Post-irradiation free radical generation: evidence from the conversion of xanthine dehydrogenase into xanthine oxidase.

The xanthine oxidoreductase (XOR) system which consists of xanthine dehydrogenase (XDH) and xathine oxidase (XO), is one of the major sources of free radicals in biological systems. The XOR system is pre-dominantly present as XDH in normal tissues and converts into the free radical generating XO-form in the damaged tissue. Therefore, the XO-form of the XOR system is expected to be mainly found in radiolytically damaged tissues. In such an event, XO may catalyze the generation of free radicals and potentiate radiation effects in the post-irradiation period. Recent findings on the effect of ionizing radiation on the XOR system in the liver of mice, peroxidative damage and lactate dehydrogenase support this possibility. From these results it has been hypothesized that free radical generating systems could be activated in the radiolytically damaged cell and in turn contribute to the cause and complications of late effects and their persistence in post-irradiation period. This aspect may have great significance in the understanding of radiation-induced damages. It may also have serious implication in various fields like radiation therapy, health physics, carcinogenesis, space travelling radiation exposures and post nuclear accident care. Further, it is suggested that efforts need to be made to search more system(s) which could be activated particularly at lower doses of radiation to generate free radicals in the post-exposure period.