3 alpha-Hydroxysteroid dehydrogenase (3 alpha-HSD) catalyzes the oxidation of the 3-hydroxyl group of steroids. The enzymatic conversion is a critical step in the enzymatic assay of urinary sulfated bile acids (SBAs), which is a valuable diagnosis index of hepatobiliary diseases. However, the source of 3 alpha-HSD for clinical applications is limited. In this study, an open reading frame (ORF) encoding a novel 3 alpha-HSD was successfully cloned from Pseudomonas aeruginosa and expressed in Escherichia coli BL21 (DE3). The recombinant protein was purified by immobilized metal ion affinity chromatography. Enzyme characterization studies revealed that the protein has 3 alpha-HSD activity and the K-m value for sodium cholate is 1.06 mmol L-1. More than 60% relative enzyme activity was observed in a wide range of pH and temperature, with an optimum pH at 8.0 and an optimum temperature at 30 degrees C. The enzyme's good thermostability under 40 degrees C would be favorable in clinical applications. Ion interference experiments indicated that Zn2+ was an activating cofactor which increased the enzyme activity 1.75-fold. With the favorable characteristics mentioned above, the new 3 alpha-HSD is a promising enzyme for clinical applications. More importantly, the present work is the first report on a 3 alpha-HSD from P. aeruginosa. (C) 2015 Elsevier Inc. All rights reserved.