The production of single strand cleavage in covalently-closed circular-DNA by the antitumour agent streptonigrin (reduced in situ by NADH) is demonstrated using the ethidium bromide fluorescence assay described previously. The degradation dependent on oxygen is completely inhibited by superoxide dismutase (EC 1.15.1.1) suggesting the intermediacy of the superoxide radical anion in the degradation. However similar complete inhibition of DNA strand breakage by catalase (EC 1.11.1.6) indicates that the hydroxyl radical (formed by interaction of superoxide with hydrogen peroxide) is the primary reactive species. Cupric ion stimulates the cleavage reaction and cobaltous ion has no effect in keeping with model studies using quinolinequinones.

Vitamin C breaks DNA only in the presence of oxygen. Superoxide dismutase has no effect on the reaction but catalase suppresses it. Superoxide also gives rise to breaks in DNA suppressible by both superoxide dismutase and catalase. The hydroxyl radical seems to be the agent responsible for strand cleavage itself.