Analysis of eumelanin-related indolic compounds

Photochemistry of 5-S-cysteinyldopa

The photochemical behavior of 5-S-cysteinyldopa (5-S-CD), a colorless product of melanocyte metabolism, was investigated in neutral phosphate buffer with biologically relevant UV radiation. Exposure of 5-S-CD to pyrex-filtered UV light (wavelengths > 320 nm) was found to induce an oxygen-dependent reaction, leading to, besides abundant polymeric materials, the benzothiazine derivatives I and II (two diastereoisomers). Superoxide dismutase exerted a small inhibitory effect on 5-S-CD consumption, whereas other active oxygen scavengers had no effect on the reaction course. Addition of glutathione as a hydrogen donor completely suppressed the reaction. With UVB light (wavelength range 280-320 nm) photolysis of 5-S-CD proceeded mainly with formation of 3,4-dihydroxyphenylalanine, arising presumably by photohomolytic cleavage of the S-CH2 bond followed by desulfuration. These results are of interest in relation to the high susceptibility of fair-complexioned individuals to actinic damage and skin cancer.

Inhibitory effect of melanin precursors on arachidonic acid peroxidation

A possible role of melanin precursors in lipid peroxidation was investigated using the lipoxygenase catalysed oxygenation of arachidonic acid (AA) as a model system. Polarographical monitoring of oxygen consumption showed that, among the metabolites examined, 5,6-dihydroxyindole (DHI) was the most active in inhibiting AA oxygenation catalysed by 15-lipoxygenase. The inhibition was found to be concentration-dependent with an IC50 value of 15 microM. Similar effects were observed in the case of the 5-lipoxygenase promoted reaction. Periodical HPLC analysis of the oxidation mixture showed that, in the presence of DHI, the rate of substrate consumption is markedly reduced. The inhibitory potency was significantly increased either by preincubation of DHI with the enzyme or by increasing the time of residence of the indole in aerated buffer solutions prior to contact with the enzyme. Addition of catalase to the incubation mixture resulted in a partial removal of DHI inhibition. From these and other experiments, an inhibition mechanism is proposed which involves inactivation of the enzyme by reactive species, especially hydrogen peroxide, arising from DHI autoxidation.

Comparative action of dopachrome tautomerase and metal ions on the rearrangement of dopachrome

A vis-a-vis comparison between the effects of dopachrome tautomerase (DCT) and metal ions, e.g., cupric ions, on the kinetics and mode of rearrangement of dopachrome has been carried out under appropriate analytical conditions. The enzyme-promoted reaction is highly stereospecific for L-dopachrome, is unaffected by metal chelators and has an optimal pH around 6.8. By contrast, the kinetics of dopachrome rearrangement catalysed by cupric ions are not dependent on the stereochemistry of the substrate, are affected by EDTA and are not influenced by the pH of the medium in the range between 5-7.5. Both cupric ions and DCT catalyse the rearrangement of dopachrome to give 5,6-dihydroxyindole-2-carboxylic acid (DICA) rather than 5,6-dihydroxyindole (DI). However, at comparable activity, the ratio of formation DICA/DI is significantly higher in the enzyme-catalysed than in the metal-catalysed reaction. These results provide an improved background to look into the mode of action of DCT and metal ions, enabling a clear cut differentiation between the effects of the two factors when both are present in biological extracts.