Effect of melanin on radiation response of CHO cells.

The effect of the presence of melanin on the response of mammalian cells to ionizing radiation was investigated in a model system utilizing the ability of Chinese hamster ovary cells to incorporate melanin by endocytosis. Cells were incubated in monolayer cultures from 2 to 20 hours with melanin prepared from 'beef eye' or synthesized by air oxidation of 3,4-dihydroxyphenylalanine. For asynchronous cultures, the survival curve parameters for cells incubated with both types of melanin were indistinguishable from those of the same cells without added melanin. The radiation response to fractionated doses of 6 Gy separated by various periods did not indicate any effect of melanin on the extent or kinetics of repair of sublethal damage. Likewise, the repair of potentially lethal damage in plateau phase cultures was unaffected by the presence of melanin. Thus the explanation for the clinical radiation resistance of melanomas in the absence of a direct radiation effect might more likely be found in consideration of other factors such as the role of melanin in oxygen consumption or in differentiation.