Xanthomonas maltophilia CBS 897.97 as a source of new 7a- and 7b-hydroxysteroid dehydrogenases and cholylglycine hydrolase: improved biotansformations of bile acids

The paper reports the partial purification and characterization of the 7β- and 7α-hydroxysteroid dehydrogenases (HSDH) and cholylglycine hydrolase (CGH), isolated from Xanthomonas maltophilia CBS 897.97. The activity of 7β-HSDH and 7α-HSDH in the reduction of the 7-keto bile acids is determined. The affinity of 7β-HSDH for bile acids is confirmed by the reduction, on analytical scale, to the corresponding 7β-OH derivatives. A crude mixture of 7α- and 7β-HSDH, in soluble or immobilized form, is employed in the synthesis, on preparative scale, of ursocholic and ursodeoxycholic acids starting from the corresponding 7α-derivatives. On the other hand, a partially purified 7β-HSDH in a double enzyme system, where the couple formate/formate dehydrogenase allows the cofactor recycle, affords 6α-fluoro-3α,6a-7β-dihydroxy-5β-cholan-24-oic acid (6-FUDCA) by reduction of the corresponding 7-keto derivative. This compound is not obtainable by microbiological route. The efficient and mild hydrolysis of glycinates and taurinates of bile acids with CGH is also reported. Very promising results are also obtained with bile acid containing raw materials.