Biosynthesis of 17α,20β-dihydroxy-4-pregnen-3-one in the ovaries of grey mullet ( Mugil cephalus) during induced ovulation by carp pituitary homogenates and an LHRH analogue

Abstract

Three female mullets received a priming injection of carp pituitary homogenate followed by a resolving injection of an LHRH analogue 24 hr later. Ovarian biopsies were obtained just prior to the first injection (phase I), 24 hr after the first injection (i.e., immediately before the second injection, phase II), and 8 hr after the second injection (phase III). Two fish (Nos. 1 and 3) spawned approximately 12 hr after the second injection. Serum levels of testosterone increased to some extent during phase II in all of the fish. Testosterone levels decreased abruptly during phase III in both fish Nos. 1 and 3. In contrast the concentration of 17α,20β-dihydroxy-4-pregnen-3-one (17α,20β-diOHprog) was undetectable in phases I and II, and increased dramatically during phase III in the same fish. In fish No. 2, which did not spawn, neither the decrease of testosterone nor the increase of 17α,20β-diOHprog was observed. Ovarian tissues of all the fish were pooled in each phase and incubated with 14C-labeled progesterone or 17α-hydroxyprogesterone to investigate steroid metabolism. During phases I and II progesterone was converted to 17α-hydroxyprogesterone, androstenedione, and testosterone. During phase III, production of these steroids decreased drastically, and in turn, synthesis of 20β-hydroxy-4-pregnen-3-one and 17α,20β-diOHprog was induced. Using 17α-hydroxyprogesterone as a substrate, androstenedione and testosterone were produced during phases I and II, whereas they decreased considerably during phase III. This was followed by the production of 17α,20β-diOHprog as the major metabolite. These results strongly suggest that the activities of 17α-hydroxylase and C 17,20-lyase are dampened and the activity of 20β-hydroxysteroid dehydrogenase is induced during final oocyte maturation and spawning. These changes agree with the observed serum profile of steroids.