Aspects physico-chimiques des espèces réactives de l’oxygène

Reactive Oxygen Species (ROS), namely hydroxyl (*OH) and superoxide (O2*-) free radicals and hydrogen peroxide (H2O2), are involved in all oxidative stress phenomena. These latter occur in numerous pathological disorders such as, for example, cardiovascular diseases, diabetes or neurodegenerative diseases. Knowledge of thermodynamic (reduction potentials) and kinetic (rate constants) properties of ROS allows to draw up a rigorous overview of the chemical reactivity of these species. Hydroxyl free radicals (*OH) are powerful oxidants (very toxic species) which attack all the biomolecular targets (DNA, proteins, lipids...), giving other free radicals localized on the targets. Superoxide free radicals (O2*-) have a more graduated reactivity, since they don't directly react with biological molecules. However, some toxicity would be attributed to them via secondary radicular reactions. Other free radicals (of peroxyl RO2* and alkoxyl RO* types), belonging also to the ROS family, contribute by enhancement to oxidative stress.

Sod and catalase inactivation by singlet oxygen and peroxyl radicals

Both superoxide dismutase and catalase are readily deactivated by singlet oxygen and by the radicals produced in the pyrolysis of 2,2'-azo-bis-(2-amidinpropano) under aerobic conditions. The rate constant for the loss of enzymatic activity induced by singlet oxygen are 3.9 x 10(7) and 2.5 x 10(7) M-1 sec-1 for SOD and catalase, respectively. The similarity between these values implies that in systems where SOD and catalase are exposed to similar singlet oxygen concentrations, it can be expected a parallel inactivation of both enzymes. The inactivation of both enzymes by the radicals produced by 2,2'-azo-bis-(2-amidinopropane) pyrolysis under aerobic conditions follows a first-order kinetics at low enzyme concentrations and a zero-order kinetics at higher concentrations. Although at low enzymatic concentrations the rate of inactivation of both enzymes is similar, this results from a compensation of effects because there are wide differences in the reactivity of both enzymes towards peroxyalkyl radicals. Catalase is considerably more reactive, but a large number of protein/radical reactive interactions are needed to inactivate one enzyme. On the other hand, the reactivity of SOD is smaller, but the average enzyme activity decreases by nearly 20% in each SOD/radical reactive interaction.