Superoxide Ion as the Cause of the Oxygen Effect

Abstract

O2 enhancement ratios (OER) was measured for the survival of Escherichia coli B after X irradiation in relationship to the membrane-associated enzyme, superoxide dismutase (SOD). Previous studies showed that the levels of this enzyme are directly related to the amount of Fe present in the growth medium. Bacteria grown in a low Fe medium had an OER of 2.35 .+-. 0.10, while bacteria grown in a high Fe medium had an OER of 1.71 .+-. 0.07. This enzyme was probably responsible for the observed radioprotective effects. Since other enzymes can also be induced by growth in an Fe medium, further experiments were performed. When bovine SOD was added to a final concentration of 1.23 .mu.g/ml to bacteria grown in a low Fe medium prior to irradiation, an OER of 1.40 .+-. 0.06 resulted. In similar control experiments using heat-inactivated SOD, only a small radioprotective effect was observed. These results suggest that the superoxide ion, O2-, is responsible for the major component of the O2 effect in bacterial cell lethality.