Effect of the sulfhydryl compound cysteamine on gamma-radiation-induced mutations in double-stranded M13 DNA

Sulfhydryl compounds can protect DNA against free-radical-induced DNA damages not only by scavenging of radicals, but also by chemical non-enzymatic repair or modification of such damages by hydrogen-donation. To investigate the influence of chemical repair and modification on mutations, induced by gamma-radiation-generated free radicals (.OH, .H), phosphate-buffered aqueous solutions of double-stranded (ds) M13 DNA were exposed to gamma-rays under N2 in the presence of 5 mM cysteamine. The exposed DNA was subsequently transfected to wild-type E. coli and mutations in the mutational target were characterized. This target in fact contains three different target sequences, i.e., the lac promoter/operator, the lacZ alpha gene and a 144 bp inframe insert. The mutation spectrum obtained was compared with those in the absence of cysteamine under N2 and N2O. In the latter case, the ratio of .OH and .H available for reacting with DNA is about the same as under N2 + cysteamine. The results show that chemical repair and/or modification by cysteamine of potentially lethal lesions takes place, leading to a much higher survival of ds M13 DNA in the presence of cysteamine than could be expected on basis of scavenging of .OH and .H alone. This higher survival appeared to be accompanied with a higher mutation induction. However, the N2 + cysteamine mutation spectrum shows a remarkable resemblance with the N2O-spectrum. This holds for the total mutation target, as well as each of the three targets, although the mutations obtained in each of the three targets under the same irradiation conditions are quite different. Thus, it can be concluded that cysteamine is mainly effective on radiation-induced potentially lethal DNA lesions, and not so much on (pre)mutagenic damages. Moreover, the type of mutation appeared to be strongly dependent on the mutational target sequence.