Effects of Oxygen and Sulphydryl-containing Compounds on Irradiated Transforming DNA

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Abstract
This paper extends our earlier observations on the effects of the sulphydryl (SH)-containing compound dithiotheitol (DTT) on the radiation response of Bacillus subtilis transforming DNA to three other SH-containing compounds–cysteamine, cysteine and glutathione (GSH). In general, all four compounds protect transforming DNA in a manner which is dependent on gassing conditions. In O2, the protection is consistent with the scavenging of OH radicals by the SH compounds, but in N2 there is additional protection which may be due to hydrogen atom donation from the SH compound to radiation-induced DNA lesions, a process which is blocked by O2. This additional protection in N2 results in an increase in the ratio of inactivation in the absence and presence of oxygen with increasing SH concentration to a maximum followed by a decrease at high SH concentrations. The maximum value of the ratio and the SH concentration at which it occurs depend on the SH compound. In particular, GSH appears to be significantly less efficient in the hydrogen-donation repair reaction with transforming DNA than are the other three SH compounds. Furthermore, on the basis of our results, we postulate the existence of a damage fixation process which occurs in the absence of O2, in competition with damage repair by SH compounds, and that this anoxic damage fixation occurs at a rate not less than 300 s−1. We also demonstrate here that the damage fixing reaction of O2 with transforming DNA radicals proceeds 200-fold faster than the competing repair reaction by hydrogen-donation from DTT.