Specificity of Naphthoquinones as Cofactor for NADPH Oxidation by Liver Microsomes

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Abstract
1.Of the variously substituted naphthoquinones tested, those having a hydroxy, acetoxy, or methylamino group at position 2 or 3 showed no effects on the oxidation of NADPH by liver microsomes. The other quinones acted as cofactors and stimulated the microsomal oxidation. 2. Essentially the same specificity was observed with partly purified NADPH-specific flavoprotein as the oxidase, except that 2- or 3-acetoxy derivatives were effective cofactors for the purified enzyme. 3.The inability of the acetoxy compounds to act as cofactors for microsomes could be explained by the presence in microsomes of an active esterase rapidly hydrolyzing the compounds to ineffective 2- or 3-hydroxynaphthoquinones. 4. Naphthoquinones effective as cofactor reacted with cysteine resulting in considerable shifts in their spectra, wherease no such interactions were observed with ineffective compounds.