capillary at 180 °C. CID experiments employed helium with a collision energy at 50% (1 V). Chromatography was performed using an Alliance 2690 HPLC system equipped with an autosampler, vacuum degasser, and column heater (Waters Co.). LC separations were performed using a Vydac C18 (100 x 1.0 mm inside diameter; 300 Å, 5 µm) column with a flow rate of 50 µl/min. Solvent A was 0.07% trifluoroacetic acid in water, and solvent B was 0.07% trifluoroacetic acid in acetonitrile. A linear gradient was employed as follows: 0 min, 1% B; 60 min, 91% B; 62 min, 100% B; 64 min, 1% B; and 90 min, 1% B.

LC/ESI/MRM/MS—Analyses were conducted on a TSQ 7000 triple-quadrupole mass spectrometer (Thermo Finnigan). ESI was conducted using a needle voltage of 4.5 kV. Nitrogen was used as the sheath (60 psi), and auxiliary (10 units) gas was used with the heated capillary at 220 °C. A collision offset energy of -30 eV and a collision cell pressure of 2.75 mT were used for CID experiments. Chromatography was also conducted using the Waters Alliance system. LC separations were performed using a Hypersil C18 column (100 x 1.0 mm inside diameter; 120 Å, 5 µm) (BD Biosciences) with a flow rate of 50 µl/min. Solvent A was 0.1% formic acid in water, and solvent B was 0.1% formic acid in acetonitrile. A linear gradient was employed as follows: 0 min, 1% B; 60 min, 91% B; 62 min, 100% B; 64 min, 1% B; and 90 min, 1% B.

NMR—1H spectra (500 MHz) were determined at 25 °C using a Varian UNITY 500 instrument. Samples were dissolved in 750 µl of [2H4]methanol and introduced into an NMR tube (200 x 5 mm outside diameter). The residual solvent peak (3.35 ppm) was used as the reference signal. Coupling constants were shown as Hz values. Acquisition conditions were as follows: spectral width of 6000 Hz, 30° pulse flip angle, 32,000 data points, and 32 transients.

Reaction of Histone H4 and Peptides with 4-Oxo-2-nonenal—Bovine histone H4 (9 nmol, 100 µl in pH 7.0 Chelex-treated 50 mM MOPS buffer) was allowed to react with 4-oxo-2-nonenal (45 nmol, 20 µl of ethanol) at 37 °C for 24 h. The sample was desalted with a protein desalting spin column (gel permeation cartridge, cutoff of 7000 Da) divided into five aliquots. The modified histone H4 was digested separately with trypsin in 0.1 M NH4HCO3, V8E in 0.1 M NH4HCO3, V8DE in 0.1 M phosphate buffer (pH 7.4), or 5% CNBr in 70% formic acid. Enzymes were used in the ratio of 1/20 (v/v). For the synthetic peptide experiments, each peptide (0.2 µmol, 200 µl in pH 7.0 Chelex-treated 50 mM MOPS buffer) was allowed to react with 4-oxo-2-nonenal, 2-[2H]-, 3-[2H]-, or 2,3-[2H2]-4-oxo-2-nonenal (0.16 µmol, 20 µl ethanol) at 37 °C for 24 h.

Reaction of Histone H4 with 13-HPODE—Bovine histone H4 (18 nmol, 200 µl in pH 7.0 Chelex-treated 50 mM MOPS) was allowed to react with 13-HPODE or [13C18]13-HPODE (500 nmol, 20 µl ethanol) in the presence of vitamin C (10 mM) at 37 °C for 24 h. The samples were desalted with protein desalting spin columns (Pierce) followed by tryptic digestion (1/40, v/v) in 0.1 M NH4HCO3 at 37 °C for 36 h.

