Hydrogen peroxide formation and stoichiometry of hydroxylation reactions catalyzed by highly purified liver microsomal cytochrome P-450

Abstract

The stoichiometry of hydroxylation reactions catalyzed by cytochrome P-450 was studied in a reconstituted enzyme system containing the highly purified cytochrome from phenobarbital-induced rabbit liver microsomes. Hydrogen peroxide was shown to be formed in the reconstituted system in the presence of NADPH and oxygen; the amount of peroxide produced varied with the substrated added. NADPH oxidation, oxygen consumption, and total product formation (sum of hydroxylated compound and hydrogen peroxide) were shown to be equimolar when cyclohexane, benzphetamine, or dimethylaniline served as the substrate. The stoichiometry observed represents the sum of two activities associated with cytochrome P-450. These are (1) hydroxylase activity: NADPH + H+ + O2 + RH --> NADP+ + H2O + ROH; and (2) oxidase activity: NADPH + H+ + O2 --> NADP+ + H2O2. Benzylamphetamine (desmethylbenzphetamine) acts as a pseudosubstrate in that it stimulates peroxide formation to the same extent as the parent compound (benzphetamine), but does not undergo hydroxylation. Accordingly, when benzylamphetamine alone is added in control experiments to correct for the NADPH and O2 consumption not associated with benzphetamine hydroxylation, the expected 1:1:1 stoichiometry for NADPH oxidation, O2 consumption, and formaldehyde formation in the hydroxylation reaction is observed.