The rate of cellular hydrogen peroxide removal shows dependency on GSH: Mathematical insight into in vivo H(2)O(2) and GPx concentrations

Although its concentration is generally not known, glutathione peroxidase-1 (GPx-1) is a key enzyme in the removal of hydrogen peroxide (H(2)O(2)) in biological systems. Extrapolating from kinetic results obtained in vitro using dilute, homogenous buffered solutions, it is generally accepted that the rate of elimination of H(2)O(2) in vivo by GPx is independent of glutathione concentration (GSH). To examine this doctrine, a mathematical analysis of a kinetic model for the removal of H(2)O(2) by GPx was undertaken to determine how the reaction species (H(2)O(2), GSH, and GPx-1) influence the rate of removal of H(2)O(2). Using both the traditional kinetic rate law approximation (classical model) and the generalized kinetic expression, the results show that the rate of removal of H(2)O(2) increases with initial GPx(r), as expected, but is a function of both GPx(r) and GSH when the initial GPx(r) is less than H(2)O(2). This simulation is supported by the biological observations of Li et al.. Using genetically altered human glioma cells in in vitro cell culture and in an in vivo tumour model, they inferred that the rate of removal of H(2)O(2) was a direct function of GPx activity × GSH (effective GPx activity). The predicted cellular average GPx(r) and H(2)O(2) for their study are approximately GPx(r) ≤ 1 μm and H(2)O(2) ≈ 5 μm based on available rate constants and an estimation of GSH. It was also found that results from the accepted kinetic rate law approximation significantly deviated from those obtained from the more generalized model in many cases that may be of physiological importance.