Kinetic studies on rat liver 11 beta-hydroxysteroid dehydrogenase.

The kinetic behavior of homogeneous rat liver 11 beta-hydroxysteroid dehydrogenase (11-HSD) was investigated. The purified enzyme catalyzed oxidation of the 11 beta-hydroxy steroids, cortisol and corticosterone, to their 11-oxo products. The reverse 11-oxoreductase was not detected. Initial velocity studies of 11 beta-dehydrogenase were consistent with a sequential bireactant mechanism. Glycyrrhetinic acid, a competitive inhibitor of corticosterone oxidation, was uncompetitive with respect to NADP+. The observed inhibition patterns were consistent with an ordered sequential mechanism with NADP+ adding to the enzyme first. Analogs of NADP+ and NAD+ did not inhibit steroid oxidation by 11-HSD, nor did the products of the 11 beta-dehydrogenase reaction slow oxidation, or catalyze reduction. Ligand binding studies generated patterns that supported the ordered sequential mechanism derived from kinetic studies. The kinetic behavior of 11-HSD is therefore similar to other alcohol dehydrogenases. The basis for the apparent inability of homogeneous 11-HSD to catalyze reduction remains to be established.