Influence of species and sex on cholesterol 7 alpha-hydroxylase activity in experimental animals

Cholesterol 7 alpha-hydroxylase activity was evaluated measuring by selected ion monitoring the amount of 7 alpha-hydroxycholesterol formed from endogenous microsomal cholesterol and from exogenously added [4-14C]cholesterol in incubations of liver microsomes. The enzyme activity was determined in female rats, hamsters and guinea pigs and results were compared with those obtained in male rats. The results suggest that there is no difference between male and females as far as the activity of the enzyme from endogenous cholesterol is concerned at least in rat liver. Moreover, saturation of the enzyme by the endogenous substrate was observed in all female animals in contrast to the male rat for which the enzyme was found to be not saturated, as already reported by others. From molar radioactivity of 7 alpha-hydroxycholesterol obtained during incubations with [4-14C]cholesterol, differences were also evidenced in the equilibration of the substrate with the endogenous pool of cholesterol. In the male rat, equilibration depended upon the concentration of radioactive cholesterol added to the medium and reached completion at 60 microM concentration. In female animals, molar radioactivity of 7 alpha-hydroxycholesterol was always linearly correlated with that of cholesterol in the medium but it was always lower than that one. This indicates that in the tested female animals only part of exogenously added cholesterol is accessible to the substrate pool of the enzyme.