NADH‐Dependent Aryl Hydrocarbon Hydroxylase in Rat Liver Mitochondrial Outer Membrane

Abstract
NADH-dependent 3,4-benzpyrene hydroxylase activity was detected in the purified mitochondrial outer membrane fraction from the livers of rats treated with 3-methylcholanthrene. The specific activity in the outer membrane fraction is nearly equal to that of microsomes, a level too high to be accounted for only by the microsomal contamination. The NADPH-dependent 3,4-benzpyrene hydroxylase activity in the outer membrane fraction is about 50% of that of microsomes. The ratio of the specific activity of NADPH to NADH-dependent 3,4-benzpyrene hydroxylase in microsomal fraction was about 3.5, while that of the outer membrane fraction was about 1.5. NADH-dependent 3,4-benzpyrene hydroxylase activity in the mitochondrial outer membrane from control rat liver was cyanide-insensitive, while that in microsomes was cyanide-sensitive. These results suggest that aryl hydrocarbon hydroxylase activity, which uses NADH almost as efficiently as NADPH, is present in the mitochondrial outer membrane fraction. The hydroxylase system is composed of cyanide-insensitive cytochrome P-450 and is inducible markedly by 3-methylcholanthrene treatment. The probable electron transfer pathways in the mitochondrial outer membrane cytochrome P-450 oxidase system are discussed.

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