Radiation-induced long-lived radicals which cause mutation and transformation.

Using electronic spin resonance (ESR), we found a new type of radical with a long life-time in cells (T1/2>20 h) and which may play a more important role in the induction of mutation and transformation than either the active, short-lived, H, or OH radicals. When cells were treated with dimethyl sulfoxide (DMSO) and l-ascorbic acid (AsA) just before irradiation, the short-lived radicals were well-scavenged. On the other hand, if cells were treated with the scavengers 20 min after irradiation, then AsA scavenged the long-lived radicals, but DMSO did not. AsA treatment 20 min after the start of irradiation drastically reduced both the frequencies of mutation at the hypoxanthine guanine phosphoribosyl transferase (HGPRT) locus in human cells and morphological transformations in mouse m5S cells, but DMSO treatment did not. In addition, AsA treatment 20 h after irradiation also reduced the mutation frequency in human cells. These results suggested that mutations and morphological transformation are probably caused by the presence of long-lived radicals in the cells, rather than by short lived radicals, and that AsA reacts efficiently with long-lived radicals, resulting in a decrease of the mutations and transformations induced.