17 beta-hydroxysteroid dehydrogenase type 1 (17 beta-HSD1) mainly catalyzes the reduction of estrone into estradiol. The enzymatic conversion is a critical step in estradiol accumulation in breast tissue, which is a valuable prognosis index of breast cancer disease. However, the source of 17 beta-HSD1 for inhibitor design is limited. In this study, the fragment encoding human 17 beta-HSD1 was successfully cloned and expressed in human embryonic kidney (HEK) 293T mammalian cells. The recombinant protein was purified by immobilized metal ion affinity chromatography yielding above 17 mg of purified 17 beta-HSD1 protein per liter of cell culture, with a specific activity of 8.54 mu moL/min/mg of protein for conversion of estradiol into estrone, with NAD(+) as cofactor at pH 9.2. Enzyme characterization studies revealed that the protein has estrogenic activity and the K-m value for estrone is about 20 nM. The recombinant protein purified from transfected HEK293T cells had higher specific activity compared to that of the enzyme purified directly from placenta. The present data show that the mammalian cell expression system can provide active 17 beta-HSD1 which is functionally identical to its natural counterpart and easy to purify in qualities suitable for its structure-function study. (C) 2017 Elsevier Inc. All rights reserved.