I. Aravindakshan Menon

COMPARISON OF THE EFFECTS OF UV‐VISIBLE IRRADIATION OF MELANINS and MELANIN‐HEMATOPORPHYRIN COMPLEXES FROM HUMAN BLACK and RED HAIR

Clinical evidence indicates that people with light skin and red hair have a higher incidence of UV radiation‐induced diseases including cancer. It is not known whether this is because of the lower protection due to the smaller amounts of eumelanin present in the skin of these people or whether the presence of pheomelanin in their skin is responsible for the higher susceptibility to carcinogenesis. Irradiation of melanoproteins from red hair with UV‐visible light has been reported to produce superoxide. Comparative studies on the formation of superoxide during the irradiation of the melanins isolated from human black and red hair (BHM and RHM, respectively) are reported in this paper. These showed that no superoxide formation could be detected in the case of the BHM. whereas the formation of superoxide during the irradiation of RHM could be definitely demonstrated. Irradiation of the RHM with NADH resulted in the oxidation of more NADH than the irradiation of NADH with BHM. The observation that RHM is an active photosensitizer indicates that this property of pheomelanin present in light skin may at least partly be responsible for the harmful effect of radiation on people with light skin and/or red hair. Administration of hematoporphyrin followed by visible irradiation is a currently used mode of therapy for cancer. The present studies have shown that hematoporphyrin is bound to both BHM and RHM. The binding of hematoporphyrin to the melanins increased the formation of superoxide by RHM and the oxidation of NADH by both the melanins. The binding of porphyrins to melanins may have an influence upon the photosensitivity in diseases such as porphyrias and may also affect the therapeutic efficacy of hematoporphyrin.

Electron transfer properties of melanin.

Abstract Melanin synthesized from mushroom tyrosinase and 3,4-dihydroxyphenylalanine has been shown to oxidize NADH and NADPH, reduce ferricyanide, oxidized forms of cytochrome c and dichlorophenolindophenol, and catalyze the coupled oxidation of NADH and reduction of ferricyanide. Kinetic studies involving the determination of initial velocity at various concentrations of substrates and product inhibition measurements have been carried out on the NADH-ferricyanide-melanin reaction. The results are consistent with a ping-pong mechanism in which one product is formed prior to the reaction of melanin with the second substrate involving the reversible oxidation and reduction of melanin during the reaction. It may be concluded that melanin is capable of acting as an electron transfer agent in several reduction-oxidation systems.

Binding of Imipramine, 8-Methoxypsoralen, and Epinephrine to Human Blue and Brown Eye Melanins

AbstractPrevious studies have shown that although human blue and brown eye melanins are similar in several respects they differ in some physical, chemical, and biological properties. This paper reports a comparison of the binding of three drugs—imipramine, 8-methoxypsoralen, and epinephrine—to the melanins isolated from human blue and brown eyes. It was found that these three drugs are bound to both of these melanins. There were differences between the binding sites and the association constants for their binding to blue and brown eye melanins. The binding of 8-methoxypsoralen to blue eye melanin showed two types of binding sites. It is suggested that the binding of drugs to eye melanins could decrease their immediate concentration in the eyes but could also result in higher concentrations for prolonged periods of time in the eyes compared to concentrations in nonmelanotic tissues. The differences between the blue and brown eye melanins in the binding of certain drugs may result in differences in the acti...