Prolactin Induction of Enzymes Controlling Luteal Cholesterol Ester Turnover

Abstract

An enzyme in extracts of luteinized rat ovaries was found which catalyzed the synthesis of cholesterol esters from palmitic acid-l-14C and cholesterol and required ATP, Mg++ and Coenzyme A for expression of activity. This enzyme, sterol acyl transferase, was associated predominantly with the 100,000 ×g pellet fraction obtained from 10,000 ×g supernatant fractions of homogenized ovaries. The enzyme which hydrolyzes cholesterol esters, sterol esterase, was assayed simultaneously in 100,000 ×g supernatant fractions. Transferase and esterase levels were both markedly reduced (p <.01) when assayed 3 days after hypophysectomy (90 and 70%, respectively). Administration of rat prolactin (2 IU; sc; b.i.d.) was completely effective in preventing the decline of both enzymes after hypophysectomy, whereas LH (NIH-LH-S14; 50 μg daily in sesame oil+5% beeswax) was without effect. In hypophysectomized animals, LH and prolactin combined were less effective in maintaining transferase activity than prolactin treatment alone (0.05 <p <.10). Progesterone (2 mg; sc; daily) alone or with estradiol-17β did not reverse the effects of hypophysectomy on the level of these enzymes. Tissue cholesterol esters were depleted 80% after hypophysectomy (p<.01). Prolactin alone or in combination with LH maintained sterol ester levels higher (p<.01) than hypophysectomized or LH treated groups. The level of free cholesterol, on the other hand, was not significantly affected by treatment. These data are in agreement with a trophic action of prolactin on rat luteal tissue.