Relationship Between Thiol-Induced Repair and Repair by Enzymes

The radioprotective effect of the addition of thiols (cysteamine and dithiothreitol, DTT) on the survival of γ-irradiated E. coli strains with deficiencies in their enzymatic repair capacities have been measured in the presence and absence of oxygen. The oxygen enhancement ratios (OER) decrease with increasing DTT concentrations in the repair wildtype to the level of the repair-deficient strain. This is interpreted as being the result of OH scavenging and a competition between oxygen and thiols for DNA radicals. At the highest thiol concentration oxygen cannot compete with chemical repair by H donation and this leads to smaller OER values. The DER and the protection by the addition of thiol (the dose modification factors, DMF) are smaller for strains with deficiencies in the enzymatic repair capacities than for strains proficient in enzymatic repair. The reason is that DNA damage produced in the absence of oxygen or in the presence of oxygen at high thiol concentrations are enzymatically relatively better repaired than that produced in the presence of oxygen.