Structural characterization of cholylcoenzyme A:glycine-taurine N-acyltransferase and a covalent substrate intermediate

Bile acid-CoA:glycine-taurine N-acyltransferase was found to catalyze a reaction in the absence of glycine or taurine in which the substrate cholyl-CoA is cleaved with the release of CoA and the formation of a covalently bound enzyme-cholate intermediate. This unstable intermediate was trapped by a rapid mixing and denaturation procedure. The denatured protein was digested with trypsin and the cholate-labeled tryptic peptide was isolated. This cholate-peptide is considered to originate from the active site region of the enzyme based on the following criteria: cholyl-CoA does not react with any of the 20 common amino acids, the hydrolysis of cholyl-CoA is known to occur on the enzyme, the lack of reaction of the enzyme with just cholate, and the fact that labeling is extensive even at low (substrate level) concentrations of cholyl-CoA. The isolated cholate-peptide was submitted to amino acid analysis. It contained 32 amino acid residues and was devoid of cysteine, methionine, and tyrosine. Amino acid analysis of the N-acyltransferase was conducted. The enzyme was also shown to possess a blocked N terminus.

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

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.

Identification of a unique mammalian species of cholyl-CoA: amino acid N-acyltransferase

The N-acyltransferase enzyme which catalyzes the conjugation of the CoA adducts of bile acids with amino acid has been partially purified from dog liver. Unlike the N-acyltransferase from five other mammalian species, the enzyme from dog liver was unable to synthesize glycine conjugates from cholyl-CoA in vitro even in the presence of 0.1 M glycine. The enzyme did catalyse the synthesis of taurine conjugates. Thus, the enzyme from dog liver resembles non-mammalian forms of the enzyme which also synthesize only taurine conjugates. However, the molecular weight of the enzyme from dog liver was found to be approximately 45 700, which is similar to values reported for other mammalian forms and considerably smaller than the molecular weight of 65 000 reported previously for a non-mammalian form of the enzyme.

Purification and characterization of cholyl-CoA: taurine N-acetyltransferase from the liver of domestic fowl (Gallus gallus)

The enzymological basis for the ability of mammalian liver to conjugate bile acids with both glycine and taurine, and for non-mammalian liver to make only taurine conjugates, was investigated. The taurine-conjugating enzyme has been purified 1200-fold from the liver of domestic fowl and its properties compared with those of the glycine/taurine-conjugating enzyme from bovine liver [Czuba & Vessey (1980) J. Biol. Chem. 255, 5296-5299]. The enzyme from both species followed a Ping Pong mechanism. The enzymes were also similar with respect to their affinity for taurine, although the enzyme from domestic fowl would not bind glycine. The affinity of both for cholyl-CoA was quite similar, too, and both enzymes were inhibited reversibly by p-mercuribenzoate. The enzymes, however, were quite different in size. The enzyme from domestic fowl had a mol.wt. of 63000-65000 by both gel filtration and sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. This is approx. 15 000 mol.wt. units larger than the enzyme from bovine liver, and suggests a loss of genome over the course of evolution as the basis for the altered specificity at the amino-acid binding site.

Kinetic characterization of cholyl-CoA glycine-taurine N-acyltransferase from bovine liver

The bile acid-conjugating enzyme cholyl-coenzyme A:glycine/taurine N-acyltransferase previously purified from bovine liver was subjected to a bisubstrate kinetic analysis. Double reciprocal plots of reaction rates as a function of variable concentrations of aminoa acid and cholyl-CoA yielded the nonintersecting type of plots typical of ping-pong reaction mechanisms. A Tetra Uni ping-pong mechanism was supported by further kinetic analysis including demonstration that the enzyme will catalyze the release of CoA from cholyl-CoA without glycine being present. This half-reaction is shown not to be the result of an acyl-CoA thiolase contaminating the N-acyltransferase preparation. The specificity of the enzyme for amino acid was found to be quite narrow; no detectable rate was found for D-Ala, L-Ala, L-Ser, L-Glu-NH2, L-ornithine, or beta-alanine. Activity toward various acyl-CoA derivatives was also tested. Phenylacetyl-CoA, benzoyl-CoA, and acetyl-CoA were not substrates. CoA derivatives of the other common bile acids were very good substrates. Conjugated bile acids were efficient competitive inhibitors of cholyl-CoA binding. Cholic acid also inhibited; however, its inhibition pattern was complex. The enzyme could be reversibly inhibited by p-mercuribenzoate and both cholyl-CoA and glycine protected. N-Ethylmaleimide and iodoacetate were not inhibitory.