Subcellular Localization of Membrane‐Bound Aryl‐Hydrocarbon Hydroxylase and NAD(P)H‐Dependent Reductase Activities in Mouse Liver

The subcellular distribution of aryl‐hydrocarbon hydroxylase, NADPH‐cytochrome c reductase, NADH:cytochrome c reductase, and NADH:cytochrome b 5 reductase activities in mouse liver was studied using the biochemical membrane markers microsomal glucose‐6‐phosphatase, mitochondrial cytochrome c oxidase, and plasma membrane 5′‐nucleotidase. The rate of appearance of activity of 3‐methylcholanthrene‐induced aryl‐hydrocarbon hydroxylase in the microsomes of C57BL/6N mice is more than twice as rapid as that in the nuclear envelope. The nuclear fraction contains less than 1% of the total cellular activities of the hydroxylase and all three reductases. All detectable basal activity of aryl‐hydrocarbon hydroxylase in the nuclear fraction of control CS7BL/6N and DBA/2N and 3‐methylcholanthrene‐treated DBA/2N mice and all detectable activities of NADPH:cytochrome c , NADH: cytochrome c , and NADH:catochrome b 5 reductase in the nuclear fraction of control and 3‐methylcholanthrene‐treated C57BL/6N and DBA/2N mice can be completely accounted for by the degree of microsomal fragment contamination (as assessed by the microsomal marker glucose‐6‐phosphatase). These data raise doubts about certain previous reports of ‘nuclear’ enzyme activities in which microsomal contamination was not taken into account. However, there is more induced activity of aryl‐hydrocarbon hydroxylase in the nuclear fraction of 3‐methylcholanthrene‐treated C57BL/6N mice than can be accounted for by the degree of microsomal membrane contribution. The murine Ah locus is known to regulate the induction by certain polycyclic aromatic chemicals of numerous drug‐metabolizing enzyme activities such as aryl‐hydrocarbon hydroxylase associated with cytochrome P 1 ‐450. The expression of 3‐methylcholanthrene‐inducible hydroxylase in nuclear membranes, like that in microsomal membranes, thus appears to be controlled by the Ah regulatory gene.