Effect of Hypophysectomy on 17α-Hydroxylase Activity in Theca Cells of the Ovarian Follicle in the Japanese Quail

The activity of 17 alpha-hydroxylase, a key enzyme for progesterone metabolism in avian follicles, was measured in cell-free homogenates of isolated theca cells of Japanese quail. Kinetic studies revealed that differences in enzyme activity in the largest and the second largest follicles were mainly due to differences in Vmax values. In order to investigate the mechanism of regulation of 17 alpha-hydroxylase activity, quail were hypophysectomized at various time periods during the ovulatory cycle, and 17 alpha-hydroxylase activity in theca homogenates was measured 4 h after surgery. Hypophysectomy at 8 h before the predicted time of ovulation, which is the time when the preovulatory luteinizing hormone (LH) surge is initiated, resulted in a significant decrease in enzyme activity. In contrast, activity was increased when the pituitary was removed 20 h before ovulation. Hypophysectomy 24 h before ovulation had no effect on enzyme activity.