Prediction of hepatobiliary transport of drugs : Contribution of carrier-mediated transport in the detoxication of xenobiotics

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The biliary excretion plays an important role in the detoxication of xenobiotics. In the present study, we clarified the mechanism for the biliary excretion of organic anions including conjugative metabolites in normal SD rats and in Eisaihyper-bilirubinemic rats (EHBR) whose multispecific organic anion transporter on the bile canalicular membrane (cMOAT) is hereditarily defective. ATP-dependent uptake of glutathione conjugate of dinitrophenol and E3040 glucuronide into the isolated bile canalicular membrane vesicles (CMV) was observed only in SD rats, but not in EHBR.In contrast, E3040 sulfate uptake into CMV was not stimulated in the presence of ATP in CMV isolated from both SD and EHBR.These results suggest that glucuronide, but not sulfate, can be the substrate for cMOAT.Furthermore, we examined the molecular feature of cMOAT.We fixed our eyes upon the fact (1) that the substrate specificity of cMOAT resembles that of multidrug resistance associated protein (MRP), a primary active transporter located on the multidrug resistance tumor cells and (2) that a series of the primary active transporters possess conserved ATP-binding cassette (ABC) region, and prepared the degenerated PCR primer for the carboxy-terminal ABC of human MRP.A 421 bp fragment was amplified from the SD rat liver cDNA.Northern blot analysis of poly (A)+RNA from SD rat liver revealed the presence of 5 kb and 8.5kb mRNA species which hybridized to this fragment. In contrast, poly (A)+ RNA from EHBR did not hybridize to this fragment. These results suggest (1) that the impaired expression of this particular region might be related to the pathogenesis of hyperbilirubinemia in EHBR and that this region might encode part of cMOAT.