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. Accepts both NADPH and NADH as cosubstrates. The conversion of the abundant bile acid DCA into isoDCA by the gut bacterium R.gnavus 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 + NADP(+) = 3-oxo-5beta-cholan-24-oate + H(+) + NADPH deoxycholate + NADP(+) = 12alpha-hydroxy-3-oxo-5beta-cholan-24-oate + H(+) + NADPH deoxycholate + NAD(+) = 12alpha-hydroxy-3-oxo-5beta-cholan-24-oate + H(+) + NADH cholate + NADP(+) = 7alpha,12alpha-dihydroxy-3-oxo-5beta-cholan-24-oate + H(+) + NADPH chenodeoxycholate + NADP(+) = 7alpha-hydroxy-3-oxo-5beta-cholan-24-oate + H(+) + NADPH kcat is 99.3 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.