A Computational Model of Endogenous Hydrogen Peroxide Metabolism in Hepatocytes, Featuring a Critical Role for GSH.

This paper presents an ordinary differential equation (ODE) model of endogenous H 2 O 2 production and elimination in hepatocytes that is unique, at the time of writing, in its ability to accurately compute intracellular H 2 O 2 concentration during incidents of oxidative stress and in its usefulness for constructing PBPK/PD models for ROS-generating xenobiotics. Versions of the model are presented for rat hepatocytes in vitro and mouse liver in vivo . A generic method is given for using the model to create PBPK/PD models which predict intracellular H 2 O 2 concentration and oxidative-stress-induced hepatocyte death; these are identifiable from in vitro data sets reporting cell mortality following xenobiotic exposure at various levels. The procedure is demonstrated for the trivalent arsenical dimethylarsinous acid (DMA III ), which is produced in liver as part of the arsenic elimination pathway. This is the first model of H 2 O 2 metabolism in hepatocytes to feature values for the endogenous rates of H 2 O 2 production by mitochondria and other organelles which are inferred from the physiology literature, and to feature a detailed, realistic treatment of GSH metabolism; the latter is achieved by incorporating a minimal version of Reed and coworkers' pioneering model of GSH metabolism in liver. Model simulations indicate that critical GSH depletion is the immediate trigger for intracellular H 2 O 2 rising to concentrations associated with apoptosis (> 1 μM ), that this may only occur hours after the xenobiotic concentration peaks ("delay effect"), that when critical GSH depletion does occur, H 2 O 2 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 that intracellular H 2 O 2 concentration > 1 μM implies critical GSH depletion. There has been speculation that ROS levels in the range associated with apoptosis simply indicate, rather than cause, an apoptotic milieu. Model simulations are consistent with this view. In a result of interest to the wider physiology community, the delay effect is shown to provide a GSH-based mechanism by which cells can distinguish transient elevations in H 2 O 2 concentration, of use in intracellular signaling, from persistent ones indicative of either pathology or the presence of toxins, the second state of affairs eventually triggering apoptosis.