Increased DNA chain breakage by combined action of bleomycin and superoxide radical.

$\text{In vitro}$ DNA chain breakage by bleomycin was enhanced by the addition of xanthine oxidase system. The effect of the xanthine oxidase system disappeared completely when superoxide dismutase was added, but not when catalase was added. From these results it is concluded that superoxide radical is one of the chemical mediators responsible for the enhancement of the DNA chain breakage action of bleomycin.