Identification of a kinetically distinct activity of 11beta-hydroxysteroid dehydrogenase in rat Leydig cells

Leydig cells are susceptible to direct glucocorticoid-mediated inhibition of testosterone biosynthesis but can counteract the inhibition through 11beta-hydroxysteroid dehydrogenase (11beta-HSD), which oxidatively inactivates glucocorticoids. Of the two isoforms of 11beta-HSD that have been identified, type I is an NADP(H)-dependent oxidoreductase that is relatively insensitive to inhibition by end product and carbenoxolone (CBX). The type I form has been shown to be predominantly reductive in liver parenchymal cells and other tissues. In contrast, type II, which is postulated to confer specificity in mineralocorticoid receptor (MR)-mediated responses, acts as an NAD-dependent oxidase that is potently inhibited by both end product and CBX. The identity of the 11beta-HSD isoform in Leydig cells is uncertain, because the protein in this cell is recognized by an anti-type I 11beta-HSD antibody, but the activity is primarily oxidative, more closely resembling type II. The goal of the present study was to determine whether the kinetic properties of 11beta-HSD in Leydig cells are consistent with type I, type II, or neither. Leydig cells were purified from male Sprague-Dawley rats (250 g), and 11beta-HSD was evaluated in Leydig cells by measuring rates of oxidation and reduction, cofactor preference, and inhibition by end product and CBX. Leydig cells were assayed for type I and II 11beta-HSD and MR messenger RNAs (mRNAs), and for type I 11beta-HSD protein. Leydig cell 11beta-HSD had bidirectional catalytic activity that was NADP(H)-dependent. This is consistent with the hypothesis that type I 11beta-HSD is present in rat Leydig cells. However, unlike the type I 11beta-HSD in liver parenchymal cells, the Leydig cell 11beta-HSD was predominantly oxidative. Moreover, analysis of kinetics revealed two components, the first being low a Michaelis-Menten constant (Km) NADP-dependent oxidative activity with a Km of 41.5 +/- 9.3 nM and maximum velocity (Vmax) of 7.1 +/- 1.2 pmol x min x 10(6) cells. The second component consisted of high Km activities that were consistent with type I:NADP-dependent oxidative activity with Km of 5.87 +/- 0.46 microM and Vmax of 419 +/- 17 pmol x min x 10(6) cells, and NADPH-dependent reductive activity with Km of 0.892 +/- 0.051 microM and Vmax of 117 +/- 6 pmol x min x 10(6) cells. The results for end product and CBX inhibition were also inconsistent with a single kinetic activity in Leydig cells. Type I 11beta-HSD mRNA and protein were both present in Leydig cells, whereas type II mRNA was undetectable. We conclude that the low Km NADP-dependent oxidative activity of 11beta-HSD in Leydig cells does not confirm to the established characteristics of type I and may reside in a new form of this protein. We also demonstrated the presence of the mRNA for MR in Leydig cells, and the low Km component could allow for specificity in MR-mediated responses.