Human estrogenic 17β-hydroxysteroid dehydrogenase:: Predominance of estrone reduction and its induction by NADPH

Human estrogenic 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD1) plays a crucial role in the last step of the synthesis of estrogens, A detailed kinetic study demonstrated that the enzyme shows about 240 fold higher specificity towards estrone reduction than estradiol oxidation at physiological pH using tri-phosphate cofactors. The k(cat)/K-m values are 96 +/- 10 and 0.4 +/- 0.1 s(-1) (mu M)(-1) respectively for the above two reactions. However, it has been shown that this difference is closely linked to the use of NADPH and NADP cofactors, A binding study using equilibrium catalysis indicated similar KD (equilibrium dissociation constant) of 11 +/- 1 and 4.7 +/- 0.9 mu M for estrone and estradiol, respectively. The binding affinity of 17 beta-HSD1 to estrone was significantly increased with a K-D of 1.6 +/- 0.2 mu M in the presence of NADP, the latter used as an analogue of the NADPH. The results of binding studies agree with the steady-state kinetics, which showed that the K-m of estrone is 12-fold lower when using NADPH as a cofactor than when using NADH, These results strongly suggest that the cofactor plays a crucial role in the stimulation of the specificity for estrogen reduction. (C) 1999 Academic Press.