The stimulation of microsomal azoreduction by flavins

The reduction of the azo dye, amaranth, by rat liver microsomes is inhibited about 90% by carbon monoxide, suggesting that the reaction largely depends on cytochrome P-450. Reducing equivalents for this reaction are supplied by NADPH. This reaction is stimulated by riboflavin, FMN and FAD, as well as by methylviologen. A large fraction of the stimulated reaction is not blocked by CO, indicating that there is a pathway of electron transfer which is independent of cytochrome P-450. Rat liver microsomes can reduce FAD, with reducing equivalents supplied by NADPH. The FADH2 thus produced is quickly oxidized by amaranth so that two FADH2 are oxidized for every amaranth reduced. The same stoichiometry is observed with photochemically prepared FADH2, formed in the absence of microsomes.