Activity of rat UGT1A1 towards benzo[a]pyrene phenols and dihydrodiols

Identification of sulfated metabolites of 4-chlorobiphenyl (PCB3) in the serum and urine of male rats

Polychlorinated biphenyls (PCBs) are legacy pollutants that exert toxicities through various mechanisms. In recent years exposure to PCBs via inhalation has been recognized as a hazard. Those PCBs with lower numbers of chlorine atoms (LC-PCBs) are semivolatile and have been reported in urban air, as well as in the indoor air of older buildings. LC-PCBs are bioactivated to phenols and further to quinone electrophiles with genotoxic/carcinogenic potential. We hypothesized that phenolic LC-PCBs are subject to conjugation and excretion in the urine. PCB3, often present in high concentrations in air, is a prototypical congener for the study of the metabolism and toxicity of LC-PCBs. Our objective was to identify metabolites of PCB3 in urine that could be potentially employed in the estimation of exposure to LC-PCBs. Male Sprague-Dawley rats (150-175 g) were housed in metabolism cages and received a single intraperitoneal injection of 600 μmol/kg body weight of PCB3. Urine was collected every 4 h; rats were euthanized at 36 h; and serum was collected. LC/MS analysis of urine before and after incubation with β-glucuronidase and sulfatase showed that sulfate conjugates were in higher concentrations than glucuronide conjugates and free phenolic forms. At least two major metabolites and two minor metabolites were identified in urine that could be attributed to mercapturic acid metabolites of PCB3. Quantitation by authentic standards confirmed that approximately 3% of the dose was excreted in the urine as sulfates over 36 h, with peak excretion occurring at 10-20 h after exposure. The major metabolites were 4'PCB3sulfate, 3'PCB3 sulfate, 2'PCB3 sulfate, and presumably a catechol sulfate. The serum concentration of 4'PCB3 sulfate was 6.18 ± 2.16 μg/mL. This is the first report that sulfated metabolites of PCBs are formed in vivo. These findings suggest a prospective approach for exposure assessment of LC-PCBs by analysis of phase II metabolites in urine.

Characterization of Rat Intestinal Microsomal UDP-Glucuronosyltransferase Activity toward Mycophenolic Acid

Mycophenolic acid (MPA) is the active immunosuppressive metabolite of the anti-organ rejection drug mycophenolate mofetil (MMF) and is implicated in the gastrointestinal toxicity associated with MMF therapy. Intestinal UDP-glucuronosyltransferases (UGT) have been proposed to provide intrinsic resistance against MMF-induced gastrointestinal toxicity by converting MPA to the inactive MPA 7-O-glucuronide. Using an optimized intestinal microsome preparation method that stabilized the intestinal MPA UGT activity, the MPA UGT activity of male Sprague-Dawley rat intestinal microsomes was characterized. A longitudinal gradient similar to that described for other phenolic compounds was observed, with the activity decreasing from the duodenum to the distal small intestine and colon. The catalytic efficiency of MPA glucuronidation decreased from the proximal to distal intestine as a result of decreasing Vmax and increasing Km. The finding that homozygous Gunn rats lack detectable intestinal MPA UGT activity indicates exclusive roles of UGT1A1, UGT1A6, and/or UGT1A7. Quantitative immunoblotting revealed a parallel between the MPA UGT activity and the content of UGT1A7-like immunoreactivity (18.7 and 7.3 microg/mg for duodenum and colon, respectively). In contrast, the lesser MPA-metabolizing UGT, UGT1A1 and UGT1A6, were lower in abundance (1.6-2.1 and 1.7-2.9 microg/mg, respectively), and their patterns of longitudinal distribution were distinct from the MPA UGT activity. These data suggest a dominant role of a UGT1A7-like enzyme, presumably UGT1A7 itself, in the catalysis of rat intestinal MPA glucuronidation. Studies are ongoing to investigate the relationship between intestinal UGT1A enzymes and susceptibility to MMF-induced gastrointestinal toxicity.