Synthesis of N{alpha}-Acetyl-HAK (AcHAK)/4-Oxo-2-nonenal Adduct— 4-Oxo-2-nonenal (4.6 mg, 16.3 µmol) in 200 µl of ethanol was added to the solution of AcHAK (8.0 mg, 20.2 µmol) in 2 ml of 50 mM MOPS buffer (pH 7). The mixture was kept at 37 °C for 24 h. The modified peptide was purified on a YMC basic (150 x 4.6 mm inside diameter; 120 Å, 5 µm) column. Solvent A was 0.07% trifluoroacetic acid in water, and solvent B was 0.07% trifluoroacetic acid in acetonitrile. A linear gradient was run as follows: 0 min, 25% B; 15 min, 100% B; 17 min, 25% B; 30 min, 25% B. The flow rate was 800 µl/min. The major peak (tR = 6.8 min) was collected, and the fraction was concentrated to 1 ml under nitrogen. The concentrated fraction was desalted by using the same HPLC column and the water/acetonitrile mobile phase without trifluoroacetic acid. The pure adduct was obtained as a white solid (~2 mg).

Chiral Separation of AcHAK/4-Oxo-2-nonenal Isomers—LC was performed using a Daicel Chiralcel OD-RH (tris(3,5-dimethylphenylcarbamoyl)cellulose-coated silica gel) column (150 x 4.6 mm inside diameter) with UV detection at 220 nm. Solvents A and B were water and acetonitrile, respectively. A linear gradient was run as follows: 0 min, 10% B; 20 min, 40% B; 22 min, 10% B; 40 min, 10% B. The flow rate was 500 µl/min. The first isomer (tR = 13.5 min; ~1 mg) and the second isomer (tR = 14.5 min; ~1 mg) were collected separately, concentrated under a nitrogen stream, and obtained as white solids.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
DISCUSSION
 REFERENCES
 
LC/ESI/MS Analysis of Intact Bovine Histone H4 —The expected molecular mass of bovine H4 based on the amino acid sequence is 11,236.2. The multiply charged ion envelope was consistent with the presence of three molecular forms. Deconvolution of the signals revealed the presence of three different post-translation modifications, including dimethylation (11,265.01 ± 1.58 Da), acetylation together with dimethylation (11,307.01 ± 1.39 Da), and diacetylation together with dimethylation (11,348.01 ± 1.20 Da). The most abundant molecular form (as judged by its molecular ion envelope intensity) was acetyl-dimethyl-histone H4.

LC/ESI/MS Analysis of Bovine Histone H4 Digests—It was possible to obtain 89% coverage of the bovine histone H4 protein (Table I). Structural assignments were all confirmed by CID analysis and comparison of spectra with ProteinProspector version 4.0.5 (MS-digest). LC/MS analysis of selected peptide fragments obtained on trypsin digestion is shown in Fig. 2A. LC/MS analysis of selected V8 peptide fragments is shown in Fig. 3B. These experiments identified the site of dimethyllysine modification as Lys20 and acetylation as Lys16. The other (minor) site of acetylation was tentatively assigned as Lys5 or Lys8, because peptides containing these amino acid residues were not detected either by LC/MS or MALDI-TOF/MS analysis.


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  		TABLE I
LC/MS/MS characterization of peptide fragments from bovine histone H4 after digestion with proteases and CNBr

 


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  		FIG. 2.
LC/MS analysis of histone H4 after tryptic digestion and before (A) and after (B) treatment with 4-oxo-2-nonenal. a, H18R19. b, L37AR39. c, G9LGK12. d, R36LAR39. e, D68AVTYTEHAK77. f, I46SGLIYEETR55. g, D24NIQGITKPAIR35.

 


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  		FIG. 3.
LC/MS analysis of histone H4 after V8 digestion before (A) and after (B) treatment with 4-oxo-2-nonenal. Peptides T54RGVLKVFLE63 (h) and N64VIRDAVTYTE74 (i) were obtained using V8E. Peptide V86VYALKRQGRTLYGFGG102 (j) was obtained using V8DE.

 

LC/ESI/MS Analysis of 4-Oxo-2-nonenal-modified Bovine Histone H4 Digests—The peptide maps were almost identical with those obtained from unmodified bovine H4. However, there was a substantial reduction in the amount of one tryptic peptide (D68AVTYTEHAK77), as shown in Fig. 2B. The V8 (Fig. 3B) and CNBr (data not shown) fragments were essentially identical.

