Recombinant superoxide dismutase reduces oxygen free radical concentrations in reperfused myocardium

It has been proposed that oxygen free radicals mediate damage that occurs during postischemic reperfusion. Recombinant human superoxide dismutase (r-h-SOD) has been shown to be effective at reducing reperfusion injury, but it is not known if this infused enzyme actually reduces oxygen free radical concentrations in the myocardial tissue. Electron paramagnetic resonance spectroscopy was used to directly measure the effect of r-h-SOD on free radical concentrations in the postischemic heart. Hearts were freeze clamped at 77°K after 10 min of normothermic global ischemia followed by 10 s of reflow with control perfusate (n = 7) or perfusate containing 60,000 U r-h-SOD (n = 7). The spectra of these hearts exhibited three different signals: signal A isotropic, g = 2.004, identical to the carbon-centered ubiquinone free radical; signal B anisotropic with axial symmetry, g(perpendicular) = 2.033, g(paralel) = 2.005, identical to the oxygen-centered alkyl peroxyl free radical; and the signal C an isotropic triplet, g = 2.000, a(n) = 24 G, similar to a nitrogen-centered free radical such as a peroxyl amine. With r-h-SOD administration the concentration of the oxygen free radical, signal B, was reduced 49% from 6.8 ± 0.3 μM to 3.5 ± 0.3 μM (P < 0.01) and the concentration of the nitrogen free radical, signal C, was reduced 38% from 3.4 ± 0.3 μM to 2.1 ± 0.3 μM (P < 0.01). The concentration of the carbon-centered free radical, signal A, however, was increased 51% from 3.3 ± 0.2 to 5.0 ± 0.2 μM (P < 0.01). Identical reperfusion with peroxide-inactivated r-h-SOD did not alter the concentrations of free radicals indicating that the specific enzymatic activity of r-h-SOD is required to decrease the concentrations of reactive oxygen free radicals. Additional measurements performed varying the duration of reflow demonstrate a burst of oxygen free radical generation peaking at 10 s of reperfusion. r-h-SOD entirely abolished this burst. These studies demonstrate that superoxide-derived free radicals are generated during postischemic reperfusion and suggest that the beneficial effect of r-h-SOD is due to its specific enzymatic scavenging of superoxide free radicals.