Periovulatory Changes in Steroid C17,20-Lyase Activity in Ovaries of Immature Rats Treated with Pregnant Mare Serum Gonadotropin

Publisher Website
Google Scholar
Abstract
The effect of LH [luteinizing hormone] on ovarian steroid formation results initially in a rise of overall steroid production, which 2-4 h later is followed by a decrease in the formation of C19-steroids. To determine the reason for this decrease, the course of 17.alpha.-hydroxyprogesterone (17.alpha.-OHP) metabolism in immature rat ovaries (10,000 7x g supernatant) stimulated to ovulate with pregnant mare serum gonadotropin (PMSG) was investigated. Employing optimal conditions the C17,20-lyase was estimated from the formation of androstenedione from 17.alpha.-OHP as substrate. Km was 61 .+-. 4 .mu.M and V varied according to the time after PMSG administration. The initial activity at 0 h was 340 pmol min-1 mg-1 protein. It decreased 48 h after PMSG to 160 pmol, followed by a slight rise at 54 h and followed by a 60% fall at 57 h, at the height of the LH-surge. The activity declined to undetectable levels at 60 and 64 h (8 h before ovulation) and remained undetectable after ovulation at 72 and 96 h after PMSG. The enzymes 5.alpha.-reductase and 20.alpha.-hydroxysteroid dehydrogenase were estimated from the formed 3.alpha.,17.alpha.-dihydroxy-5.alpha.-pregnan-20-one and 17.alpha.,20.alpha.-dihydroxy-4-pregn-en-3-one, respectively. In the unstimulated ovary the 5.alpha.-reductase was 405 pmol; it decreased to 110 pmol 48 h after PMSG and continued to decrease. The 20.alpha.-hydroxysteroid dehydrogenase activity which was undetectable in untreated rats, rose to 330 pmol 60 h after PMSG, 8 h before ovulation. The changes in enzyme activities when tracer amounts of [3H]17.alpha.-OHP were used agreed with the pattern of kinetic studies. From these incubations 8 metabolites were isolated, one of them, 5.alpha.-pregnane-3.alpha.,17.alpha.,20.alpha.-triol, has not been previously identified. The decrease in C17,20-lyase activity limit the formation of C19-steroids in preovulatory follicles shortly before ovulation.