Reaction of D68AVTYTEHAKRKT80 with 4-Oxo-2-nonenal— MALDI/MS analysis of the reaction product revealed that the MH+ had increased from 1519.8 Da for the unmodified peptide to 1637.8 Da after reaction with 4-oxo-2-nonenal. This finding corresponded to a mass increase of 118 Da. The modified peptide was isolated and digested with trypsin. This resulted in the formation of peptide fragments with MH+ of 1408.51 Da (modified D68AVTYTEHAKR78) and 1252.42 Da (modified D68AVTYTEHAK77), respectively (Fig. 4A).



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  		FIG. 4.
MALDI-TOF/MS spectra of modified peptides after digestion. A, modified D68AVTYTEHAKRKT80 after tryptic digestion. B, modified D68AVTYTEHAK77 after V8E digestion.

 

Reaction of D68AVTYTEHAK77 with 4-Oxo-2-nonenal—Analysis of the modified peptide by MALDI/MS revealed the presence of a modified peptide with an MH+ of 1252.41. This finding was identical with the peptide observed in the digestion of 4-oxo-2-nonenal-modified D68AVTYTEHAKRKT80. Digestion of the modified peptide with V8E resulted in the formation of a peptide with an MH+ of 473.29 Da (Fig. 4B), as determined by MALDI/MS. This result corresponded to a modified form of H75AK77. LC/ESI/MS confirmed the MALDI data by showing the presence of a singly charged ion at m/z 473.1 (MH+). However, CID analysis provided no structurally informative ions. Major ions that were observed were at m/z 456 (MH+-NH3), 445 (MH+-CH=NH), and 428 (MH+-NH3-CH=NH).

Reaction of AcHAK with 4-Oxo-2-nonenal—Analysis of the reaction by LC/MS revealed a time-dependent decrease in MH+ at m/z 397. There was a concomitant increase in the formation of a compound with a retention time of 13.9 min and an MH+ at m/z 515. After 24 h, there was one major ion detected in the LC/MS total ion chromatogram (Fig. 5a). There was a substantial reduction in the amount of AcHAK, as evidenced from the intensity of the ion chromatogram for MH+ (m/z 397) (Fig. 5b). An intense ion chromatogram was observed for MH+ of the modified peptide at m/z 515 (Fig. 5c). CID of MH+ of the major product (m/z 515) resulted in the formation of a series of product ions at m/z 487 (MH+-CH=NH), 444 (MH+-C5H11), 428 (MH+-CH3CONH2-CH=NH) (Fig. 6A). This fragmentation pattern provided no structurally significant ions, in contrast to the unmodified AcHAK peptide. CID of MH+ from AcHAK (m/z 397) resulted in the formation of ions at m/z 269 (b2+H2O), 251 (b2), 218 (y2), 180 (b1), 152 (a1), and 147 (y1) (Fig. 6B).



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  		FIG. 5.
LC/MS analysis of the reaction mixture of AcHAK with 4-oxo-2-nonenal. a, total ion chromatogram (m/z 200-800). b, MH+ (m/z 397) for the residual AcHAK. c, MH+ (m/z 515) for the modified AcHAK.

 


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  		FIG. 6.
LC/MS/MS analyses of AcHAK and AcHAK adduct. A, atropisomers formed in the reaction of 4-oxo-2-nonenal with AcHAK. CID was conducted on MH+ at m/z 515. B, unmodified AcHAK. CID was conducted on MH+ at m/z 397.

 

