The role of hydroxyl radical quenching in the protection by acetate and ethylenediaminetetraacetate of supercoiled plasmid DNA from ionizing radiation-induced strand breakage.

A supercoiled plasmid of 7300 base pairs was isolated and exposed in various aqueous environments to 60Co gamma-radiation. Conversion of the supercoiled form to the relaxed circular and linear forms was monitored by agarose gel electrophoresis and quantified by fluorescence scanning of the gel. Acetate, which has been reported to affect the conformation of DNA in solution, decreased the radiosensitivity of the supercoil in a concentration-dependent manner. Acetate, formate, and azide anions, as well as mannitol, all protected the supercoil from relaxation in approximate proportion to the rate at which their solutions quench the hydroxyl radical. At concentrations greater than 300 mmol dm-3, however, the efficiency of acetate radioprotection is reduced. Disodium ethylenediaminetetraacetate protected the supercoil more efficiently than would be expected from the published value of its rate constant for quenching the hydroxyl radical.