METABOLISM OF ACETAMINOPHEN AND PHENACETIN BY ISOLATED RAT HEPATOCYTES - A SYSTEM IN WHICH THE SPATIAL-ORGANIZATION INHERENT IN THE LIVER IS DISRUPTED

  • 1 January 1985
    • journal article
    • research article
    • Vol. 13  (1) , 42-50
Abstract
The kinetics of sulfation of 3H-acetaminophen [an analgesic toxic in high doses] (preformed metabolite) and 14C-acetaminophen (generated metabolite) during the parallel incubations of tracer concentrations of 3H-acetaminophen and 14C-phenacetin in isolated rat hepatocytes and rat 9000 g supernatant fractions from liver and from isolated hepatocytes were compared to results obtained from rat liver perfusion studies. The isolated rat hepatocytes and the subcellular fractions represent in vitro systems whose metabolic activities are apparently homogeneous and contrast the in situ rat liver perfusion system where differential distribution of drug metabolizing activities exists within the rigid architecture of the intact organ. The ratio of sulfation (intrinsic clearance = 23.8 ml/min per liver) to O-deethylation (intrinsic clearance = 11.7 ml/min per liver) activities within these isolated hepatocytes (2.2-2.7) and subcellular fractions (3.72) were similar, but were reversed in the perfused liver preparation (0.2-0.5) depending on the model of hepatic drug clearance used for the calculation. The sulfation to O-deethylation activity in the isolated rat hepatocyte system was independent of cell concentration (3.8 to 11 .times. 106 cells/ml) or the method of isolation of hepatocytes (by normal digestion or retrograde digestion). The reason for the reversal of lower O-deethylation activity in the subcellular and cellular systems remains unknown. A discrepancy between the extent of further metabolism of 14C-acetaminophen, a generated primary metabolite, and the extent of metabolism of 3H-acetaminophen, a preformed metabolite, was found in the isolated rat hepatocytes, a phenomenon explained by the succession of kinetic events in the formation and metabolism of the metabolite. The data were predicted adequately by computer simulations on the time course of phenacetin, acetaminophen (as a generated or preformed metabolite) and acetaminophen sulfate by employing the rate constants for the disappearance of phenacetin and (preformed) acetaminophen observed in isolated rat hepatocytes and strongly suggest a lack of diffusional barrier for preformed acetaminophen in reaching enzymatic sites.