Metal reduction reactions of the melanins: histochemical studies.

Abstract

In general the melanins are argentaffin and, except in hair cortex, reduce ferric ferricyanide. Cutaneous and ocular melanins reduce acid silver nitrate, more promptly in guinea pigs than in man and rhesus monkeys. This reduction is slower than that of ammoniacal silver nitrate. It may be accelerated so as to occur in one or two minutes by prior reduction of the melanins by sodium hydrosulfite. Neuromelanin reduces acid silver nitrate very slowly (2-3 days) and prior hydrosulfite reduction does not induce rapid reductivity for acid silver. Acetylation has no influence on the reductivity of melanin for acid or ammoniacal silver or ferric ferricyanide. But, if applied to cutaneous and ocular melanins after reduction with hydrosulfite, it then greatly impairs the reduction of acid silver nitrate, but not of ammoniacal silver. Acetylation similarly influences the reductivity of catechol toward the acid and ammoniacal silver solutions. Oxidation has two general effects on the melanins, an irreversible bleaching by the more drastic oxidants, and a reversible abolition or impairment of their reducing capacity toward acid and ammoniacal silver solutions and ferric ferri cyanide. The oxidants vary in their effect on the silver and ferric ferricyanide reactions, so that some are more effective against the argentaffin reactions, others against the ferric ferricyanide reaction. Likewise the cutaneous, ocular and nerve cell melanins show differences in loss of reductivity with the various oxidants. Tumor melanins seem somewhat less reactive to silver than adjacent normal tissue melanins. Ocular tumor melanin resembles chorioidal melanin more than the epithelial melanins of the retina, ciliary and iris. The argentaffin reaction of cutaneous and ocular melanin is probably assignable to the presence of quinhydrone groupings which may be reversibly oxidized to quinones and reduced to diphenols. This type of grouping does not appear to be present in neuromelanin. Neuromelanin, from this and from the other recorded differences, appears to be a distinct substance, perhaps not even closely related chemically to the other melanins.