Characterization of 4-Oxo-2-nonenal-modified AcHAK—The modified AcHAK peptide was purified to homogeneity by reversed-phase HPLC. The 1H NMR showed a series of signals in the aromatic region, {delta} (ppm) 6.01 (1H, d, J = 3.0 Hz; Ha from pyrrole): 6.10 (1H, d, J = 3.0 Hz; Ha from pyrrole); 6.58 (1H, d, J = 3.0 Hz; Hb from pyrrole), 6.62 (1H, d, J = 3.0 Hz; Hb from pyrrole); 6.68 (1H, s; Hc from imidazole), 6.74 (1H, s; Hc from imidazole); and 7.53 (1H, s; Hd from imidazole), 7.54 (1H, s; Hd from imidazole) (Fig. 7). This result suggested that there was a mixture of two isomers present. Chiral chromatography on a Daicel Chiralcel OD-RH was used to separate the two isomers (Fig. 8). NMR for the first eluting isomer ({delta} ppm) was as follows: 0.86 (3H, t, J = 6.5 Hz; CH3 from C5H11), 1.33 (3H, d, J = 7.0 Hz; CH3 from A), 1.0-1.8 (12H, m; {alpha}-, {beta}-, {gamma}-CH2 from lysine and CH2CH2CH2 from C5H11), 1.99 (3H, s; CH3CO), 2.25 (1H, m; CH from C5H11), 2.55 (1H, m; CH from C5H11), 3.00 (1H, q, J1 = 15.0 Hz, J2 = 7.5 Hz; {beta}H-1 from H), 3.10 (1H, d, J = 15.0 Hz; {beta}H-2 from H), 3.91 (1H, d, J = 10.5 Hz; {epsilon}H-1 from K), 3.96 (1H, brs; {epsilon}H-2 from K), 4.20 (1H, brs, {alpha}H from K), 4.67 (1H, brs; H{alpha} from H), 6.10 (1H, d, J = 3.0 Hz; Ha from pyrrole), 6.62 (1H, d, J = 3.0 Hz; Hb from pyrrole), 6.74 (1H, s; Hc from imidazole), and 7.53 (1H, s; Hd from imidazole). NMR for the second eluting isomer ({delta} ppm) was as follows: 0.84 (3H, t, J = 7.0 Hz; CH3 from C5H11), 1.30 (3H, d, J = 7.0 Hz; CH3 from A), 1.0-1.9 (12H, m; {alpha}-, {beta}-, {gamma}-CH2 from lysine and CH2CH2CH2 from C5H11), 1.97 (3H, s; CH3CO), 2.54 (1H, m; CH from C5H11), 2.66 (1H, m; CH from C5H11), 2.99 (1H, d, J1 = 15.0 Hz; {beta}H-2 from H), 3.07 (1H, q, J1 = 14.0 Hz, J2 = 8.5; {beta}H-1 from H), 3.94 (1H, d, J = 13.7 Hz; {epsilon}H-1 from K), 4.01 (1H, q, J1 = 13.7 Hz, J2 = 7.0 Hz, {epsilon}H-2 from K), 4.17 (1H, t, J = 6.1 Hz; {alpha}H from K), 4.55 (1H, brs; H{alpha} from H), 6.01 (1H, d, J = 3.0 Hz; Ha from pyrrole), 6.58 (1H, d, J = 3.0 Hz; Hb from pyrrole), 6.68 (1H, s; Hc from imidazole), and 7.54 (1H, s; Hd from imidazole).



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  		FIG. 7.
1H NMR spectrum showing the aromatic region of 4-oxo-2-nonenal-modified AcHAK purified by reversed-phase HPLC.

 


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  		FIG. 8.
Separation of 4-oxo-2-nonenal-modified AcHAK isomers by chiral phase chromatography using a Daicel Chiralcel OD-RH column.

 

Purified isomer 1 and purified isomer 2 were heated at 50 °C in water for 150 h. Analysis by chiral chromatography showed that eventually an equilibrium was established in which there was 40% of isomer 1 and 60% of isomer 2 present.

LC/MS Analysis of the Reaction of AcHAK with Deuterated 4-Oxo-2-nonenal Analogs—Treatment of AcHAK with 3-[2H]-4-oxo-2-nonenal resulted in the formation of an adduct with identical LC/ESI/MS characteristics to the adduct obtained from reaction of the protium form of 4-oxo-2-nonenal with AcHAK. The molecular ion region revealed ions at m/z 515.3 (MH+ protium form; 100%), 516.3 (24%), 517.3 (4%), and 518.3 (1%) (Fig. 9A). The product from 2-[2H]-4-oxo-2-nonenal was different in the molecular ion region. Ions were observed at m/z 515.3 (MH+ of protium form; 68%), 516.3 (MH+ of deuterium form; 100%), 517.3 (32%), and 518.3 (6%) (Fig. 9B). The molecular ion region obtained from the 2,3-[2H2]-4-oxo-2-nonenal-derived adduct was essentially identical with that obtained from 2-[2H]-4-oxo-2-nonenal. Ions were observed at m/z 515.3 (MH+ protium form; 51%), 516.3 (MH+ deuterium form; 100%), 517.3 (32%), and 518.3 (6%).



