This chapter reviews the historical perspective into the conceptual evolution of 11β-hydroxysteroid dehydrogenase (HSD) from its pedestrian origin as an enzyme that catalyzes reversible inactivation of corticosteroids to its currently more prestigious role as mediator of steroid-receptor interactions. An enzyme responsible for catalyzing the oxidation of cortisol to cortisone is found in rat liver and named “l lβ-hydroxy dehydrogenase.” The transformations catalyzed by this enzyme are illustrated in the chapter. The catalysis of 11-oxidation and 11-oxoreduction is not uniformly distributed among tissues. In liver, 11-oxoreduction is the dominant activity; in most other tissues, it is 11β-hydroxy oxidation. The recognition of clinical disorders whose symptomatology could be rationalized as being because of defects in 11-HSD expression. It helps in the development of the tools—antibodies, complementary DNA (cDNA)—that facilitate exploration of the enzyme at the molecular level. 11-HSD protects cells against the toxic effects of excess corticosteroid. The enzyme also serves a conservationist function, as the oxidized form of the steroid can be reduced by 11-oxoreductase to its active reduced form thus contributing to the circulating cortisol, and providing a buffer against the changes in blood level caused by paroxysmal secretion of the adrenal.
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