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Abstract
Although its concentration is generally not known, glutathione peroxidase-1 (GPx-1) is a key enzyme in the removal of hydrogen peroxide (H2O2) 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 H2O2in vivo by GPx is independent of glutathione concentration (GSH). To examine this doctrine, a mathematical analysis of a kinetic model for the removal of H2O2 by GPx was undertaken to determine how the reaction species (H2O2, GSH, and GPx-1) influence the rate of removal of H2O2. Using both the traditional kinetic rate law approximation (classical model) and the generalized kinetic expression, the results show that the rate of removal of H2O2 increases with initial GPxr, as expected, but is a function of both GPxr and GSH when the initial GPxr is less than H2O2. 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 H2O2 was a direct function of GPx activity×GSH (effective GPx activity). The predicted cellular average GPxr and H2O2 for their study are approximately GPxr ≤1 µm and H2O2≈5µm 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.
| Acronyms | ||
| concentration of species j | = | Cj |
| rate of change of species j | = | dCj/dt |
| 2′,7′-dichlorodihydrofluorescein | = | DCFH2 |
| glutathione peroxidase | = | GPx |
| classic (cytosolic) glutathione peroxidase | = | GPx-1 |
| oxidized glutathione peroxidase | = | GPxo |
| reduced glutathione peroxidase, the form that reacts with hydroperoxides | = | GPxr |
| glutathione-enzyme complex | = | GS-GPx |
| glutathione | = | GSH |
| glutathione disulphide | = | GSSG |
| reaction rate constant of reaction i | = | ki |
| ordinary differential equation | = | ODE |
| reactive oxygen species | = | ROS |
| overall time constant | = | τ |
| initial concentration of species i | = | [i]0 |
| Acronyms | ||
| concentration of species j | = | Cj |
| rate of change of species j | = | dCj/dt |
| 2′,7′-dichlorodihydrofluorescein | = | DCFH2 |
| glutathione peroxidase | = | GPx |
| classic (cytosolic) glutathione peroxidase | = | GPx-1 |
| oxidized glutathione peroxidase | = | GPxo |
| reduced glutathione peroxidase, the form that reacts with hydroperoxides | = | GPxr |
| glutathione-enzyme complex | = | GS-GPx |
| glutathione | = | GSH |
| glutathione disulphide | = | GSSG |
| reaction rate constant of reaction i | = | ki |
| ordinary differential equation | = | ODE |
| reactive oxygen species | = | ROS |
| overall time constant | = | τ |
| initial concentration of species i | = | [i]0 |