Gonadal hormone-induced changes in hepatic microsomal carboxylesterase in rats

Hepatic microsomal carboxylesterase (E.C. 3.1.1.1) from rat liver microsomes showed a different capacity for the hydrolysis of various substrates. In castrated male rats, the enzyme activities towards p-nitrophenylacetate and malathion were decreased. When testosterone propionate was administered to castrated male rats, the activities of p-nitrophenylacetate and malathion hydrolases were reversely increased. However, in ovariectomized female rats, the carboxylesterase activities showed substrate-dependent changes, i.e., increase in p-nitrophenylacetate and malathion hydrolases and decrease in acetanilide and isocarboxazid hydrolases. When estradiol benzoate was administered to ovariectomized female rats, the activities of p-nitrophenyl-acetate and malathion hydrolases were decreased; and acetanilide and isocarboxazid hydrolases were increased. These results suggest that hepatic microsomal carboxylesterases may be, at least in part, regulated by gonadal hormones which exert different effects on the several isozymes of carboxylesterases.