Characteristics and regulation of 11 beta-hydroxysteroid dehydrogenase of proximal and distal nephron

Enzymatic properties of the enzyme 11 beta-hydroxysteroid dehydrogenase (11-HSD), which confers mineralocorticoid selectivity, have been explored in the aldosterone-sensitive collecting duct (CCD) and the aldosterone-insensitive Pars Recta (PR) of the rat kidney. After incubation of freshly isolated tubular segments with [3H]corticosterone (3H-B) or [3H]dehydrocorticosterone (3H-A), the rate of transformation of 3H-B into 3H-A (dehydrogenase activity), or the reverse reaction (reductase activity) were measured by HPLC, Vmax for dehydrogenase activity was found to be 8- to 10-fold higher in CCD than PR. The enzyme functions over a very wide range (0.1-5000 nM) of corticosterone concentration. In CCD, enzyme kinetics suggest either the presence of two 11-HSD forms, differing by their affinity for corticosterone, or complex kinetics. Addition of NAD or NADP to permeabilized tubules revealed that dehydrogenase activity is NAD-dependent in CCD and NADP-dependent in PR. Cofactor addition was ineffective in intact tubules. CCD exhibited an exclusive dehydrogenase activity, whereas in PR dehydrogenase and reductase activity were found. No regulation of dehydrogenase activity could be evidenced in adrenalectomized rats receiving or not aldosterone, corticosterone or dexamethasone, for 2 h, 3 days or 4 days. We conclude that 11-HSD in the CCD and PR differs by its Vmax and cofactor dependence. Corticosteroid hormones do not influence 11-HSD activity.