MOLECULAR-CLONING OF RAT-LIVER 3-ALPHA-HYDROXYSTEROID DEHYDROGENASE AND IDENTIFICATION OF STRUCTURALLY RELATED PROTEINS FROM RAT LUNG AND KIDNEY

3alpha-Hydroxysteroid dehydrogenase and related enzymes play important roles in the metabolism of endogenous compounds including androgens, corticosteroid, prostaglandins and bile acids, as well as drugs and xenobiotics such as benzo(a)pyrene. Complementary DNA clones encoding 3alpha-hydroxysteroid dehydrogenase were isolated from a rat liver cDNA lambda gtl1 expression library using monoclonal antibodies as probes. A full-length cDNA clone of 1286 base pairs contained an open reading frame encoding a protein of 322 amino acids with an estimated M(w) of 37 kD. When expressed in E. coli, the encoded protein migrated to the same position on SDS-polyacrylamide gels as the enzyme in rat liver cytosols. The protein expressed in bacteria was highly active in androsterone oxidation in the presence of NAD as cofactor and this activity was inhibited by indomethacin, a potent inhibitor of 3alpha-hydroxysteroid dehydrogenase. The predicted amino acid sequence of 3alpha-hydroxysteroid dehydrogenase was related to sequences of several other aldo-keto reductases such as bovine prostaglandin F synthase, human chlordecone reductase, human aldose reductase, human aldehyde reductase and frog lens epsilon-crystallin, suggesting that these proteins belong to the same gene family. Recently, we have found that monoclonal antibodies against 3alpha-hydroxysteroid dehydrogenase also recognized multiple antigenically related proteins in rat lung, kidney and testis. Further screening of liver, lung and kidney cDNA libraries using these monoclonal antibodies as probes resulted in the isolation of additional five different cDNAs encoding proteins with high degree of structural homology to rat liver 3alpha-hydroxysteroid dehydrogenase.