Kinetic Studies on Ovarian C-17,20-Lyase Activity: Effect of Luteinizing Hormone Surge

Publisher Website
Google Scholar
Abstract
It has been shown that 3 h after the preovulatory gonadotropin surge, an abrupt decrease occurs in follicular C-17,20-lyase (lyase) activity which causes a decrease in C19-steroid production. To determine the reason for the reduced lyase activity, we used rats that were induced to ovulate by means of PMSG administration. In these rats, a 54% decrease in lyase activity occurred at the peak of the LH surge. When the gonadotropin surge and ovulation were blocked by pentobarbitone the decrease was prevented. Administration of LH to the pentobarbitone-blocked rats reduced lyase activity to well below the level reached after the endogenous gonadotropin surge. In cycling proestrous rats as well, human CG (hCG) decreased the lyase activity, as measured in isolated follicles 3 h after hCG administration. Out of three potential inhibitory steriods for lyase activity; progesterone, 3α,17α-dihydroxy-5α-pregnen-20-one, and 17α,20α-di-hydroxy-4-pregnen-3-one, only the last compound inhibited competitively ovarian lyase activity. The inhibition constant (Ki) value was 29 μm. In order to explore which of the two activities of the lyase complex is regulated by the gonadotropin, a double label double substrate experiment was conducted using [14C]progesterone with [3H]17α-hydroxyprogesterone (17α-OHP). With this assay procedure, we could determine in the same experiment the site of stimulation, the preferred substrate, and the amount of conversion. The conversion of progesterone to 17α-OHP, as well as the conversion to androstenedione were significantly inhibited throughout the reaction by the gonadotropin. Thus, the changes in ovarian lyase after hCG mimic those of 17α-hydroxylase. The labeling pattern of androstenedione showed that the ovarian lyase complex catalyzed the conversion preferentially from progesterone. Whereas the 3H/14C ratio in androstenedione varied between 0.29 to 0.76, the ratios in the 17α-OHP were from 5 to 22. Thus, the exogenous 17α-OHP did not equilibrate with the product formed from progesterone. The effect of the hCG was to decrease the preference of progesterone over 17α-OHP as substrate. It is concluded that: 1) the LH of the surge inhibits both the 17α-hydroxylase and lyase activities. 2) The ovarian lyase complex shows a preference for progesterone as a substrate rather than 17α-OHP. 3) 17α-OHP is not an obligatory intermediate in androstenedione production in ovarian tissue. 4) hCG affects the ovarian lyase complex by shifting the relative preference of substrates towards 17α-OHP. (Endocrinology117: 2376–2382, 1985)