Effects of oxygen and sulphydryl-containing compounds on irradiated transforming DNA. Part I. Actions of dithiothreitol.

The actions and interactions of oxygen and the sulphydryl-containing compound dithiothreitol (DTT) upon the radiation sensitivity of the biological activity of purified Bacillus subtilis transforming DNA have been examined. It has previously been shown that the sensitivity of transforming DNA irradiated in dilute solution is less when irradiation is performed in 100 per cent O2 than when in 100 per cent N2, i.e. O2 protects transforming DNA with a dose-modifying factor of about 0.7. DTT protects transforming DNA in a manner that is dependent on DTT concentration and on gassing conditions. In O2 the DTT protection can largely be attributed to the scavenging of .OH radicals by the DTT, but in anoxia DTT exerts a further protective effect which results in an increasing oxygen enhancement ratio (o.e.r.) with increasing DDT concentration to a maximum o.e.r. of about 14 at 2-5 mM DTT. This additional protective effect of DTT is attributable to hydrogen atom donation from DTT to DNA radicals, thus chemically repairing the DNA. Oxygen appears to block this chemical repair reaction.