Corticosteroidogenesis in the toad Bufo arenarum H: evidence for a precursor role for an aldosterone 3 beta-hydroxy-5-ene analogue (3 beta, 11 beta, 21-trihydroxy-20-oxo-5-pregnen-18-al)

Steroid production in toads

In Bufo arenarum, androgen biosynthesis occurs through a complete 5-ene pathway, including 5-androstane-3beta,17beta-diol as the immediate precursor of testosterone. Besides, steroidogenesis changes during the breeding period, turning from androgens to C(21)-steroids such as 5alpha-pregnan-3alpha,20alpha-diol, 3alpha-hydroxy-5alpha-pregnan-20-one and 5alpha-pregnan-3,20-dione. In B. arenarum, steroid hormones are not involved in hCG-induced spermiation, suggesting that the steroidogenic shift to C(21)-steroids during the breeding be not related to spermiation. The activity of 17-hydroxylase-C(17-20) lyase (CypP450(c17)) decreases during the reproductive season, suggesting that this enzyme would represent a key enzyme in the regulation of seasonal changes. However, the increase in the affinity for pregnenolone of 3beta-hydroxysteroid dehydrogenase (3alphaHSD)/isomerase could also be involved. Moreover, the reduction in CypP450(c17) leading to a reduction in C(19)-steroids, among them dehydroepiandrosterone (DHE), would contribute to the conversion of pregnenolone into progesterone, avoiding the non-competitive inhibition exerted by DHE on this transformation. Additionally, CypP450(c17) possesses a higher affinity for pregnenolone than for progesterone, explaining the predominance of the 5-ene pathway for testosterone biosynthesis. Animals in reproductive condition showed a significant reduction in circulating androgens, enhancing the physiological relevance of all the in vitro results. The in vitro effects of mGnRH and hrFSH on testicular steroidogenesis revealed that both hormones inhibited CypP450(c17) activity. In summary, these results demonstrate that, in B. arenarum, the change in testicular steroidogenesis during the reproductive period could be partially due to an FSH and GnRH-induced decrease in CypP450(c17) activity.

Effect of salt acclimatization on 3 beta-hydroxysteroid dehydrogenase/isomerase activity in the interrenal of Bufo arenarum

In amphibians, aldosterone (Aldo) is particularly important in the regulation of Na(+) exchange by skin and urinary bladder. In previous works we studied a key enzyme in Aldo biosynthesis, the 3 beta-hydroxysteroid dehydrogenase/isomerase (3 beta HSD/I), in the interrenals of Bufo arenarum. In those works a dual localization of the 3 beta HSD/I in both microsomes and mitochondria was described. The mitochondrial, but not the microsomal, enzyme prefers the immediate Aldo precursor, 3 beta-analogue of aldosterone, as substrate. In this order, the enzyme 3 beta HSD/I would be not only a key enzyme for the synthesis of Aldo but additionally, due to its microsomal and mitochondrial localization, a possible target for the regulation of Aldo biosynthesis. With this rationale in mind, we have used in vivo and in vitro approaches to study Aldo regulation. In the present investigation the levels of Aldo were determined in plasma of winter (W) and summer (S) toads subjected to different saline concentrations (0.125 and 0.15 M) or kept on wet land. Saline hyperosmotically treated toads had significantly lower levels than isoosmotically treated toads. These results are consistent with the response in mammals, in which salt loading provokes a reduction in Aldo secretion. In W toads, plasmatic corticosterone (B) concentration was higher than Aldo concentration, whereas in S toads, B/Aldo ratio approached unity. The reduction of Aldo levels after saline dehydration was due to a decline in its biosynthesis. K(m) and V(max) values for 3 beta HSD/I were calculated for mitochondrial and microsomal fractions obtained from animals acclimated to 0.15 M NaCl or kept on land. As previously described, V(max) differs between W and S toads. However, only mitochondrial V(max) changed as a consequence of saline adaptation, suggesting that the mitochondrial enzyme could be involved in the regulation of Aldo biosynthesis.

