Bile acid induction of 7 alpha- and 7 beta-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 7 alpha/7 beta-hydroxysteroid dehydrogenase and NAD-dependent 7 alpha-hydroxysteroid dehydrogenase which were active against conjugated and unconjugated bile acids. No measurable activity could be found when deoxycholic acid (3 alpha, 12 alpha-dihydroxy-5 beta-cholan-24-oic acid) was used as substrate. No 7 beta-hydroxysteroid dehydrogenase activity and only a trace of 7 alpha-hydroxysteroid dehydrogenase activity could be demonstrated when bile acid was deleted from the growth medium. If bile acid was added after the time of inoculation, the amounts of 7 alpha/7 beta-hydroxysteroid dehydrogenase were greatly reduced. Enzyme enhancement was blocked by addition of rifampicin. The 7 alpha/7 beta-hydroxysteroid dehydrogenase components had pH optima of approximately 10.5. Both the 7 alpha/7 beta-hydroxysteroid dehydrogenase activities were heat-labile, with the 7 beta-component 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-ketodeoxycholic acid, cholic acid, chenodeoxycholic acid, and deoxycholic acid. Both 7-ketolithocholic acid and ursodeoxycholic 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 deoxycholic acid suppressed enzyme induction.(ABSTRACT TRUNCATED AT 250 WORDS)