Formation of urso- and ursodeoxy-cholic acids from primary bile acids by a Clostridium limosum soil isolate.

A gram-positive, rod-shaped anaerobe (isolate F-14) was isolated from soil. This organism was identified by cellular morphology as well as by fermentative and biochemical data as Clostridium limosum. Isolate F-14 formed ursocholic acid (UC) and 7-ketodeoxycholic acid (7-KDC) from cholic acid (CA), and ursodeoxycholic acid (UDC) and 7-ketolithocholic acid (7-KLC) from chenodeoxycholic acid (CDC) in whole cell cultures, but did not transform deoxycholic acid (DC). No hydrolysis or transformation occurred when either taurine- or glycineconjugated bile acids were incubated with F-14. The type strain of Closfridium limosum (American Type Culture Collection 25620) did not transform bile acids. The structures of ursocholic, ursodeoxycholic, 7-ketodeoxycholic, and 7-ke- tolithocholic acids were verified by mass spectroscopy and by thin-layer chromatography using Komarowsky's spray reagent. The organism transformed cholic and chenodeoxycholic acids at concentrations of 20 mM and 1 mM, respectively; higher concentrations of bile acids inhibited growth. Optimal yields of ursocholic and ursodeoxycholic acids were obtained at 9- 24 hr of incubation and depended upon the substrate used. Increasing yields of 7-ketodeoxycholic and 7-ketolithocholic acids, and decreasing yields of ursocholic and ursodeoxycholic acids were observed with longer periods of incubation. Culture pH changed with time and was characterized by a small initial drop (0.2-0.4 pH units) and a subsequent increase to a pH (8.1-8.2) that was above the starting pH (7.4).M We propose that the following pathways are associated with the F-14 soil isolate: cholic acid 7-ketodeoxycholic acid ursocholic acid and chenodeoxycholic acid S 7-ketolithocholic acid e ursodeoxycholic acid, with increasing dominance of the back reaction of the second step on aging (oxygenation) of the culture.-Sutherland, J. D., L. V. Holdeman, C. N. Williams, and I. A. Macdonald. Formation of uno- and ursodeoxy- cholic acids from primary bile acids by a Clostridium limosum soil isolate.). Lipid Res. 1984. 25: 1084-1089.