GLUTATHIONE DEPLETION KINETICS WITH ACETAMINOPHEN - A SIMULATION STUDY

A tubular-flow model with known published parameters on glutathione (GSH) synthesis, degradation, and transport was developed to examine the distributed-in-space bimolecular reaction of GSH conjugation with acceptor substrates. Simulations were performed to obtain the vascular and intracellular GSH concentrations in the absence and presence of acetaminophen. Zonal localization of GSH was found to be effectively modified upon varying the activities for GSH synthesis and degradation along the sinusoidal flow path. A periportal (zone 1) GSH distribution resulted when GSH synthetic activity was distributed anterior to the degradation activity (models A and D); a perivenous (zone 3) GSH enrichment existed when these activities were reversed (model B), whereas when the synthetic and degradation activities for GSH were homogeneously distributed (model C), GSH concentration was unchanged in all zones. Although the zonation of GSH was model-dependent (models A-D), only minor differences were found to exist for the length-averaged tissue GSH concentration (5.8-6 mu mol/g liver) and at the outflow of the liver (similar to 15 mu M). With acetaminophen, a substrate known to deplete GSH via its reactive intermediate, N-acetyl-p-quinoneimine (NAPQI), acinar GSH patterns were not greatly perturbed at concentrations <1 mM. At 10 mM acetaminophen, however, differential patterns of GSH zonal depletion were observed among models, although there was virtually no difference in the length-averaged intracellular GSH concentration (3 mu mol/g liver) nor in the formation of the acetaminophen GSH adduct, with the latter being rate-limited by the bioactivation of acetaminophen to NAPQI. All models were consistent with previously published sulfation, glucuronidation, and GSH conjugation rates for acetaminophen in the single-pass perfused rat liver. Inclusion of a perivenous distribution of the cytochrome P450 and glutathione S-transferase or GST (positive or plus gradients along the sinusoid) revealed that accumulation of NAPQI and depletion of GSH were very sensitive to the cytochrome P450 activity gradient, less so to GST, and not at all to the zonation of sulfation and glucuronidation activities. Model A, which was most compatible physiologically with the known periportal zonation of GSH in the liver, depicted a lower perivenous GSH distribution and accumulation of NAPQI in zone 3, and predicted the highest zone 3 toxicity for acetaminophen. The simulated results suggest that the average tissue GSH concentration, normally determined in liver homogenate, and the formation of acetaminophen GSH adduct are poor indices of the extent of toxic exposure, and that the lower GSH but higher cytochrome P450 contents in zone 3 are contributing factors for the observed zonal toxicity with acetaminophen.