Cholesteryl esterase and endogenous cholesteryl ester pools in ovaries from maturing and superovulated immature rats

Results from the assay of cholesteryl esterase (EC 3.1.1.13) with radiolabelled substrate are difficult to interpret if endogenous cholesteryl ester is present. We overcame this problem by using an isotope dilution method to measure the endogenous pool sizes of cholesteryl ester in subcellular fractions of the ovary. This permitted calculation of the total cholesteryl esterase activity of the mitochondrial microsomal, and cytosolic fractions of the ovary. At all stages of ovarian development most cholesteryl esterase acitivity was found in the cytosol, and generally there was more activity in the microsomes than the mitochondria. The cholesteryl esterase in all three fractions exhibited higher activity with cholesteryl oleate as substrate than with cholesteryl palmitate. Increases in cholesteryl esterase activity and endogenous ester concentration were found at two stages of ovarian development; firstly after initiation of follicular growth by gonadotropin in the immature ovary, and secondly during luteinization. The increases were observed in all three sub-cellular fractions. Administration of human choriogonadotropin to rats which possessed luteinized ovaries resulted in activation of the mitochondrial and microsomal cholesteryl esterase but not the cytosolic enzyme.