THE EFFECT OF BILE-ACID STRUCTURE ON THE ACTIVITY OF BILE ACID-COA - GLYCINE TAURINE-N-ACYLTRANSFERASE

  • 1 January 1982
    • journal article
    • research article
    • Vol. 257  (15) , 8761-8765
Abstract
Bile acid-CoA:glycine/taurine N-acyltransferase from bovine liver was analyzed for its ability to conjugate a variety of different bile acid-CoA analogs. A complete steady state bisubstrate kinetic analysis was conducted for each analog. 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 analogs 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 analogs 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 analog, chenodeoxycholyl-CoA, was a surprisingly inefficient substrate. The relative rates of formation of glycine vs. taurine conjugates also varied with changes in structure. This indicates that certain bile acids are more likely to be conjugated with taurine, and others with glycine.