The 11β-hydroxysteroid dehydrogenase type 2 activity in human placental microsomes is inactivated by zinc and the sulfhydryl modifying reagent N-ethylmaleimide

Proper glucocorticoid exposure in utero is vital to normal fetal organ growth and maturation. The human placental 11beta-hydroxysteroid dehydrogenase type 2 enzyme (11beta-HSD2) catalyzes the unidirectional conversion of cortisol to its inert metabolite cortisone. thereby controlling fetal exposure to maternal cortisol. The present study examined the effect of zinc and the relatively specific sulfhydryl modifying reagent N-ethylinaleimide (NEM) on the activity of beta-HSD2 in human placental microsomes. Enzyme activity. reflected by the rate of conversion of cortisol to cortisone, was inactivated by NEM (IC50 = 10 muM), while the activity was markedly increased by the sulfhydryl protecting reagent dithiothreitol (DTT; EC50 = I mM). Furthermore, DTT blocked the NEM-induced inhibition of 11beta-HSD2 activity. Taken together, these results suggested that the sulfhydryl (SH) group(s) of the microsomal 11beta-HSD2 may be critical for enzyme activity. Zn2+ also inactivated enzyme activity (IC50= 2.5 muM), but through a novel mechanism not involving the SH groups. In addition, prior incubation of human placental microsomes with NAD(+) (cofactor) but not cortisol (substrate) resulted in a concentration-dependent increase (EC50 = 8 muM) in 11beta-HSD2 activity, indicating that binding of NAD(+) to the microsomal 11beta-HSD2 facilitated the conversion of cortisol to cortisone. Thus, this finding substantiates the previously proposed concept that a compulsorily ordered ternary complex mechanism may operate for 11beta-HSD2, with NAD(+) binding first, followed by a conformational change allowing cortisol binding with high affinity. Collectively, the present results suggest that cellular mechanisms of SH group modification and intracellular levels of Zn2+ may play an important role in regulation of placental 11beta-HSD2 activity. (C) 2002 Elsevier Science B.V. All rights reserved.