On the stoichiometry of the oxidase and monooxygenase reactions catalyzed by liver microsomal cytochrome P-450. Products of oxygen reduction

This laboratory has recently reported that, in a reconstituted enzyme system containing alcohol-induced isozyme 3a of liver microsomal cytochrome P-450, the sum of acetaldehyde generated by the monooxygenation of ethanol and of hydrogen peroxide produced by the NADPH oxidase activity is inadequate to account for the O₂ and NADPH consumed. Studies on the stoichiometry have revealed the occurrence of an additional reaction involving an overall 4-electron transfer to molecular oxygen which is presumed to yield water: O₂ + 2 NADPH + 2H⁺ → 2 H₂O + 2 NADP⁺. The occurrence of a peroxidase reaction in which free H₂O₂ is reduced to water by NADPH was ruled out. When the 4-electron oxidase activity is taken into account, measurements of NADPH oxidation and O₂ consumption are in accord with the amounts of products formed in the presence of various P-450 isozymes, either in the absence or presence of typical substrates, including those which undergo hydroxylation, N- or O-demethylation, or oxidation of hydroxymethyl to aldehyde groups. Of the substrates examined, some had no effect on the oxidase reaction yielding hydrogen peroxide or the 4-electron oxidase reaction, some were inhibitory, and some were stimulatory, but the same substrate did not necessarily have the same effect on the two reactions.