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  		FIG. 9.
Molecular ion region of AcHAK treated with deuterated 4-oxo-2-nonenals. A, 3-[2H]-4-oxo-2-nonenal. B, 2-[2H]-4-oxo-2-nonenal.

 

LC/MS/MS Analysis of 4-Oxo-2-nonenal-modified D68AVTYTEHAKR78—The CID spectrum of MH2+2 at m/z 705 resulted in the generation of product ions at m/z 1234.5 (b10), 1123.7 (y8), 1022.6 (y7), 859.5 (y6), 758.4 (y5), 647.5 (y10+2), 629.4 (y4), 612.1 (y9+2), 562.4 (y8+2), and 511.9 (y7+2) (Fig. 10A). An identical spectrum was obtained from D68AVTYTEHAKR78 isolated from the tryptic digestion of 4-oxo-2-nonenal-modified histone H4 (Fig. 10B). An LC/MRM/MS method was developed using the transition 704.8 (MH2+2) -> m/z 1123.6 (y8). Using a Hypersil reversed-phase column with a linear water/acetonitrile gradient, the major adduct eluted at 42.9 min. It was possible to separate two additional isomers with retention times of 44.0 and 46.2 min (Fig. 11A, upper). As expected, there were no signals in the channel corresponding to the 13C9 analog (709.3 -> m/z 1132.6) (Fig. 11A, lower).



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  		FIG. 10.
Product ion spectrum of MH2+2 (at m/z 705) of modified D68AVTYTEHAKR78. A, from 4-oxo-2-nonenal-treated D68AVTYTEHAKR78. B, tryptic peptide from 4-oxo-2-nonenal-treated bovine histone H4.

 


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  		FIG. 11.
LC/MRM/MS analysis of modified D68AVTYTEHAKR78. The upper channel corresponds to 4-oxo-2-nonenal-modified D68AVTYTEHAKR78 (704.8 -> m/z 1123.6) and the lower channel corresponds to [13C9]-4-oxo-2-nonenal-modified D68AVTYTEHAKR78 (709.3 -> m/z 1132.6). The absolute signal intensities (digital-to-analog converter units) are shown on the right-hand side of each chromatogram. A, 4-oxo-2-nonenal-treated D68AVTYTEHAKR78. B, tryptic peptides from 13-HPODE-modified bovine histone H4. C, tryptic peptides from [13C18]13-HPODE-modified bovine histone H4.

 

LC/MRM/MS Analysis of D68AVTYTEHAKR from Tryptic Digestion of 13-HPODE-modified Histone H4 —Homolytic 13-HPODE decomposition was induced by vitamin C in the presence of histone H4. The modified protein was then subjected to tryptic digestion followed by LC/MRM/MS analysis of 704.8 -> m/z 1123.6 (modified D68AVTYTEHAKR78) and 709.3 -> m/z 1132.6 (modified [13C9]D68AVTYTEHAKR78). The major isomer observed in the reaction of 4-oxo-2-nonenal with D68AVTYTEHAKR78 (Fig. 11A, upper) at a retention time of 42.9 min was observed at a similar retention time of 42.6 min (Fig. 11B, upper). The two other isomers were present at retention times of 43.6 and 45.9 min, respectively. Their retention times were also similar to the later eluting adducts observed in the reaction of 4-oxo-2-nonenal with D68AVTYTEHAKR78 (Fig. 11A, upper). However, these adducts were present in much lower abundance. There was a minor signal in the channel corresponding to the 13C9 analog (709.3 -> m/z 1132.6), which was not derived from the interaction of 13-HPODE with histone H4 (Fig. 11B, lower). A similar experiment was then conducted in which homolytic decomposition of [13C18]13-HPODE was induced by vitamin C in the presence of histone H4. The modified protein was then subjected to trypsin digestion. No signals were detected by LC/MRM/MS analysis in the channel corresponding to the 12C isomers (Fig. 11C, upper). The channel corresponding to the 13C9 isomers (Fig. 11C, lower) revealed the presence of a major adduct with an identical retention time to that observed previously (Fig. 11, A and B, upper). Minor isomers of the 12C analog were observed at 43.4 and 45.7 min, respectively (Fig. 11C, lower).

