EFFICIENT HEPATIC-UPTAKE AND CONCENTRATIVE BILIARY-EXCRETION OF A MERCAPTURIC ACID

The role of the Liver in the disposition of circulating mercapturic ac ids was examined in anesthetized rats and in the isolated perfused rat liver using S-2,4-dinitrophenyl-N-acetylcysteine (DNP-NAC) as the mod el compound. When DNP-NAC was infused into the jugular vein (150 pr OO nmol over 60 min)it was rapidly and nearly quantitatively excreted as DNP-NAC into bile (42-36% of the dose) and urine (48-62% of dose). So me minor metabolites were detected in bile (<4%), with the major metab olite coeluting on HPLC with the DNP conjugate of glutathione (DNP-SG) . Isolated rat Livers perfused single pass with 3 mu M DNP-NAC removed 72 +/- 9% of this mercapturic acid from perfusate. This rapid DNP-NAC uptake was unaffected by sodium omission, or by L-cysteine, L-glutama te, L-cystine, or N-acetylated amino acids, but was decreased by inhib itors of hepatic sinusoidal organic anion transporters (oatp), indicat ing that DNP-NAC is a substrate for these transporters. The DNP-NAC re moved from perfusate was promptly excreted into bile, eliciting a dose -dependent choleresis. DNP-NAC itself constituted similar to 75% of th e total dose recovered in bile, reaching a concentration of 9 mM when livers mere perfused in a recirculating mode with an initial DNP-NAC c oncentration of 250 mu M. Other biliary metabolites included DNP-SG, D NP-cysteinylglycine, and DNP-cysteine. DNP-SG was likely formed by a s pontaneous retro-Michael reaction between glutathione and DNP-NAC. Sub sequent degradation of DNP-SG by biliary gamma-glutamyl-transpeptidase and dipeptidase activities accounts for the cysteinylglycine and cyst eine conjugates, respectively. These findings indicate the presence of efficient hepatic mechanisms for sinusoidal uptake and biliary excret ion of circulating mercapturic acids in rat Liver and demonstrate that the liver plays a role in their whole body elimination.