A Computational Model of Hydrogen Peroxide Production in Liver and its Removal by Catalase and GSH-reliant Enzymes that Can Predict Intracellular Hydrogen Peroxide Concentration and Cell Death During Incidents of Extreme Oxidative Stress: (1) Applications

I present a simple computational model of hydrogen peroxide metabolism in hepatocytes and oxidative stress-induced hepatocyte death that is unique, among existing models of cellular hydrogen peroxide metabolism, in its ability to accurately model hydrogen peroxide dynamics during incidents of extreme oxidative stress such as occur in the toxicological setting. Versions of the model are presented for rat hepatocytes in vitro and mouse liver in vivo. This is the first model of cellular hydrogen peroxide metabolism 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 that causes oxidative stress in hepatocytes, using experimental data on hepatocyte mortality resulting from in vitroexposure to the xenobiotic at various concentrations. The result is a PBPK/PD model that predicts intracellular hydrogen peroxide concentration and oxidative stress-induced hepatocyte death; both in vitro and in vivo (liver of living animal) PBPK/PD models can be produced. I demonstrate the procedure for the ROS-generating trivalent arsenical DMAIII. Simulations of DMAIII exposure using the model indicate that critical GSH depletion is the immediate trigger for intracellular hydrogen peroxide rising to concentrations associated with apoptosis (> 1 microM), that this may only happen hours after intracellular DMAIII peaks (``delay effect"), that when it does happen, hydrogen peroxide concentration is seen to rise 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 hydrogen peroxide concentration > 1 microM implies 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 hydrogen peroxide concentration > 1 microM and extreme GSH depletion co-occur/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 hydrogen peroxide concentration will eventually trigger critical GSH depletion and hydrogen peroxide concentration 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 hydrogen peroxide concentration within such an extremely narrow range.