LC/MS Analysis of the Reaction of D68AVTYTEHAKR78 with Deuterated 4-Oxo-2-nonenal Analogs—Treatment of D68AVTYTEHAKR78 with 3-[2H]-4-oxo-2-nonenal resulted in the formation of an adduct with identical LC/ESI/MS characteristics (data not shown) to the adduct obtained from reaction of the protium form of 4-oxo-2-nonenal with D68AVTYTEHAKR78 (Fig. 12A). The molecular ion region of the major (first eluting) adduct revealed ions at m/z 1408.7 (MH+ protium form; 100%), 1409.7 (81%), 1410.7 (30%), 1411.8 (10%), and 1412.6 (2%) (Fig. 12A). The major product from 2-[2H]-4-oxo-2-nonenal was different in the molecular ion region. Ions were observed at m/z 1408.7 (MH+ of protium form; 53%), 1409.7 (MH+ of deuterium form; 100%), 1410.7 (61%), 1411.7 (23%), and 1412.8 (8%) (Fig. 12B). The molecular ion region obtained from the major 2,3-[2H2]-4-oxo-2-nonenal-derived adduct was essentially identical with the major adduct obtained from 2-[2H]-4-oxo-2-nonenal. Ions were observed at m/z 1408.7 (MH+ protium form; 88%), 1409.7 (MH+ deuterium form 100%), 1410.7 (44%), 1411.7 (19%), and 1412.8 (3%). The less abundant adducts retained deuterium during the reaction.



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  		FIG. 12.
Molecular ion region of D68AVTYTEHAKR78 treated with deuterated 4-oxo-2-nonenals. A, 3-[2H]-4-oxo-2-nonenal. B, 2-[2H]-4-oxo-2-nonenal.

 


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
REFERENCES
 
It has long been thought that 4-hydroxy-2-nonenal is the major lipid hydroperoxide-derived aldehydic bifunctional electrophile that reacts with DNA (18) and proteins (19). However, it has now been recognized that 4-oxo-2-nonenal is also a major product of homolytic lipid hydroperoxide decomposition (12, 20), and that it is more reactive than 4-hydroxy-2-nonenal toward the DNA-bases dGuo (13, 21), 2'-deoxyadenosine (22, 23), and 2'-deoxycytidine (24), as well as selected amino acids (14) and proteins (25, 26). The precise mechanism for 4-oxo-2-nonenal formation from lipid hydroperoxides is still somewhat controversial. However, there is unequivocal evidence that 4-hydroperoxy-2-nonenal is the precursor of both 4-oxo-2-nonenal (10) and 4-hydroxy-2-nonenal (10, 27) and that 4-hydroperoxy-2-nonenal is a major product of homolytic lipid hydroperoxide decomposition (12, 27). Studies on 4-hydroxy-2-nonenal- and 4-oxo-2-nonenal-derived modifications to proteins have been mainly concerned with the ability of these bifunctional electrophiles to induce cross-linking of proteins (19, 25). We have now focused on the possibility that modifications to amino acids within a single protein can occur. Our studies have used the histone H4 protein because we originally hypothesized that modifications could arise from 4-oxo-2-nonenal-mediated reactions with arginine residues (14) in this arginine-rich protein (Fig. 1 B). Such modifications could potentially modulate transcriptional activation in a manner similar to mechanisms that have been reported for other histone alkylation pathways (28).

Bovine H4 was found to consist of three molecular forms. The major form contained a dimethyllysine modification of Lys20 and acetylation of Lys16 (Fig. 1B). Bovine histone H4 was treated with 4-oxo-2-nonenal, and digestion of the modified proteins was conducted with CNBr and a series of proteases including trypsin, V8E, and V8DE. Comparison of the LC/MS profiles of the tryptic fragments of unmodified histone H4 proteins (Fig. 2A) with the tryptic fragments after treatment with 4-oxo-2-nonenal (Fig. 2B) revealed one major difference. The tryptic fragment D68AVTYTEHAK77 was reduced in intensity by more than 2 orders of magnitude (Fig. 2B, e). The presence of V8E peptide fragment N64VIRDAVTYTE74 in the untreated protein (Fig. 3A, upper) as well as the 4-oxo-2-nonenal-treated protein (Fig. 3B, upper) upon LC/MS analysis suggested that a modification had occurred in the sequence H75AKRK79. This finding meant that a structural motif within the histone protein was directing the reaction rather than a single amino acid such as arginine.

