Glucuronidation of polychlorinated biphenylols and UDP-glucuronic acid concentrations in channel catfish liver and intestine

Polychlorinated biphenylols (OH-PCBs) are potentially toxic polychlorinated biphenyl metabolites that can be eliminated by glucuronidation, catalyzed by UDP-glucuronosyltransferases (UGTs). OH-PCBs with a 3,5-dichloro-4-hydroxy substitution pattern have been detected in blood from humans and wildlife, suggesting slow elimination. In this study we assessed the glucuronidation of 4-OHPCBs with zero, one, or two chlorine atoms flanking the 4-hydroxyl group and zero to four chlorine atoms in the aphenolic ring in microsomes from channel catfish liver and proximal intestine. Product formation was quantitated with [C-14] UDP-glucuronic acid ( UDPGA). Physiological concentrations of UDPGA were measured in preparations of liver and intestine. When the OH-PCB concentrations were varied in the presence of saturating UDPGA concentrations, glucuronidation V-max values were higher in hepatic than in intestinal microsomes (0.40 - 3.4 and 0.12 - 0.78 nmol/min/mg of protein, respectively), whereas the Km values were generally lower for intestine (0.042 - 0.47 mM) than for liver (0.11 - 1.64 mM). In both tissues Vmax values with 3,5-dichloro-4-OH-PCBs were lower than with the corresponding 3-chloro-4-OH-PCBs. Varying the UDPGA concentrations in the presence of saturating concentrations of OH-PCB showed that the Km for UDPGA was lower in intestine (27 mu M) than in liver (690 mu M). The measured concentration of UDPGA in catfish liver (246 - 377 nmol/g) was lower than the Km for UDPGA, suggesting that in vivo rates of glucuronidation may be suboptimal, whereas in intestine the measured UDPGA concentration (71 - 258 nmol/g) was higher than the Km for UDPGA. Although liver has a greater glucuronidation capacity than proximal intestine, the properties of intestinal UGTs in channel catfish enable them to efficiently glucuronidate low concentrations of OH-PCBs.