11β-hydroxysteroid dehydrogenase mediates reductive metabolism of xenobiotic carbonyl compounds

Abstract

The enzyme 11β-hydroxysteroid dehydrogenase (11β-HSD) is considered to confer mineralocorticoid specificity on the non-selective Type I adrenocorticoid receptor by converting active 11-hydroxyglucocorticoids to receptor-inactive 11-oxo metabolites, in mineralocorticoid target tissues like the kidney. However, 11β-HSD is also present in the liver, where it may regulate steroid exposure to the glucocorticoid Type II receptor. Because of the much higher activities compared to that in kidney, liver 11β-HSD possibly has additional functions besides the metabolism of glucocorticoids. In the present investigation we have isolated 11β-HSD from mouse liver microsomes and demonstrate that the homogeneously purified enzyme is also capable of catalyzing the reductive metabolism of xenobiotic carbonyl compounds such as metyrapone, p-nitroacetophenone and p-nitrobenzaldehyde. Enzyme kinetic studies revealed that, in addition to NADP +, mouse liver 11β-HSD also accepts NAD + as cosubstrate for glucocorticoid 11β-dehydrogenation. NADH as cosubstrate for 11-oxoreduction plays only a minor role compared to that with NADPH, a fact which is also true for xenobiotic carbonyl reduction. Inhibition experiments revealed strong sensitivity of xenobiotic carbonyl reduction to glucocorticoids. The competitive nature of this inhibition suggests that both glucocorticoids and xenobiotic carbonyl substances bind to the same catalytically active site of 11β-HSD. High enzyme activities were also found in microsomal fractions of the ovary and adrenal gland but, although expressing considerable glucocorticoid 11-dehydrogenation activity (one third that of liver), almost no carbonyl reduction was detectable in kidney microsomes. Immunoblot analysis with polyclonal antibodies directed against the liver 11β-HSD did not yield an immunological crossreaction in the same tissues. In conclusion, corresponding to the cytosolic aldo-keto reductases, microsomal 11β-HSD of liver may be considered to play a role in the phase I biotransformation of pharmacologically relevant carbonyl substances as well as protecting organisms against toxic carbonyl compounds by converting them to less lipophilic and more soluble and conjugatable metabolites. Discrepancies in bioactivity together with the lack of response to anti-liver 11β-HSD antibodies strongly indicate the existence of distinct forms of 11β-HSD to be present in kidney, adrenal gland and ovary. The ability of xenobiotic carbonyl reduction might be another distinguishing feature among the various 11β-HSD isozymes.