Bile acid induction of 7a- and 76-hydroxysteroid dehydrogenases in Clostridium limosum

When grown in the presence of bile acids, two strains of Clostridium limosum were found to contain significant amounts of NADP-dependent 7a/7/3-hydroxysteroid dehydrogenase and NAD-dependent 7a-hydroxysteroid dehydrogenase which were active against conjugated and unconjugated bile acids. No measurable activity could be found when deoxycholic acid (3a, 12a-dihydroxy-5/3-cholan-24-oic acid) was used as substrate. No 78-hydroxysteroid dehydrogenase activity and only a trace of 7a-hydroxysteroid dehydrogenase activity could be demon- strated when bile acid was deleted from the growth medium. If bile acid was added after the time of inoculation, the amounts of 7a/7@-hydroxysteroid dehydrogenase were greatly reduced. Enzyme enhancement was blocked by addition of rifampicin. The 7a/78-hydroxysteroid dehydrogenase components had pH optima of approximately 10.5. Both the 7a/7/3-hydroxysteroid dehydrogenase activities were heat-labile, with the 78-compo- nent being the more stable of the two. When ranked according to the level of enzymes induced, the order in increasing bile acid induction power on an equimolar scale (0.4 mM) was: 7-keto- deoxycholic acid, cholic acid, chenodeoxycholic acid, and deoxycholic acid. Both 7-ketolithocholic acid and ursodeoxy- cholic acid were ineffective as enzyme inducers. Optimal induction was achieved with high concentrations of cholic acid (5 mM) and a harvest time of 24 hr. Addition of ursodeoxycholic acid to medium containing optimal concentrations of deoxy- cholic acid suppressed enzyme induction. With regard to the NADP-dependent 7a-hydroxysteroid dehydrogenase activity, a dihydroxy bile acid had a higher apparent K,,, value than a tri- hydroxy bile acid; chenodeoxycholic acid (0.42 mM) versus cholic acid (0.14 mM). In contrast, with the NADP-dependent 78-hydroxysteroid dehydrogenase activity, a trihydroxy bile acid had a higher K, value; ursocholic acid (0.076 mM) versus urso- deoxycholic acid (0.018 mM). The NAD-dependent 7a-hydroxy- steroid dehydrogenase activity with cholic acid as substrate had the highest apparent K,,, value of 0.62 mM. - Suthcrland, J. D., and C. N. Williams. Bile acid induction of 7a- and 78- hydroxysteroid dehydrogenases in Clostridium limosum. J. Lipid RCS. 1985. 26: 344-350.