Molecular Pharmacology, Vol 15, 428-438, Copyright © 1979 by the American Society for Pharmacology and Experimental Therapeutics

Active Oxygen in Liver Microsomes: Mechanism of Epinephrine Oxidation

¹ Department of Pharmacology, Yale University School of Medicine, New Haven, Connecticut 0651O

Under normal conditions, during the oxidation of NADPH, liver microsomes are shown to generate hydrogen peroxide but not appreciable amounts of superoxide anion. This latter species can be detected, however, if iron pyrophosphate is present. The NADPH-supported microsomal oxidation of epinephrine is non-self-propagating, i.e., requires an external catalyst. It requires NADPH in a stoichiometry of 1 per adrenochrome formed, while utilizing 3 oxygens and producing 3 peroxides. This reaction is inhibited by superoxide dismutase but not by catalase and does not appear to diminish the background rate of NADPH mediated hydrogen peroxide formation, a reaction utilizing equal amounts of oxygen and NADPH for a 1:1:1 stoichiometry with hydrogen peroxide formation. Adrenochrome formation always exhibits a lag period. This is shown here to be due to accumulation of an intermediate of epinephrine oxidation. From the data presented it is concluded that if free superoxide anion is released to the aqueous medium, it is only in small amounts and rapidly dismutes. Epinephrine does not reduce cytochrome P-450. NADH does not support the oxidation of epinephrine to adrenochrome to any appreciable extent, but does allow peroxide formation to occur, presumably by other routes. A schematic mechanism of epinephrine oxidation is shown demonstrating the observed 1:3 stoichiometry of adrenochrome:peroxide formation.