Variability of human hepatic UDP-glucuronosyltransferase activity

The availability of a unique series of liver samples from human subjects, b oth control patients (9) and those with liver disease (6; biliary atresia ( 2), retransplant, chronic tyrosinemia type I, tyrosinemia, Wilson's disease ) allowed us to characterize human hepatic UDP-glucuronosyltransferases usi ng photoaffinity labeling, immunoblotting and enzymatic assays. There was w ide inter-individual variation in photoincorporation of the photoaffinity a nalogs, [P-32]5-azido-UDP-glucuronic acid and [P-32]5-azido-UDP-glucose and enzymatic glucuronidation of substrates specific to the two subfamilies of UDP-glucuronosyltransferases. However, the largest differences were betwee n subjects with liver disease. Glucuronidation activities toward one substr ate from each of the UDP-glucuronosyltransferases subfamilies, 1A and 2B, f or control and liver disease, respectively, were 1.7-4.5 vs 0.4-4.7 nmol/mg x min for hyodeoxycholic acid (2B substrate) and 9.2-27.9 vs 8.1-75 nmol/m g x min for p-chloro-m-xylenol (1A substrate). Microsomes from a patient wi th chronic tyrosinemia (HL32) photoincorporated [P-32]5-azido-UDP-glucuroni c acid at a level 1.5 times higher than the other samples, was intensely ph otolabeled by [P-32]5-azido-UDP-glucose and had significantly higher enzyma tic activity toward p-chloro-m-xylenol. Immunoblot analysis using anti-UDP- glucuronosyltransferase antibodies demonstrated wide inter-individual varia tions in UDP-glucuronosyltransferase protein with increased UDP-glucuronosy ltransferase protein in HL32 microsomes, corresponding to one of the bands photolabeled by both probes. Detailed investigation of substrate specificit y, using substrates representative of both the 1A (bilirubin, 4-nitrophenol ) and 2B (androsterone, testosterone) families was carried out with HL32, H L38 (age and sex matched control) and HL18 (older control). Strikingly incr eased (5-8-fold) glucuronidation activity was seen in comparison to HL18 on ly with the phenolic substrates. The results indicate that one or more phen ol-specific UDP-glucuronosyltransferase 1A isoforms are expressed at above normal levels in this tyrosinemic subject.