Radioprotective effect of superoxide dismutase on model phospholipid membranes.

1. Hydroperoxide formation in model membranes was measured via the net increase in absorbance at 232 nm after exposure to X-rays or 137Cs gamma rays in the presence and absence of bovine superoxide dismutase and other radical scavengers. 2. Membranes X-irradiated in air to 4200 rad at 210 rad/min exhibited a large increase in absorbance, a major portion of which was O2--mediated since active superoxide dismutase at 1 mug/ml reduced it by more than 80% to the level observed in N2O. In N2 the change in absorbance was smaller than in N2O but not in proportion to the halving in OH production. 3. The net absorbance of membranes exposed to a constant dose from 137Cs increased with decreasing dose rate. A minor component of this effect was due to exposure protraction with decreasing dose rates while the major component was attributed to long chain reactions initiated by ionizing radiation. A corollary effect was also observed, namely, that with reducing dose rate the dose required to elicit a constant absorbance change decreased. Both aspects were abolished by superoxide dismutase at 1 mug/ml. 4. The enzyme protected membranes after an acute exposure and from low level radiation at natural background while its inactivated form sensitized.