The complete stoichiometry of the metabolism of the cytochrome b (cyt b)-requiring substrate, methoxyflurane, by purified cytochrome P-450 2B4 was compared to that of another substrate, benzphetamine, which does not require cyt b for its metabolism. Cyt b invariably improved the efficiency of product formation. That is, in the presence of cyt b a greater percentage of the reducing equivalents from NADPH were utilized to generate substrate metabolites, primarily at the expense of the side product, superoxide.

With methoxyflurane, cyt b addition always resulted in an increased rate of product formation, while with benzphetamine the rate of product formation remained unchanged, increased or decreased. The apparently contradictory observations of increased reaction efficiency but decrease in total product formation for benzphetamine can be explained by a second effect of cyt b. Under some experimental conditions cyt b inhibits total NADPH consumption. Whether stimulation, inhibition, or no change in product formation is observed in the presence of cyt b depends on the net effect of the stimulatory and inhibitory effects of cyt b. When total NADPH consumption is inhibited by cyt b, the rapidly metabolized, highly coupled (50%) substrate, benzphetamine, undergoes a net decrease in metabolism not counterbalanced by the increase in the efficiency (2-20%) of the reaction. In contrast, in the presence of the slowly metabolized, poorly coupled (0.5-3%) substrate, methoxyflurane, inhibition of total NADPH consumption by cyt b was never sufficient to overcome the stimulation of product formation due to an increase in efficiency of the reaction.

CiteULike

Complore

Connotea

Del.icio.us

Digg

Reddit

Technorati

This article has been cited by other articles:

				N. N. Bumpus and P. F. Hollenberg Investigation of the Mechanisms Underlying the Differential Effects of the K262R Mutation of P450 2B6 on Catalytic Activity Mol. Pharmacol., October 1, 2008; 74(4): 990 - 999. [Abstract] [Full Text] [PDF]

				H. Zhang, D. Hamdane, S.-C. Im, and L. Waskell Cytochrome b5 Inhibits Electron Transfer from NADPH-Cytochrome P450 Reductase to Ferric Cytochrome P450 2B4 J. Biol. Chem., February 29, 2008; 283(9): 5217 - 5225. [Abstract] [Full Text] [PDF]

				H. Zhang, S.-C. Im, and L. Waskell Cytochrome b5 Increases the Rate of Product Formation by Cytochrome P450 2B4 and Competes with Cytochrome P450 Reductase for a Binding Site on Cytochrome P450 2B4 J. Biol. Chem., October 12, 2007; 282(41): 29766 - 29776. [Abstract] [Full Text] [PDF]

				C. W. Locuson, L. C. Wienkers, J. P. Jones, and T. S. Tracy CYP2C9 Protein Interactions with Cytochrome b5: Effects on the Coupling of Catalysis Drug Metab. Dispos., July 1, 2007; 35(7): 1174 - 1181. [Abstract] [Full Text] [PDF]

				C. W. Locuson, J. M. Hutzler, and T. S. Tracy Visible Spectra of Type II Cytochrome P450-Drug Complexes: Evidence that "Incomplete" Heme Coordination Is Common Drug Metab. Dispos., April 1, 2007; 35(4): 614 - 622. [Abstract] [Full Text] [PDF]

				T. A. Clarke, S.-C. Im, A. Bidwai, and L. Waskell The Role of the Length and Sequence of the Linker Domain of Cytochrome b5 in Stimulating Cytochrome P450 2B4 Catalysis J. Biol. Chem., August 27, 2004; 279(35): 36809 - 36818. [Abstract] [Full Text] [PDF]

				I. Fleming Cytochrome P450 and Vascular Homeostasis Circ. Res., October 26, 2001; 89(9): 753 - 762. [Abstract] [Full Text] [PDF]

				M. A. Shultz, D. Morin, A.-M. Chang, and A. Buckpitt Metabolic Capabilities of CYP2F2 with Various Pulmonary Toxicants and Its Relative Abundance in Mouse Lung Subcompartments J. Pharmacol. Exp. Ther., April 13, 2001; 296(2): 510 - 519. [Abstract] [Full Text]

				A. Bridges, L. Gruenke, Y.-T. Chang, I. A. Vakser, G. Loew, and L. Waskell Identification of the Binding Site on Cytochrome P450 2B4 for Cytochrome b5 and Cytochrome P450 Reductase J. Biol. Chem., July 3, 1998; 273(27): 17036 - 17049. [Abstract] [Full Text] [PDF]

				V. M. Guzov, H. L. Houston, M. B. Murataliev, F. A. Walker, and R. Feyereisen Molecular Cloning, Overexpression in Escherichia coli, Structural and Functional Characterization of House Fly Cytochrome b5 J. Biol. Chem., October 25, 1996; 271(43): 26637 - 26645. [Abstract] [Full Text] [PDF]