Inhibition of Streptomyces hydrogenans 3α,20β-hydroxysteroid dehydrogenase by licorice-derived compounds and crystallization of an enzyme-cofactor-inhibitor complex

Streptomyces hydrogenans 3α, 20β-hydroxysteroid dehydrogenase reduces the C20 ketone on glucocorticoids and progestins. We find that two licorice-derived compounds, glycyrrhizic acid and carbenoxolone, inhibit this enzyme with μM K_is. Inhibition is competitive, indicating that these compounds are binding at or close to the catalytic site. Carbenoxolone's high aqueous solubility and affinity for 3α,20β-hydroxysteroid dehydrogenase enabled us to prepare crystals of a carbenoxolone-NADH-enzyme ternary complex, which preliminary X-ray analysis indicates has a crystal structure that is significantly different from that of the 3α,20β-hydroxysteroid dehydrogenase-NADH complex. A comparison of the tertiary structures of these two complexes should prove useful in understanding this enzyme's catalytic mechanism, as well as those of two homologous enzymes, mammalian 11β-hydroxysteroid dehydrogenase and 15-hydroxyprostaglandin dehydrogenase that also are inhibited by carbenoxolone.