Hepatic microsomal electron transport with particular reference to azoreductase activity.

The reduction of the polar azo compound amaranth by hepatic microsomal NADPH-dependent electron transport components was investigated under anaerobic conditions. This activity was increased by riboflavin, FMN and FAD. Azoreductase activity in the absence or presence of flavins was increased or decreased, respectively, by type I or type II cytochrome P-450 substrates and ligands, and inhibited by carbon monoxide. Pretreatment of rats with cobaltous chloride, carbon tetrachloride or allylisopropylacetamide decreased azoreductase activity. Phenobarbitone or 3-methylcholanthrene increased the specific activity of the microsomal azoreductase in the absence of flavin, but had no effect on the flavin supplemented activity. Alkylation of the microsomal fraction with 2-bromo-4'-nitro-acetophenone or proteolytic digestion by trypsin decreased both azoreductase activities. Azoreductase activity was reconstituted from purified NADPH-cytochrome c (P-450) reductase, cytochrome P-450 and dilauroylphosplmtidylcholine liposomes. Possible mechanisms of interaction between the components of the NADPH-dependent electron transfer system and flavins and amaranth are discussed. It is proposed that cytochrome P-450 is the terminal electron donor during the microsomal reduction of amaranth and flavins.