Sex-related differences in diurnal activities and development of hepatic microsomal 3-hydroxy-3-methylglutaryl coenzyme A reductase and cholesterol 7alpha-hydroxylase.

The activities of hepatic microsomal 3-hydroxy-3-methylglutaryl CoA reductase and cholesterol 7alpha-hydroxylase were consistently higher (up to 3-fold) in female compared to male rats fed 2% cholestyramine for 8 h daily. In all animals studied, enzymic activities were highest 6 h after feeding began. However, 85% of the rise in cholesterol 7alpha-hydroxylase activity occurred in the 6 h before and 89% of the rise in 3-hydroxy-3-methylglutaryl CoA reductase activity occurred in the 6 h after feeding started. Sex-related differences in both enzymic activities first became apparent at the time of sexual maturity. Enzymic activities before weaning were generally low and a late-suckling (13--20 days) rise in cholesterol 7alpha-hydroxylase was not accompanied by a rise in 3-hydroxy-3-methylglutaryl CoA reductase. For all of these studies we assayed cholesterol 7alpha-hydroxylase at two concentrations of exogenous cholesterol to obviate problems relating to size of the cholesterol pool.