A novel NADP +-dependent dehydrogenase activity for 7α/β- and 11β-hydroxysteroids in human liver nuclei: A third 11β-hydroxysteroid dehydrogenase

Human tissue from uninvolved liver of cancer patients was fractionated using differential centrifugation and characterized for 11βHSD enzyme activity against corticosterone, dehydrocorticosterone, 7α- and 7β-hydroxy-dehydroepiandrosterone, and 7-oxo-dehydroepiandrosterone. An enzyme activity was observed in nuclear protein fractions that utilized either NADP + or NAD +, but not NADPH and NADH, as pyridine nucleotide cofactor with K m values of 12 ± 2 and 390 ± 2 μM, compared to the K m for microsomal 11βHSD1 of 43 ± 8 and 264 ± 24 μM, respectively. The K m for corticosterone in the NADP +-dependent nuclear oxidation reaction was 102 ± 16 nM, compared to 4.3 ± 0.8 μM for 11βHSD1. The K cat values for nuclear activity with NADP + was 1687 nmol/min/mg/μmol, compared to 755 nmol/min/mg/μmol for microsomal 11βHSD1 activity. Inhibitors of 11βHSD1 decreased both nuclear and microsomal enzyme activities, suggesting that the nuclear activity may be due to an enzyme similar to 11βHSD Type 1 and 2.