A computational model of hydrogen peroxide production in liver and its removal by catalase and GSH-reliant enzymes that can predict intracellular H2O2 concentration and cell death during incidents of extreme oxidative stress
I present a simple computational model of H2O2 metabolism in hepatocytes and oxidative stress-induced hepatocyte death that is unique, among existing models of cellular H2O2 metabolism, in its ability to accurately model H2O2 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 H2O2 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 causing oxidative stress in hepatocytes, using experimental data on hepatocyte mortality resulting from in vitro exposure to the xenobiotic at various concentrations. The result is a PBPK/PD model that predicts intracellular H2O2 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 H2O2 rising to concentrations associated with apoptosis (> 1 {micro}M), that this may only occur hours after intracellular DMAIII peaks ("delay effect"), that when it does occur, H2O2 concentration 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 H2O2 concentration > 1 {micro}M 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 H2O2 concentration > 1 {micro}M and 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 H2O2 concentration will eventually trigger critical GSH depletion and H2O2 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 H2O2 concentration 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.