Enzyme inactivation by ultraviolet irradiation and protective effect of melanin in vitro

Photodestruction of pheomelanin: role of oxygen

Pheomelanin, the red-brown polymeric pigment in the skin and hair of red-headed humans, is composed of a protein fraction covalently bound to a colored chromophore. Photolysis of aerated aqueous phemelanin solutions, isolated from human red hair, results in destruction of the chromophore and liberation of the peptide fraction. The rate of photolysis depends on the pH and the concentration of both pigment and oxygen and is slightly inhibited by the enzyme superoxide dismutase (superoxide:superoxide oxidoreductase EC 1.15.1.1). Pheomelanin photolyzed in the presence of nitroblue tetrazolium results in the formation of a blue diformazan, whether or not oxygen is present. Superoxide dismutase inhibits the aerobic photoreduction of nitroblue tetrazolium whereas, in the absence of oxygen, no inhibition is observed. These experiments strongly suggest the involvement of superoxide in the aerobic photolysis of pheomelanin and point out a possible mechanism for ultraviolet-induced cell damage in redheads.

Effect of clinically relevant irradiation regimes on human malignant melanomas grown in athymic nude mice

Human tumors grown subcutaneously in the thymus deficient nude mice are used as a model system to compare the effectiveness of clinically relevant irradiation regimes. Procedures for local irradiation of the tumors with 60Co radiation are described in detail. A preliminary test of the regression of two malignant melanomas, following three clinically relevant fractionation schedules, is performed. The overall treatment time and the nominal standard dose (NSD) are kept equal for the irradiation regimes chosen.

ULTRAVIOLET LIGHT AND LYSOSOME ACTIVITY

The abdominal skin of guinea pigs was irradiated by a Xenon short arc lamp with glass filters. Acid phosphatase and beta-glucuronidase activity increased in the irradiated skin. The change of acid phosphatase is slow and mild in intensity, while that of beta-glucuronidase is quick and high in degree. The activity of acid phosphatase is influenced equally by light with filters of UV-25, UV-27, UV-29, and UV-31, while that of beta-glucuronidase is affected more mildly by light with a UV-31 filter than with any of the other three filters. These findings suggest that there is a close relationship between cutaneous changes induced by ultraviolet light and changes in lysosome activity, and that energy required to induce changes in enzyme activity is much less than energy required to produce erythema.

Mobility gaps: a mechanism for band gaps in melanins

The semiconductor behavior of melanins is reviewed and compared with quantum mechanical models of conduction in amorphous solids. The available data are consistent with extensions of Mott's basic model for amorphous semiconductors, whereas they are inconsistent with crystalline semiconductor models. An investigation of the specific conduction mechanisms operative in melanins in terms of the amorphous model should reveal important aspects of the band structure.