Oxidation of NADH by melanin and melanoproteins.

Abstract The oxidation of NADH by melanin was employed as a means for studying the properties of melanin isolated from melanoma. The NADH-oxidizing activity of melanin isolated from B16 melanoma (tumour-melanin) was compared with that of the melanin synthesized by the action of monophenol monooxygenase (tyrosinase, EC 1.14.18.1) on 3,4-dihydroxyphenylalanine (dopa-melanin). Unlike dopa-melanin, the tumour-melanin did not oxidize NADH. When the proteins associated with the tumour-melanin were hydrolyzed by acid treatment, the resulting product readily oxidized NADH. Conversely, addition of bovine serum albumin to the melanin-synthesizing system or to the melanin already synthesized decreased the oxidation of NADH by the melanin. Similarly, polyarginine and polylysine decreased the NADH oxidizing activity while polyglutamate and polyaspartate did not have any effect. The results indicate that basic components of the proteins bound to melanin may be blocking the active sites of melanin involved in the oxidation of NADH. It is postulated that the electron transfer properties of melanin may be important in its protective role against radiation and toxic free radicals and that the inactive melanoprotein, as it occurs in the cell, may be converted to active melanin upon dissociation or degradation of the protein by agents such as irradiation.