ACYL-COA-BINDING AND ACYLATION OF UDP-GLUCURONOSYLTRANSFERASE ISOFORMS OF RAT-LIVER - THEIR EFFECT ON ENZYME-ACTIVITY

When [C-14]arachidonoyl-CoA was incubated with crude extracts of rat l iver microsomes, [C-14]arachidonic acid was incorporated into many pro teins, suggesting that modification of these proteins with fatty acid, i.e. acylation, occurred. Using [C-14]arachidonoyl-CoA labelling assa y, 50 and 53 kDa proteins were purified from rat liver microsomes to n ear homogeneity by sequential chromatography on Red-Toyopearl, hydroxy apatite, heparin-Toyopearl, Blue-Toyopearl and UDP-hexanolamineagarose . Acylation of the 50 and 53 kDa proteins occurred in the absence of a ny other protein, suggesting that these molecules catalyse autoacylati on. The acylation was dependent on the length of the incubation period and the concentration of [C-14]arachidonoyl-CoA. The 50 and 53 kDa pr oteins also had acyl-CoA-binding activity; initial rates of acyl-CoA b inding and acylation were 0.25 and 0.004 min(-1) respectively. The pro teins also had weak but distinct acyl-CoA-hydrolysing activity (0.006 min(-1)). These results suggest that the proteins catalysed the sequen tial reactions of binding to acyl-CoA, autoacylation, and hydrolysis o f fatty acid. N-terminal amino acid sequencing analysis showed these p roteins to be UDP-glucuronosyltransferase (UDPGT) isoforms. UDPGT acti vity was inhibited by arachidonoyl-CoA. These results suggest that bin ding of acyl-CoA and acylation of UDPGT isoforms regulate the enzyme a ctivities, implying a possible novel function for fatty acyl-CoA in gl ucuronidation, which is involved in the metabolism of drugs, steroids and bilirubin.