INHIBITION OF UDP-GLUCURONOSYLTRANSFERASE ACTIVITY BY FATTY ACYL-COA - KINETIC-STUDIES AND STRUCTURE-ACTIVITY RELATIONSHIP

We previously identified and purified UDP-glucuronosyltransferase (UGT ) isoforms as targets of protein acylation from rat liver microsomes ( Yamashita et al., Biochem J 312: 301-308, 1995). The acylation of UGT isoforms occurred upon incubation with acyl-CoA without another protei n acyltransferase, suggesting that it was autoacylation. The study rev ealed the interaction of UGT isoforms with acyl-CoA. In the present st udy, the effects of fatty acyl-CoA on UGT activities were examined tho roughly, using a rat liver microsomal and purified enzyme fractions. T he UGT activities of both fractions were inhibited by acyl-CoA in a co ncentration-dependent manner. The effect of acyl-CoA was observed on t he activities toward various substrates, suggesting that the effect sh ows the wide spectrum of the isoforms of UGT. To assess the mechanism underlying the inhibition of UGT activity by acyl-CoA, the relationshi p of the inhibition, acyl-CoA binding to the proteins, and changes in the tertiary structure of the enzyme were examined. The kinetics of th ese phenomena were related closely with each other. Furthermore, the i nhibition of UGT activity was specified for acyl-CoA, though a structu rally related compound, acyl-3'-dephosphoCoA, had no inhibitory effect . The results suggested that the specific binding of acyl-CoA to UGT i soforms induced conformational changes of the enzymes and resultant in hibition of UGT activity. (C) 1997 Elsevier Science Inc.