ABSTRACT:Gunn rats glucuronidate acetaminophen (APAP) at reduced rates and show increased susceptibility to APAP-induced hepatotoxicity. This defect is presumed to involve UDP-glucuronosyltransferase (UGT) 1A6, which is nonfunctional in Gunn rats, but it is currently unclear whether other 1A family members are also involved. In humans, two 1A isoforms are known to be active (1A6 and 1A9) but 1A6 form has a 25-fold lower apparent K m (2 mM). Rat liver microsomal APAP UGT activity is induced by in vivo treatment with ␤-naphthoflavone or oltipraz, an effect correlating with induction of 1A6 and 1A7. To address a possible role of 1A7 in APAP glucuronidation relative to other 1A forms, cDNAs encoding UGTs 1A1, 1A5, 1A6, 1A7, and 1A8 were expressed in human embryonic kidney cells and the contents of expressed enzyme in prepared membrane fractions determined by quantitative immunoblotting. At 2.5 mM APAP, 1A7 showed the highest specific activity (2.8 nmol/min/nmol 1A7 protein), followed by 1A6 (1.1 nmol/min/nmol), and 1A8 (0.27 nmol/min/nmol). 1A1 and 1A5 were essentially inactive. Kinetic comparisons indicated 1A7 had a similar apparent K m as 1A6 (4.7 versus 3.9 mM, respectively) but a 2.4-fold higher catalytic activity. These data suggest that in rats, 1A7 plays a major role in APAP glucuronidation and contributes to protection against APAP-induced hepatotoxicity. The involvement of other UGTs besides 1A6 is further underscored by the presence of significant residual APAP-glucuronidating activity by Gunn rat hepatocytes, indicating the activity of an unknown UGT2 family member. APAP1 (Tylenol) is a widely used over-the-counter analgesic/ antipyretic drug and model hepatotoxin. Although it is considered safe at normal doses, at higher doses it is associated with a predictable, dose-dependent centrilobular hepatotoxicity (Black, 1984) by a mechanism involving its metabolism to a toxic quinone imine (Cohen and Khairallah, 1997;Bessems and Vermeulen, 2001). APAP undergoes detoxification by competing phase 2 conjugation reactions, glucuronidation and sulfation, which convert APAP to nontoxic conjugates for elimination in bile or urine. The main type of APAP conjugation in most species, including rat and human, is glucuronidation. It has been proposed that individuals who metabolize APAP at slower rates are at higher risk of developing hepatotoxicity, based partly on the known greater susceptibility of animals that are deficient in the glucuronidation of phenols, such as cats (Feloidae) (Court and Greenblatt, 2000;Bessems and Vermeulen, 2001) and the Gunn rat Wells, 1988, 1989).A common assumption has been that the increased susceptibility of cats and Gunn rats to APAP toxicity is due to inactive UGT1A6, a major liver-expressed phenol UGT. The wild-type 1A6 2 enzyme from rat and human is effective in APAP catalysis (Bock et al., 1993), and in rats, 1A6 is induced by 3-methylcholanthrene (Munzel et al., 1994;Bock et al., 1999), which results in increased APAP glucuronidation (Gregus et al., 1990; Bock et al., 1993). In ca...