GLUTATHIONE DEPLETION KINETICS WITH ACETAMINOPHEN - A SIMULATION STUDY

A tubular-flow model with known published parameters on glutathione (G SH) synthesis, degradation, and transport was developed to examine the distributed-in-space bimolecular reaction of GSH conjugation with acc eptor substrates. Simulations were performed to obtain the vascular an d intracellular GSH concentrations in the absence and presence of acet aminophen. Zonal localization of GSH was found to be effectively modif ied upon varying the activities for GSH synthesis and degradation alon g the sinusoidal flow path. A periportal (zone 1) GSH distribution res ulted when GSH synthetic activity was distributed anterior to the degr adation activity (models A and D); a perivenous (zone 3) GSH enrichmen t existed when these activities were reversed (model B), whereas when the synthetic and degradation activities for GSH were homogeneously di stributed (model C), GSH concentration was unchanged in all zones. Alt hough the zonation of GSH was model-dependent (models A-D), only minor differences were found to exist for the length-averaged tissue GSH co ncentration (5.8-6 mu mol/g liver) and at the outflow of the liver (si milar to 15 mu M). With acetaminophen, a substrate known to deplete GS H via its reactive intermediate, N-acetyl-p-quinoneimine (NAPQI), acin ar GSH patterns were not greatly perturbed at concentrations <1 mM. At 10 mM acetaminophen, however, differential patterns of GSH zonal depl etion were observed among models, although there was virtually no diff erence in the length-averaged intracellular GSH concentration (3 mu mo l/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 N APQI and depletion of GSH were very sensitive to the cytochrome P450 a ctivity gradient, less so to GST, and not at all to the zonation of su lfation and glucuronidation activities. Model A, which was most compat ible physiologically with the known periportal zonation of GSH in the liver, depicted a lower perivenous GSH distribution and accumulation o f NAPQI in zone 3, and predicted the highest zone 3 toxicity for aceta minophen. The simulated results suggest that the average tissue GSH co ncentration, normally determined in liver homogenate, and the formatio n of acetaminophen GSH adduct are poor indices of the extent of toxic exposure, and that the lower GSH but higher cytochrome P450 contents i n zone 3 are contributing factors for the observed zonal toxicity with acetaminophen.