Properties of DNA Irradiated in the Presence of the Protective Agent Bis(2-guanidoethyl) Disulphide (GED)

Summary

Structural alterations were measured after gamma-irradiation of dilute solutions of salmon-sperm DNA and were correlated with the enhancement of template activity at 7 kR, as well as its inhibition at higher exposures as previously described. Formation of single-stranded regions in irradiated DNA was demonstrated by measurements of hyperchromicity and T_m in the presence of formaldehyde, and by the rate and quantity of ¹⁴C-formaldehyde binding. The radioprotective agent GED showed little and marked protection against this damage at 7 kR and 28 kR, respectively. Ultracentrifugation analysis demonstrated the formation of double- and single-stranded breaks in a ratio of 1 : 6·5 without GED and 1 : 10 with GED at both 7 kR and 28 kR. It is concluded that enhancement of template activity at 7 kR is due to the production of single-stranded regions and single-stranded breaks which stimulate the activity of DNA polymerase. The protection by GED results in reducing the number of double-stranded breaks and other detrimental changes which inhibit template activity. At 28 kR and higher exposures, the protection is considerable but not complete, and the enhancement may be masked by an increasing number of double-stranded breaks and other changes which inhibit template activity.