Nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate-dependent reduction of mammalian hepatic microsomal cytochrome b5: some properties of the enzyme system catalyzing the endogenous reduction of pyridine nucleotides.

1. An enzyme system is present in mouse hepatic microsomes which is capable of reducing NAD+ and NADP+ and, consequently, of reducing cytochrome b5 in the absence of other externally added cofactors. 2. A similar enzyme system is also present in hepatic microsomes of rat and rabbit. 3. The rates of reduction of cytochrome b5 and auto-oxidation of reduced cytochrome b5 were, however, much slower in NAD+ supplemented reaction media than in those supplemented with NADH. 4. The effects of sulfhydryl reagents, chelating agents and divalent cations are reported.