ROLE OF MELANIN AS A SCAVENGER OF ACTIVE OXYGEN SPECIES

The protective role of melanin, either synthetic or derived from a met astatic lung melanoma nodule, was studied in terms of its ability to i nteract with active oxygen species (O2-, H2O2, RO., ROO., etc.). Both melanins showed the ability to react with O2.. The superoxide dismutas e-like activity corresponds to 21 and 10 U/mg for synthetic and tumor melanin, respectively. The latter value accounts for about 8% of the s uperoxide dismutase activity of cultured melanoma cells. Neither type of melanin showed catalase-like or glutathione peroxidase-like activit y. Both types of melanin reacted with RO. and ROO. radicals as determi ned by inhibition of the lipid peroxidation reaction of rat liver homo genates. The spontaneous lipid peroxidation of rat liver homogenate wa s inhibited up to 90% and 80% by synthetic and tumor melanin with half -maximal effects at 2.5 and 5.5 mug melanin/ml, respectively. The 2,2- azobis-(2 amidino propane) (AAPH)-initiated lipid peroxidation of rat liver homogenate was inhibited up to 30% and 20% by synthetic and tumo r melanin, with half maximal effect at 120 and 500 mug melanin/ml, res pectively. Both types of melanin were able to protect the in vitro ina ctivation of glucose oxidase, which occurs in the presence of AAPH-gen erated radicals.