It has been suggested that 20 -hydroxysteroid dehydrogenase (20 -HSD) is a T-cell differentiation marker in mice. In the human, this enzyme has generally been associated with types 1 and 2 17 -HSDs, which belong to the short-chain alcohol dehydrogenase family, whereas the rat, rabbit, pig and bovine 20 -HSDs are members of the aldoketo reductase superfamily, which also includes the 3 -HSD family. In this study, we report the cloning, from a human skin cDNA library, of a cDNA that shows, after transfection into human embryonic kidney (HEK-293) cells, high 20 -HSD activity but negligible 3 -and 17 -hydroxysteroid dehydrogenase activities. A comparison of the amino acid sequence of the human 20 -HSD with those of other related 20 -and 3 -HSDs indicates that the human 20 -HSD shares 79·9, 68·7 and 52·3% identity with rabbit, rat and bovine 20 -HSDs, whereas it shows 97, 84 and 65% identity with human type 3, type 1 and rat 3 -HSDs. In contrast, the enzyme shares only 15·2 and 15·0% identity with type 1 and type 2 human 17 -HSDs. DNA analysis predicts a protein of 323 amino acids, with a calculated molecular weight of 36 767 Da. In intact transfected cells, the human 20 -HSD preferentially catalyzes the reduction of progesterone to 20 -hydroxyprogesterone with a K m value of 0·6 µM, the reverse reaction (oxidation) being negligible. In a cell cytosolic preparation, the enzyme could use both NADPH and NADH as cofactors, but NADPH, which gave 4-fold lower K m values, was preferred. We detected the expression of 20 -HSD mRNA in liver, prostate, testis, adrenal, brain, uterus and mammary-gland tissues and in human keratinocyte (HaCaT) cells. The present study clearly indicates that the genuine human 20 -HSD belongs to the aldoketo reductase family, like the 20 -HSDs from other species.
