Expression and Kinetic Properties of a Recombinant 3α-Hydroxysteroid/Dihydrodiol Dehydrogenase Isoenzyme of Human Liver

Human liver cytosol contains multiple forms of 3α-hydroxysteroid dehydrogenase and dihydrodiol dehydrogenase with hydroxysteroid dehydrogenase activity, and multiple cDNAs for the enzymes have been cloned from human liver cDNA libraries. To understand the relationship of the multiple enzyme forms to the genes, a cDNA, which has been reported to code for an isoenzyme of human liver 3α-hydroxysteroid/dihydrodiol dehydrogenase, was expressed in Escherichia coli. The recombinant enzyme showed structural and functional properties almost identical to those of the isoenzyme purified from human liver. In addition, the recombinant isoenzyme efficiently reduced 5α-dihydrotestosterone and 5β-dihydrocortisone, the known substrates of human liver 3α-hydroxysteroid dehydrogenase and chlordecone reductase previously purified, which suggests that these human liver enzymes are identical. Furthermore, the steady-state kinetic data for NADP†-linked (S)-1-indanol oxidation by the recombinant isoenzyme were consistent with a sequential ordered mechanism in which NADP+ binds first. Phenolphthalein inhibited this isoenzyme much more potently than it did the other human liver dihydrodiol dehydrogenases, and was a competitive inhibitor (K1=20 nM) that bound to the enzyme-NADP+complex.