Involved in the modification of secondary bile acids into iso-bile acids (3beta-bile acids) via epimerization of the 3-OH group through a 3-oxo-intermediate. Catalyzes the oxidation of deoxycholate (DCA) and lithocholate (LCA) to yield 12-alpha-hydroxy-3-oxo-5-beta-cholan-24-oate (3-oxo-DCA) and 3-oxo-5-beta-cholan-24-oate (3-oxo-LCA), respectively. Is also able to catalyze the oxidation of cholate (CA) and chenodeoxycholate (CDCA) into 3-dehydrocholate (3-oxo-CA) and 7-alpha-hydroxy-3-oxo-5-beta-cholan-24-oate (3-oxo-CDCA), respectively. Can also catalyze the reverse reactions in vitro. Prefers NADH to NADPH as cosubstrate. The conversion of the abundant bile acid DCA into isoDCA by the gut bacterium E.lenta favors the growth of the keystone commensal genus Bacteroides, since isoDCA is less cytotoxic than its parent compound, DCA; iso-bile acids have thus a potential role in modulating gut community composition. lithocholate + NAD(+) = 3-oxo-5beta-cholan-24-oate + H(+) + NADH deoxycholate + NAD(+) = 12alpha-hydroxy-3-oxo-5beta-cholan-24-oate + H(+) + NADH cholate + NAD(+) = 7alpha,12alpha-dihydroxy-3-oxo-5beta-cholan-24-oate + H(+) + NADH chenodeoxycholate + NAD(+) = 7alpha-hydroxy-3-oxo-5beta-cholan-24-oate + H(+) + NADH kcat is 291 min(-1) with deoxycholate as substrate. Transformation of DCA is more efficient at pH 10 than pH 7. Belongs to the short-chain dehydrogenases/reductases (SDR) family.