Paracetamol metabolism in the isolated perfused rat liver with further metabolism of a biliary paracetamol conjugate by the small intestine

Abstract

The isolated, perfused rat liver metabolized paracetamol to glucuronide. sulphate and glutathione conjugates. Sulphate conjugation was the preferred route of metaholism at the drug concentrations studied, but formation of glutathione conjugate became increasingly prominent at higher paracetamol concentrations. Sulphate conjugation was saturated at a paracetamol concentration in the perfusatc of 5 mM. while formation of glucuronide or glutathione conjugates was not yet saturated at 10 mM. The sulphate conjugate was predominantly excreted into the perfusate whereas the excretion pattern for the glucuronide and glutathione conjugates changed with time. Initially, both these conjugates were almost exclusively excreted in the bile, after 90 min perfusion mainly into the perfusate. Preformed paracetamol conjugates were not transferred from perfusate to bile by the isolated perfused liver. Freshly isolated intestinal cells rapidly converted paracetamol- S-glutathione to paracetamol- S-cysteine which was slowly acetylated to the, N-acetylcysteine derivative. Methionine stimulated, and serine-borate inhibited, the breakdown of paracetamol- S-glutathione by intestinal cells, indicating the involvement of γ-glutamyltranspeptidase in the reaction. Biliary paracctamol- S-glutathione was metabolized similarly by the small intestine in situ; the subsequent appearance of the cysteine conjugate in plasma revealed that breakdown of the glutathione conjugate occurred before or during passage through the intestinal wall. Direct absorption of paracetamol- S-cysteine from the intestinal lumen to the portal blood was verified by instillation of this derivative in the intestinal lumen in situ. Analysis of plasma also indicated enterohepatic circulation and reabsorption of biliary paracetamol and paracetamol glucuronide from the intestinal lumen.