Interindividual variability in acetaminophen glucuronidation by human liver microsomes: Identification of relevant acetaminophen UDP-glucuronosyltransferase isoforms

Interindividual variability in acetaminophen (APAP) glucuronidation may con tribute to differences in susceptibility to APAP intoxication in humans. Th e purpose of this study was to identify the relevant UDP-glucuronosyltransf erase (UGT) isoforms mediating APAP-UGT activity in human liver microsomes (HLMs). APAP-UGT activities and enzyme kinetics were determined using HLMs from 56 donors and nine recombinant human UGTs. Activities mediated by UGT1 A1, UGT1A4, UGT1A9, and UGT2B7, and relative UGT1A6 protein content were qu antified using 20 livers. More than 15-fold variation in liver microsomal A PAP-UGT activities was observed with a distribution skewed toward lower act ivities. Although most UGTs could glucuronidate APAP, UGT1A1, UGT1A6, and U GT1A9 were most active. UGT1A6 was a relatively high-affinity (K-m = 2.2 mM ), low-capacity enzyme; UGT1A1 was intermediate in affinity (K-m = 9.4 mM) and capacity; and UGT1A9 was a low-affinity (K-m = 21 mM), high-capacity en zyme. K-m values were similar to UGT1A1 in high- and intermediate-activity HLMs (6-10 mM) and UGT1A9 in low-activity HLMs (10-55 mM). APAP-UGT activit ies correlated best with propofol-UGT (r = 0.85; UGT1A9) and bilirubin-UGT (r = 0.66; UGT1A1) activities, but poorly with UGT1A6 protein (r = 0.30). A kinetic model was constructed from these data that identified UGT1A9 as th e predominant APAP-UGT (>55% total activity) in HLMs over a clinically rele vant APAP concentration range (50 muM-5 mM). UGT1A1 was also predicted to c ontribute substantially at toxic concentrations (>1 mM; >28% activity), whe reas UGT1A6 was most active at relatively low concentrations (<50 <mu>M; >2 9% activity).