Abstract
Introduction Steroid hormones bound to their specific nuclear receptors exert diverse physiological effects on gene expression in target tissues. One important mechanism by which cells modify hormone action is by modulating the structure of the steroid hormone ligand. 17β-Hydroxysteroid dehydrogenase (17β-HSD) is an enzyme involved in the activation, reducing 17-keto steroids to 17β-hydroxy steroids, and inactivation, oxidizing 17β-hydroxy steroids to 17-keto steroids, of androgens and estrogens (Fig. 1). Its reductase activity increases the affinity of the cognate receptors for their ligands, whereas 17β-hydroxy steroids are inactivated by the dehydrogenase activity. A general observation is that gonadal tissues convert 17-keto steroids to 17β-hydroxy steroids, where the opposite is observed for extragonadal tissues. It is unlikely that the redox potential of a given cell determines the direction of the reaction. More likely is that differential expression of 17β-HSD isozymes determines the net activity in a given tissue. The recent cloning of multiple

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