A series of peptides from the region containing the H75AKRK79 sequence was then synthesized and subjected to reaction with 4-oxo-2-nonenal. The peptide D68AVTYTEHAKRKT80 showed an increase in mass from 1519.8 to 1637.8 Da (corresponding to the addition of 118 Da or C9H10). Trypsin cleavage resulted in the formation of peptide fragments (D68AVTYTEHAKR78 and D68AVTYTEHAK77) that each contained the C9H10 modification (Fig. 4A). The synthetic tryptic fragment D68AVTYTEHAK77 then was subjected to reaction with 4-oxo-2-nonenal. Exactly the same increase in mass (from 1134 to 1252 Da) was observed as in the tryptic fragment from the 4-oxo-2-nonenal-modified D68AVTYTEHAKRKT80. These data suggested that the modification had occurred on H75AK77. Protease digestion of the modified peptide with V8E provided H75AK77 (473.29 Da) in which the C9H10 was still present (Fig. 4B). This result confirmed that it was indeed the HAK motif that had directed addition of the 4-oxo-2-nonenal to the histone protein. Unfortunately, there were no structurally significant product ions when the HAK peptide was subjected to CID and MS/MS analysis. In fact, the pattern of product ions was very reminiscent of cyclic peptides.

Treatment of the small model peptide AcHAK with 4-oxo-2-nonenal resulted in the addition of C9H10 (+118 Da) (Fig. 5c) and a product ion spectrum (from CID of MH+ at m/z 515) that was again reminiscent of a cyclic peptide (Fig. 6A). The CID spectrum was quite different from that obtained from the unmodified AcHAK (Fig. 6B), where the expected series of y and b ions was observed (16, 17). 1H NMR studies revealed that the modified peptide was, in fact, a mixture of two isomers (Fig. 7). The isomers were subsequently separated by preparative chiral chromatography (Fig. 8). The first eluting isomer had signals that were diagnostic for the presence of a substituted pyrrole ring with a doublet at 6.62 ppm (Fig. 7) coupled to a doublet at 6.10 ppm (with a coupling constant of 3.0 Hz). The presence of a second heteroaromatic ring was confirmed by the presence of an upfield singlet at 6.74 ppm (Fig. 7) and a downfield singlet at 7.53 ppm, signals that are diagnostic of a substituted imidazole ring. The later eluting isomer had an identical pattern of signals except that they were shifted slightly downfield. The isomers were found to inter-convert when heated at 50 °C, which suggested that they were atropisomers (29). We believe that the atropisomerism is a consequence of the attachment of an imidazole ring (from histidine) at position 3 of the 4-oxo-2-nonenal followed by lysine-mediated pyrrole ring formation (Scheme I). Conducting the reaction of AcHAK with deuterium-labeled 4-oxo-2-nonenal analogs and analyzing the products by LC/MS established the regio-selectivity of the reaction. Reaction with 3-[2H]-4-oxo-2-nonenal resulted in the formation of cyclic peptide atropisomers with no deuterium (Fig. 9A), whereas 2-[2H]-4-oxo-2-nonenal (Fig. 9B) and 2,3-[2H2]-4-oxo-2-nonenal (data not shown) each resulted in products that had lost 50% of the deuterium content.



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  		SCHEME I.
Proposed mechanism for the formation of the cyclic modification of AcHAK with 3-[2H]-4-oxo-2-nonenal.

