Radioprotective effect of free radical scavenging enzymes.

Ionizing radiation is an important treatment modality in the management of head and neck squamous cell carcinoma. The superoxide (O2-) and hydroxyl (OH.) radicals produced from oxygen and the radio-hydrolysis of water are responsible for most of the DNA and lipid membrane injury caused by radiotherapy. Superoxide dismutase (SOD) and catalase (CAT) are intracellular enzymes that scavenge the superoxide and hydroxyl radicals respectively. The effect of intravenous SOD and CAT on acute and delayed radiation injury was investigated in a rat model. Catalase was shown to reduce the severity of radiation-induced changes in both the vascular endothelium and squamous epithelium. SOD, alone or with CAT, showed no radioprotective effect. As intravenous catalase does not penetrate intracellularly it should have no effect on the tumoricidal effect of radiation. Further investigation of catalase as an agent to reduce the acute side-effects of radiotherapy is warranted.