Factors influencing steroid hydroxylases in liver microsomes

Abstract

Liver microsomal enzymes oxidatively metabolize drugs, insecticides and steroids in the presence of TPNH and oxygen to compounds that are more polar than the substrate. Several factors listed below which alter the activity of the oxidative drug-metabolizing enzymes in liver microsomes, similarly alter the activity of liver microsomal enzymes which oxidize steroids to a mixture of oxidized products: (1) Phenobarbital and insecticides such as chlordane and DDT when administered to immature rats for 4 days increase several-fold the oxidation of both steroid hormones and drugs by liver microsomes; (2) Adult male rats have a much greater microsomal steroid and drug hydroxylase activity than do immature male rats; (3) Microsomal steroid and drug oxidation is greater in livers of adult male rats than in livers of adult female rats, whereas livers from female mice have at least as much activity as those from male mice; (4) The in vitro addition of 10 −4 m SKF 525-A or organophosphate insecticides, such as Chlorthion, results in an inhibition of steroid and drug oxidation by liver microsomes. These similarities in the factors which influence drug and steroid oxidation suggest that steroid hormones are normally occurring substrates for oxidative drug metabolizing enzymes in liver microsomes. More detailed studies on the regulation of individual hydroxylation reactions suggested that testosterone is hydroxylated in the 6β-, 7α- and 16α- positions of the steroid nucleus by more than one enzyme system. The in vitro addition of organophosphate insecticides such as Chlorothion (10 −5 m) inhibits the liver microsomal 16α-hydroxylation but has no effect on the 6β- or 7α-hydroxylation of testosterone. Comparisons of testosterone hydroxylase levels in immature and adult male rats revealed that the 16α-hydroxylase is low or absent in the immature male rat and rises to high levels in the adult, whereas 6β-hydroxylase activity increases to a lesser extent and 7α-hydroxylase activity does not increase at all. Treatment of immature male rats with phenobarbital, phenylbutazone, DDT or chlordane for several days increases markedly the liver 16α-hydroxylase activity but causes much smaller increases in the 6β- or 7α-hydroxylation of testosterone. These results suggest that the 16α-hydroxylation of testosterone is catalyzed by a different enzyme system than that required for the 6β- and 7α-hydroxylation. Further evidence for separate enzyme systems controlling the hydroxylation of steroids in various positions comes from experiments on species differences in steroid metabolism. The 7α-hydroxylation of testosterone by liver microsomal enzymes was not found to occur in the dog whereas the hydroxylation of testosterone in the 16α- and 6β-position did occur. When dogs were treated with phenylbutazone chronically, testosterone 16α- and 6β-hydroxylase activity in liver microsomes increased, whereas no 7α-hydroxylase activity could be found even in the phenylbutazone-treated dogs. Furthermore, when progesterone was incubated with liver microsomes from male and female rats, female rabbits, female guinea pigs and female mice, a different pattern of oxidized metabolites of progesterone was obtained for each species so that the chromatographic profile of metabolites was a characteristic fingerprint for each species studied.