Involved in the multi-step bile acid 7alpha-dehydroxylation pathway that transforms primary bile acids to secondary bile acids in the human gut (PubMed:23836456, PubMed:16299351). Catalyzes the oxidation of C3-hydroxyl group of CoA conjugated bile acids generating a C3-oxo bile acid intermediate. Can use choloyl-CoA, chenodeoxycholoyl-CoA, deoxycholoyl-CoA, and lithocholoyl-CoA as substrates with similar efficiency. Highly prefers NAD over NADP as cosubstrate. Also catalyzes the reverse reactions; in vitro, the preferred direction of reaction depends on the pH. Has very little activity with unconjugated (non-CoA) bile acid substrates (PubMed:23836456). a 3alpha-hydroxy bile acid CoA + NAD(+) = a 3-oxo bile acid CoA + H(+) + NADH choloyl-CoA + NAD(+) = 7alpha,12alpha-dihydroxy-3-oxochol-24-oyl-CoA + H(+) + NADH chenodeoxycholoyl-CoA + NAD(+) = 7alpha-hydroxy-3-oxochol-24-oyl-CoA + H(+) + NADH deoxycholoyl-CoA + NAD(+) = 12alpha-hydroxy-3-oxocholan-24-oyl-CoA + H(+) + NADH lithocholoyl-CoA + NAD(+) = 3-oxocholan-24-oyl-CoA + H(+) + NADH kcat is 10 min(-1) with choloyl-CoA as substrate. kcat is 12 min(-1) with chenodeoxycholoyl-CoA as substrate. kcat is 11 min(-1) with lithocholoyl-CoA as substrate. kcat is 9.5 min(-1) with deoxycholoyl-CoA as substrate. kcat is 8.3 min(-1) with 3-oxocholoyl-CoA as substrate. kcat is 10 min(-1) with 3-oxochenodeoxycholoyl-CoA as substrate. kcat is 7.3 min(-1) with 3-oxolithocholoyl-CoA as substrate. kcat is 9.1 min(-1) with 3-oxodeoxycholoyl-CoA as substrate (at pH 8.7 and 37 degrees Celsius). The direction of reaction is pH dependent in vitro. At pH 7.3, clearly the reduction reaction is preferred with an order of magnitude higher velocity than the oxidation reaction. At pH 8.7, the velocities of the oxidation and reduction reaction are comparable. At pH 9.9 the velocity of the reduction reaction is about 60% of oxidation reaction. Lipid metabolism; bile acid biosynthesis. Homotetramer. Induced by the C24 primary bile acids cholic acid (CA) and chenodeoxycholic acid (CDCA). There are three genes for BaiA proteins: baiA1 is identical to baiA3 and encodes a polypeptide sharing 92% identity with baiA2 gene product. Reaction mechanism seems to proceed via a nicotinamide-OH(-) adduct, which is proposed to be involved in proton relay instead of hydride transfer. Belongs to the short-chain dehydrogenases/reductases (SDR) family.