7α-OH epimerisation of bile acids via oxido-reduction with Xanthomonas maltophilia

The microbial 7α-OH epimerisation of cholic, chenodeoxycholic, and 12-ketochenodeoxycholic acids (7α-OH bile acids) with Xanthomonas maltophilia CBS 827.97 to corresponding 7β-OH derivatives with scarcity of oxygen is described. With normal pressure of oxygen the 7-OH oxidation products are obtained. No biotransformations are achieved in anaerobic conditions. The microbial 7α-OH epimerisation is achieved by oxidation of 7-OH function and subsequent reduction. Partial purification, in fact, of the enzymatic fraction revealed the presence of two hydroxysteroid dehydrogenases (HSDH) α- and β-stereospecific together with a glycocholate hydrolase. On the basis of these results a further application is the microbial reduction of 6α-fluoro and 6β-fluoro-3α-hydroxy-7-oxo-5β-cholan-24-oic acid methyl esters to the corresponding 7α-OH and 7β-OH derivatives.