Udp-Glucuronosyltransferase Isoforms Catalyzing Glucuronidation of Hydroxy-Polychlorinated Biphenyls in Rat

Abstract

ABSTRACT:Polychlorinated biphenyls (PCBs) are highly toxic environmental contaminants that can cause irreversible damage in humans and wildlife. The mechanism of toxicity is not clear, but biotransformation products such as hydroxy PCBs (OH-PCBs) are a major concern. Efforts to elucidate the metabolism of PCBs and their metabolites, however, have paid little attention to the structure of the compound to be eliminated. The objectives of this study were to clarify organ tissue distribution of the glucuronidation activities toward OH-PCBs and to determine the UDP-glucuronosyltranseferase (UGT) isoforms responsible for glucuronidation in relation to the OH-PCB structure. 2,4,6-Trichlorobiphenyl and 2,3,4,5-tetrachlorobiphenyl were incubated in primary culture of rat hepatocytes, and the metabolites were identified by HPLC. Organ tissue glucuronidation activities toward 10 OH-PCBs were investigated by reactions of microsomes prepared from brain, liver, small and large intestine, lung, kidney, and testis tissues. To determine substrate specificity of the isoforms toward the OH-PCBs, rat UGT isoforms UGT1A1, UGT1A3, UGT1A5, UGT1A6, UGT1A7, UGT2B1, UGT2B3, and UGT2B12 were expressed in yeast strain AH22. Glucuronidation of the PCBs was found to be contingent on their hydroxylation. The organ tissues had strong glucuronidation activities toward the OH-PCBs tested; and most OH-PCBs were glucuronidated by UGT1A1, UGT1A6, and UGT2B1, all of which were substrate-specific. In conclusion, glucuronidation activities of UGT1A1, UGT1A6, and UGT2B1 toward OH-PCBs is relative to expression of the isoforms in each tissue, and glucuronidation intensity of the isoforms is relative to the structure of the OH-PCB to be glucuronidated.

Effects of food natural products on the biotransformation of PCBs

“…It is noteworthy that UDPGA concentrations of >2mM were needed for maximal rates of glucuronidation in rat hepatic microsomes. The apparent K m and V max values reported in Table 2 were obtained with UDPGA concentrations of 3 mM (Daidoji et al, 2005) or 4 mM (Tampal et al, 2002). Hepatic concentrations of UDPGA in rats are 0.2-0.4 mM (Goon and Klaassen, 1992), thus, actual rates of glucuronidation of OH-PCBs in rat liver are likely to be lower than the V max values.…”

“…The effects of food natural products on OH-PCB glucuronidation have not been studied, but OH-PCBs that are substrates for UGT1A1, or other isoforms induced by these natural products, should be more rapidly glucuronidated following exposure to flavonoids. To date, only one study has published information on the specific UGT isoforms that metabolize OH-PCBs (Daidoji et al, 2005). Expressed human UGT2B1 metabolized eight of the eleven OH-PCBs studied, UGT1A6 metabolized three of the eleven and UGT1A1 catalyzed glucuronidation of ten of the eleven OH-PCBs.…”

“…One study (Tampal et al, 2002) found apparent K m values for OH-PCB between 0.1 and 0.27 mM and V max from 0.31 to 31.3 nmol/min/mg, while another (Daidoji et al, 2005) reported OH-PCB apparent K m values between 0.03 and 0.14 mM and V max from 3 to 11 nmol/min/mg. The rat studies showed a general trend towards lower V max values with higher numbers of Cl atoms in the OH-PCB.…”

“…Glucuronidation of several OH-PCBs has been studied in rat liver microsomes, with expressed human UGTs and in catfish intestinal and hepatic microsomes (Tampal et al, 2002;Daidoji et al, 2005;Sacco, 2006).…”

Influence of chronic hepatic failure on disposition kinetics of valproate excretion through a phase II reaction in rats treated with carbon tetrachloride

“…1). Daidoji et al [17] reported that the expression of UGT1A1 mRNA in normal rat liver is higher than that of UGT1A6 mRNA, while UGT1A7 mRNA is only slightly expressed. We also confirmed that the expression of UGT1A1 mRNA in liver is the highest among the three isoforms (data not shown).…”