 

The peptide obtained from reaction of 4-oxo-2-nonenal with synthetic D68AVTYTEHAKR78 was analyzed by LC/MS/MS. CID was conducted on the doubly charged MH2+2 ion at m/z 705 (Fig. 10A). A rich series of y ions (16, 17) down to the y4 ion at the modified histidine moiety was produced. A weak b10 ion, which arose by loss of the unmodified terminal arginine, was also present. An identical spectrum was observed from the tryptic peptide obtained by digestion of 4-oxo-2-nonenal-modified histone H4 (Fig. 10B). A sensitive and selective LC/MRM/MS method for the modified D68AVTYTEHAKR78 was then established (Fig. 11A). Finally, histone H4 was treated with a 13-HPODE/vitamin C system to generate 4-oxo-2-nonenal in situ (10). The modified D68AVTYTEHAKR78 isomers resulting from tryptic digestion (Fig. 11B, upper) were identical with the synthetic D68AVTYTEHAKR78 treated with 4-oxo-2-nonenal (Fig. 11A, upper). However, D68AVTYTEHAKR78 obtained from [13C18]13-HPODE was 9 Da higher in molecular mass (Fig. 11C, lower). Interestingly, the minor isomers were present in much lower abundance in the 68AVTYTEHAKR78 adduct derived from histone H4 when compared with the model peptide. This finding suggests that the tertiary structure of the protein adds further constraints on the cyclization reaction to produce mainly the imidazole-pyrrole atropisomers.

These studies confirm that the HAK motif is a target for the lipid hydroperoxide-derived bifunctional electrophile, 4-oxo-2-nonenal. We anticipate that the H113AK115 motif in the carboxyl-terminal region of bovine histone H3 (30) will be modified in a similar manner. The ability to modify histone proteins to a novel cyclic structure could have significant functional consequences. A similar modification to histidine and lysine was reported in model studies with protein side chains (25). Alkylation of histone lysine residues results in transcriptional silencing, whereas acetylation results in transcriptional activation (28). It is noteworthy that methylation of Lys79 in the carboxyl terminus of histone H3 was reported recently (31). The HAK motif in other proteins involved in transcriptional regulation such as human histone deacetylases 1, 2, and 3 (H141AK143, H142AK144, and H135AK137) (32) are also targets for lipid hydroperoxide-derived bifunctional electrophiles such as 4-oxo-2-nonenal. Furthermore, we have already demonstrated that the sequence KAH and the replacement of alanine by other neutral amino acids such as proline were able to undergo the same type of modification (data not shown). Therefore, nuclear receptor proteins such as human peroxisome proliferator-activated receptor {gamma}-1 and {gamma}-2 that contain the K95PH97 and K125PH127 motifs, respectively (33), are also potential targets for lipid hydroperoxide-derived bifunctional electrophiles. Cyclic peptides, as exemplified by vancomycin and cyclosporin, often have potent biological activity (34, 35). The finding that cyclic peptides result from reactions with 4-oxo-2-nonenal provides a new way to prepare novel peptide structures that can be tested for biological activity.


   FOOTNOTES
 
* This work was supported by National Institutes of Health Grant CA95586. 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

{ddagger} To whom correspondence should be addressed: Center for Cancer Pharmacology, 1254 BRB II/III, 421 Curie Blvd., University of Pennsylvania, Philadelphia, PA 19104-6160. Tel.: 215-573-9880; Fax: 215-573-9889; E-mail: ian{at}spirit.gcrc.upenn.edu.

1 The abbreviations used are: PUFA, polyunsaturated fatty acid; HPODE, hydroperoxyoctadecadienoic acid; LOX, lipoxygenase; COX, cyclooxygenase; 13-HPODE, 13(S)-hydroperoxy-(Z,E)-9,11-octadecadienoic acid; 9-HPODE, 9(R)-hydroperoxy-(E,Z)-10,12-octadecadienoic acid; MOPS, 3-morpholinopropanesulfonic acid; HPLC, high pressure liquid chromatography; ESI/MS, electrospray ionization mass spectrometry; MALDI-TOF, matrix-assisted laser desorption/ionization time-of-flight; LC, liquid chromatography; CID, collision-induced dissociation; MRM, multiple reaction monitoring; AcHAK, N{alpha}-acetyl-HAK; MH+, protonated molecular ion; , doubly charged protonated molecular ion; dGuo, 2'-deoxyguanosine; V8E, S. aureus V8 in bicarbonate buffer to cleave at Glu; V8DE, S. aureus V8 in phosphate buffer to cleave at Asp and Glu. Back



   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
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