The effect of bile acid structure on the activity of bile acid-CoA:glycine/taurine-N-acetyltransferase.

Abstract

Bile acid-CoA:glycine/taurine N-acetyltransferase from bovine liver was analyzed for its ability to conjugate a variety of different bile acid-CoA analogues. A complete steady state bisubstrate kinetic analysis was conducted for each analogue. The enzyme demonstrated strict specificity for the normal 4-substituted pentanoic acid side chain; shortening the side chain by 1 methylene group (norcholyl-CoA) completely eliminates enzymatic activity, and extending the side chain by 1 methylene group (homocholyl-CoA) causes a 30-fold decrease in activity at Vmax. These effects of side chain modification were not related to decreased binding affinity as much as to decreases in the rates of the bond-breaking and bond-making steps. Bile acid-CoA analogues with a variety of ring substitutions involving keto and hydroxyl groups were also examined. Varying the position of substitution and the nature of the substituent had major effects on both the Km and Vmax terms. The analogues with the highest activities at Vmax were 7-dehydrocholyl-CoA and the "allo" bile acid 5 alpha,6-ketolithocholyl-CoA. However, in both cases, the high activity is obtained at the expense of binding energy. The most efficient substrates were 7-ketolithocholyl-CoA and 3-dehydrocholyl-CoA. The more common analogue, chenodeoxycholyl-CoA, was a surprisingly inefficient substrate. The relative rates of formation of glycine versus taurine conjugates were also found to vary with changes in structure. This indicates that certain bile acids are more likely to be conjugated with taurine, and others with glycine.