The effects of gamma-irradiation on solutions of catalase, apocatalase and haematin

Marked difference in cytochrome c oxidation mediated by HO(*) and/or O(2)(*-) free radicals in vitro

Cytochrome c (cyt c) is an electron carrier involved in the mitochondrial respiratory chain and a critical protein in apoptosis. The oxidation of cytochrome c can therefore be relevant biologically. We studied whether cytochrome c underwent the attack of reactive oxygen species (ROS) during ionizing irradiation-induced oxidative stress. ROS were generated via water radiolysis under ionizing radiation (IR) in vitro. Characterization of oxidation was performed by mass spectrometry, after tryptic digestion, and UV-visible spectrophotometry. When both hydroxyl and superoxide free radicals were generated during water radiolysis, only five tryptic peptides of cyt c were reproducibly identified as oxidized according to a relation that was dependent of the dose of ionizing radiation. The same behavior was observed when hydroxyl free radicals were specifically generated (N(2)O-saturated solutions). Specific oxidation of cyt c by superoxide free radicals was performed and has shown that only one oxidized peptide (MIFAGIK+16), corresponding to the oxidation of Met80 into methionine sulfoxide, exhibited a radiation dose-dependent formation. In addition, the enzymatic site of cytochrome c was sensitive to the attack of both superoxide and hydroxyl radicals as observed through the reduction of Fe(3+), the degradation of the protoporphyrin IX and the oxidative disruption of the Met80-Fe(3+) bond. Noteworthy, the latter has been involved in the conversion of cyt c to a peroxidase. Finally, Met80 appears as the most sensitive residue towards hydroxyl but also superoxide free radicals mediated oxidation.

Radiation-induced changes in purified prothrombin and thrombin

The effect of gamma-irradiation on purified prothrombin and thrombin in aqueous solution has been assessed with reference to bifunctional activities, e.g., clotting and esterase functions, physico-chemical changes in structure, and kinetics. The inactivation curves indicated that the clotting activity was more susceptible to gamma-radiation than the esterolytic function in both the proteins. Prothrombin was comparatively more sensitive to radiation than thrombin. The irradiation of prothrombin (100 kR) caused modifications in the protein resulting in reduced formation of thrombin after activation by Factor Xa. The modifications caused by irradiation were assessed in these proteins by changes in spectral characteristics, levels of tryptophan and disulphides, electrophoretic mobility and amino acid composition. Radiation-induced changes in thrombin were reflected in its kinetic behaviour. The clotting activity of thrombin was almost completely lost at 100 kR, while esterolysis was relatively less affected. The modification of tyrosine and tryptophan residues in thrombin influenced the clotting activity, while these were not involved for esterolysis. Histidine had involvement in both these activities.

Concentration-dependent inactivation of superoxide dismutase

The inactivation yield of superoxide dismutase (superoxide: superoxide oxidoreductase, EC 1.15.1.1) from bovine erythrocytes, when Co60-gamma-irradiated in air, N2 or N2O-saturated solutions, increases exponentially with the initial enzyme concentration. In aerated solutions at less than or equal to 10 micrometers, the inactivation process continues in a concentration-dependent manner in the subsequent 72 h. This post-irradiation effect is inhibited by catalase. Above 10 micrometers, radiation-induced inactivation of the enzyme is partially reversed in a concentration-dependent manner and is not affected by catalase. In aerated and N2O-saturated solutions, competitive scavenging of radiation chemical species by catalase and EDTA in combination reduces the inactivation yield by 80%; the residual yield remains dependent on enzyme concentration. Radiation-induced loss of copper and zinc initially exhibits a linear dose-response relationship and is less severe than the drop in enzyme activity.