Comamonas testosteroni TA441 degrades steroids such as testosterone via aromatization of the A ring, followed by meta-cleavage of the ring. In the DNA region upstream of the meta-cleavage enzyme gene tesB, two genes required during cholic acid degradation for the inversion of an alpha-oriented hydroxyl group on C-12 were identified. A dehydrogenase, SteA, converts 7 alpha,12 alpha-dihydroxyandrosta-1,4-diene-3,17-dione to 7 alpha-hydroxyandrosta-1,4-diene-3,12,17-trione, and a hydrogenase, SteB, converts the latter to 7 alpha,12 beta-dihydroxyandrosta-1,4-diene-3,17-dione. Both enzymes are members of the short-chain dehydrogenase/reductase superfamily. The transformation of 7 alpha,12 alpha-dihydroxyandrosta-1,4-diene-3,17-dione to 7 alpha,12 beta-dihydroxyandrosta-1,4-diene-3,17-dione is carried out far more effectively when both SteA and SteB are involved together. These two enzymes are encoded by two adjacent genes and are presumed to be expressed together. Inversion of the hydroxyl group at C-12 is indispensable for the subsequent effective B-ring cleavage of the androstane compound. In addition to the compounds already mentioned, 12 alpha-hydroxyandrosta-1,4,6-triene-3,17-dione and 12 beta-hydroxyandrosta-1,4,6-triene-3,17-dione were identified as minor intermediate compounds in cholic acid degradation by C. testosteroni TA441.