Seasonal dynamics, kinetics, and effects of 2-hydroxyestradiol-17β on some steroidogenic enzymes in the ovary of the catfish Heteropneustes fossilis.

The synthesis of bioactive steroids is catalyzed by an array of enzymes of diverse properties and actions. In the present study, seasonal dynamics and kinetics of key steroidogenic enzymes, 17α-hydroxylase (Cyp17a), 3β-hydroxysteroid dehydrogenase (Hsd3b), 20α-hydroxysteroid dehydrogenase (Hsd20a) and 20β-hydroxysteroid dehydrogenase (Hsd20b) were investigated in the female catfish Heteropneustes fossilis. Further, the effects of the estrogen metabolite 2-hydroxyestradiol-17β (2-OHE2) and human chorionic gonadotropin (hCG) on activities of the above enzymes, cytochrome P450 aromatase (Cyp19a1), and steroid products including testosterone and cortisol were determined. The enzymes under investigation showed significant seasonal variations across the annual ovarian cycle with low activity in the gonad resting phase. The enzymes Hsd3b and Cyp17a showed high activity during early oogenesis but the activities of Hsd20a and Hsd20b were higher towards late oogenesis in the spawning phase. Hsd3b and Cyp17a elicited high apparent Km values (low substrate affinity) and high apparent Vmax in the vitellogenic phase compared to the postvitellogenic phase. Hsd20a did not elicit any significant differences in the kinetic parameters between the two phases. Hsd20b showed high apparent Km values (low substrate affinity) and high Vmax in the postvitellogenic phase. The incubation of ovarian slices with 2-OHE2 for 24 h increased dose-dependently Hsd3b, Cyp17a, Hsd20a and Hsd20b activities, similar to hCG. The levels of the corresponding C21 steroid products, progesterone (P4), 17α-hydroxyprogesterone (17-OHP4), 17, 20α-dihydroxy-4-pregnen-3-one (17,20α-DP) and 17,20β-dihydoxy-4-pregnen-3-one (17,20β-DP), and cortisol were elevated. However, 2-OHE2 decreased significantly the C19 and C18 steroids, testosterone and E2 levels, and Cyp19a activity. The co-incubation with hCG and 2-OHE2 produced a synergistic effect on the enzyme activities except that of CYP19a. The co-incubation reversed the inhibitory effect of 2-OHE2. The data show that 2-OHE2 exerts a dual role on steroidogenesis, stimulating the C21 pathway and inhibiting the C19-C18 pathway, resulting in the steroidogenic shift. [ABSTRACT FROM AUTHOR]