Androgen-dependent renal microsomal cytochrome P-450 responsible for N-hydroxylation and mutagenic activation of 3-methoxy-4-aminoazobenzene in the BALB/c mouse

Abstract

A murine renal microsomal enzyme responsible for the mutagenic activation of 3-methoxy-4-aminoazobenzene (3-MeO-AAB) was characterized by its catalytic activity for the mutagenic and metabolic conversion of 3-MeO-AAB. Incubation of 3-MeO-AAB with a renal or hepatic microsome fraction from male BALB/c mice in the presence of NADPH and NADH yielded N-hydroxy and 4'-hydroxy metabolites of 3-MeO-AAB as determined by two-dimensional thin layer chromatography, and the enzyme responsible for the N-hydroxylation was named 3-MeO-AAB N-hydroxylase. A mutagenicity test using Salmonella typhimurium TA98 bacteria as a tester strain has revealed that N-hydroxy-3-MeO-AAB is a potent direct mutagen but that 4'-hydroxy-3-MeO-AAB is not mutagenic. Although 3-MeO-AAB N-hydroxylase activity in liver microsomes showed no sex difference, the enzyme activity in the kidney was detected from male mice but not from females. However, administration of testosterone to female mice induced the enzyme in the kidney. Castration of male mice depressed the activity of 3-MeO-AAB N-hydroxylase in renal microsomes but it little affected the hepatic activity, and on administration of testosterone to the castrated mice the depressed renal microsomal activity recovered to a normal level. The activity of 3-MeO-AAB hydroxylase and the amount of cytochrome P-450 in renal microsomes showed a close correlation. Both renal and hepatic microsomes required NADPH as a main cofactor to mutagenize 3-MeO-AAB and to yield N-hydroxy-3-MeO-AAB from 3-MeO-AAB, and the enzyme activity was strongly inhibited by 7,8-benzoflavone. When the activities of renal and hepatic 3-MeO-AAB N-hydroxylase were compared on the basis of the amount of cytochrome P-450, the renal type enzyme showed about 8 times greater activity than hepatic type enzyme. These results indicate that the kidney contains an androgen-dependent microsomal 3-MeO-AAB hydroxylase which is different from an isozyme present in the liver and which is a new type of cytochrome P-450 isozyme.