The Effect of Radioprotective Agents and of Tissues on the Radiation-Induced Oxidation of Dopa to Melanin

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Abstract
Using the dopa system as assay, anti-oxidant mechanisms in tissue have been investigated and the behavior of tissue compared to that of radio-protective agents. Both tissues and radio-protective agents inhibit the autoxidation of dopa. A close parallelism exists between the inhibition by the radio-protectors of the UV-induced, the X-ray induced, and the autoxidation of dopa to melanin. On the other hand, the tissue inhibitor acts only on the autoxidative process. This is explicable on the basis that the radiations act directly (absorption of UV quanta by dopa; oxidation by OH and HO2 radicals of the dopa in case of X-rays) while the autoxidation is a metal-catalyzed process. The tissue inhibitor probably acts by metal-binding, whereas the action of the radio-protective chemicals, though most were metal-binding agents, must be explained on the grounds of their inherent chemical reactivity. H2O2 does not, but OH radicals generated from peroxide do oxidize dopa to melanin. With either type of radiation true aftereffects, following cessation of the radiation could not be observed. The tissue autoxidation inhibitor is destroyed by X-rays and by UV; destruction is more rapid in the presence of an oxidizable substrate.