Human chorionic gonadotropin-induced spermiation in Bufo arenarum is not mediated by steroid biosynthesis

This study employed an in vitro system to identify potential steroidal mediators of spermiation in Bufo arenarum. Testicular fragments were incubated for 2 h at 28 degrees. Spermiation was induced by 10 IU human chorionic gonadotropin (hCG) and the effect of different inhibitors of steroid biosynthesis was analyzed. Cyanoketone (10(-5)-10(-6) M), an inhibitor of 3-oxo-4-ene steroid biosynthesis, did not block hCG-inducing activity even when biosynthesis of 3-oxo-4-ene steroids and its reduced metabolites was inhibited by 95%. Aminogluthetimide at a concentration that inhibited testosterone biosynthesis (10(-4) and 10(-5) M) did not alter hCG actions. Similar results were obtained with spironolactone, an inhibitor of 17alpha-hydroxylase/17-20 lyase activity. No spermiation-inducing activity was found with different steroids (progesterone, 17-hydroxypregnenolone, 17, 20alpha/beta-dihydroxy-4-pregnene-3-one, estradiol, testosterone, etc.). It is concluded that spermiation induced by hCG is not steroid mediated in B. arenarum.

Kinetic properties of microsomal 3beta hydroxysteroid dehydrogenase-isomerase from the testis of Bufo arenarum H

3beta-hydroxysteroid dehydrogenase 5-ene isomerase (3betaHSD/I) activity is necessary for the biosynthesis of hormonally active steroids. A dual distribution of the enzyme was described in toad testes. The present study demonstrates that in testicular tissue of Bufo arenarum H., microsomal 3betaHSD/I has more affinity for dehydroepiandrosterone (DHEA) than for pregnenolone (K(m)=0.17+/-0. 03 and 1.02 microM, respectively). The Hill coefficient for the conversion of DHEA and pregnenolone were 1.04 and 1.01, respectively. The inclusion of DHEA in the kinetic analysis of pregnenolone conversion affected V(max) while K(m) was not modified, suggesting a non-competitive inhibition of the conversion of pregnenolone. K(i) was calculated from replot of Dixon's slope for each substrate concentration. K(i) from the intercept and the slope of this replot were similar (0.276+/-0.01 and 0.263+/-0.02 microM) and higher than the K(m) for DHEA. The K(m) and K(i) values suggest the presence of two different binding sites. When pregnenolone was present in the assays with DHEA as substrate, no effect was observed on the V(max) while K(m) values slightly increased with pregnenolone concentration. Consequently, pregnenolone inhibited the transformation of DHEA in a competitive fashion. These studies suggest that, in this species, the microsomal biosyntheses of androgens and progesterone are catalysed by different active sites.

Subcellular localization of 3 beta hydroxysteroid dehydrogenase isomerase in testis of Bufo arenarum H

3 Beta-hydroxysteroid dehydrogenase 5-ene isomerase (3 beta HSD/I) catalyzes an essential step in the biosynthesis of steroid hormones and is usually considered to be mainly microsomal, although there is a dual distribution of the enzyme in toad interrenals. The present study demonstrates that in the testicular tissue, as in interrenals of Bufo arenarum H., 3 beta HSD/I is both mitochondrial and microsomal. The conversion of dehydroepiandrosterone to androstenedione takes place only in microsomes while pregnenolone is converted to progesterone in both microsomes and mitochondria. Kinetic constants of 3 beta HSD/I were determined by the oxidation of pregnenolone and dehydroepiandrosterone. The preferred substrate of the microsomal 3 beta HSD/I enzyme was dehydroepiandrosterone (K(m) = 0.17 microM and 0.53 microM for dehydroepiandrosterone and pregnenolone, respectively) not only during the breeding season but also in the non-breeding period (K(m) = 0.49 microM and 2.9 microM for dehydroepiandrosterone and pregnenolone, respectively).