A Computational Model of Hydrogen Peroxide Production in Liver and its Removal by Catalase and GSH-reliant Enzymes that Can Predict Intracellular H2O2Concentration and Cell Death During Incidents of Extreme Oxidative Stress: (1) Applications to PBPK/PD Modeling of the Trivalent Arsenical DMAIII, (2) Insights Obtained into (a) the Role of Critical GSH Depletion in Apoptosis and (b) How Intracellular H2O2Concentration is So Tightly Regulated

Abstract

AbstractI present a simple computational model of H2O2metabolism in hepatocytes and oxidative stress-induced hepatocyte death that is unique, among existing models of cellular H2O2metabolism, in its ability to accurately model H2O2dynamics during incidents of extreme oxidative stress such as occur in the toxicological setting. Versions of the model are presented for rat hepatocytesin vitroand mouse liverin vivo. This is the first model of cellular H2O2metabolism to incorporate a detailed, realistic model of GSH synthesis from its component amino acids, achieved by incorporating a minimal version of Reed and coworkers’ pioneering model of GSH metabolism in liver. I demonstrate a generic procedure for coupling the model to an existing PK model for a xenobiotic causing oxidative stress in hepatocytes, using experimental data on hepatocyte mortality resulting fromin vitroexposure to the xenobiotic at various concentrations. The result is a PBPK/PD model that predicts intracellular H2O2concentration and oxidative stress-induced hepatocyte death; bothin vitroandin vivo(liver of living animal) PBPK/PD models can be produced. I demonstrate the procedure for the ROS-generating trivalent arsenical DMAIII. Simulations of DMAIIIexposure using the model indicate that critical GSH depletion is the immediate trigger for intracellular H2O2rising to concentrations associated with apoptosis (>1µM), that this may only occur hours after intracellular DMAIIIpeaks (“delay effect”), that when it does occur, H2O2concentration rises rapidly in a sequence of two boundary layers, characterized by the kinetics of glutathione peroxidase (first boundary layer) and catalase (second boundary layer), and finally, that intracellular H2O2concentration>1µMimplies critical GSH depletion. Franco and coworkers have found that GSH depletion is central to apoptosis through mechanisms independent of ROS formation and have speculated that elevated ROS may simply indicate, rather than cause, an apoptotic milieu. Model simulations are consistent with this view, as they indicate that intracellular H2O2concentration>1µMand extreme GSH depletion cooccur/imply each other; however, I note that this does not rule out a direct role for elevated ROS in the apoptotic mechanism. Finally, the delay effect is found to underlie a mechanism by which a normal-as-transient but pathological-as-baseline intracellular H2O2concentration will eventually trigger critical GSH depletion and H2O2concentration in the range associated with apoptosis, if and only if it persists for hours; this helps to rigorously explain how cells are able to maintain intracellular H2O2concentration within such an extremely narrow range.DISCLAIMER: The views presented in this article do not necessarily reflect those of the U.S. Food and Drug Administration or the National Toxicology Program.