Sulphydryl compounds in melanogenesis

Neutral pH and copper ions promote eumelanogenesis after the dopachrome stage

The diversity of pigmentation in the skin, hair, and eyes of humans has been largely attributed to the diversity of pH in melanosomes with acidic pH being proposed to suppress melanin production. Tyrosinase has an optimum pH of 7.4 and its activity is suppressed greatly at lower pH values. The first step of eumelanogenesis is the oxidation of tyrosine to dopachrome (DC) via dopaquinone. However, how eumelanogenesis is controlled by pH beyond this stage is not known. In this study, we examined the effects of pH (5.3-7.3) on the conversion of DC to 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) and the subsequent oxidation of DHI and DHICA to form eumelanin. The effects of Cu(2+) ions on those reactions were also compared. The results indicate that an acidic pH greatly suppresses the late stages of eumelanogenesis and that Cu(2+) ions accelerate the conversion of DC to DHICA and its subsequent oxidation.

Relationship of melanin degradation products to actual melanin content: application to human hair

Methods not only for characterizing but also for quantitating melanin subtypes from the two types of melanin found in hair--eumelanin and pheomelanin--have been established. In relation to testing for drugs of abuse in hair, these methods will allow for correction of drug binding to specific melanin subtypes and will serve to improve drug measurement in hair. 5,6-Dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) make up the majority of the eumelanin polymer while benzothiazene units derived from 2-cysteinyl-S-Dopa (2-CysDopa) and 5-cysteinyl-S-Dopa (5-CysDopa) compose the majority of the pheomelanin polymer. Our results show that: (1) pyrrole-2,3-dicarboxylic acid (PDCA) and pyrrole-2,3,5-tricarboxylic acid (PTCA), markers for DHI and DHICA units, respectively, are produced in 0.37 and 4.8% yields, respectively, when melanins are subjected to alkaline hydrogen peroxide degradation, (2) 3-aminotyrosine (3AT) and 4-amino-3-hydroxyphenylalanine (AHP), markers for 2-CysDopa and 5-CysDopa, respectively, are produced in 16 and 23% yield, respectively, when subjected to hydriodic acid hydrolysis, and (3) that black human hair contains approximately 99% eumelanin and 1% pheomelanin, brown and blond hair contain 95% eumelanin and 5% pheomelanin; and red hair contains 67% eumelanin and 33% pheomelanin. These data will allow deeper investigation into the relationship between melanin composition and drug incorporation into hair.

The roles of neuromelanin, binding of metal ions, and oxidative cytotoxicity in the pathogenesis of Parkinson's disease: a hypothesis

A characteristic feature of both Parkinson's disease (idiopathic paralysis agitans) and normal aging is loss of pigmented neurons in the substantia nigra. This has been found to correlate with the accumulation of neuromelanin and with oxidative stress in this brain region, but a clear association between these factors has not been established. Based on our recent demonstration that neuromelanin is a true melanin, containing bound metal ions in situ, we present a general model for its accumulation in vivo and the hypotheses (1) that it has a cytoprotective function in the sequestration of redox-active metal ions under normal conditions but (2) that it has a cytotoxic role in the pathogenesis of Parkinson's disease. Thus, neuromelanin accumulates normally through the autooxidation of catecholamines and serves tightly to bind redox-active metal ions, processes which would accelerate under conditions of intracellular or extracellular oxidative stress. Based on the known properties of melanin, however, neuromelanin also has the potential for exacerbating oxidative stress, eg by generating H2O2 when it is intact or by releasing redox-active metal ions if it loses its integrity; these reactions also would modulate the reactivity of the neuromelanin. By overwhelming intracellular antioxidative defense mechanisms, such a positive-feedback cycle could turn a condition of chronic or repeated oxidative stress in vulnerable neurons into an acute crisis, leading to cellular death. If the cumulative stress in duration and/or degree is severe enough, neuronal depletion could be sufficient to cause Parkinson's disease during life. One possible trigger for this cascade is suggested by the increased nigral iron contents in postmortem parkinsonian brains and the correlation of this disease with urban living where exposure to heavy metal ions is high: the saturation of neuromelanin with redox-active metal ions. Parkinson's disease therefore may be a form of accelerated aging in the substantia nigra associated with environmental toxins in which neuromelanin has a central, active role.

A reexamination of the melanin formation assay of tyrosinase and an extension to estimate phaeomelanin formation

This paper presents some modifications of the melanin formation assay for tyrosinase from the point of view of both eu- and phaeomelanosynthesis. On the one hand, eumelanosynthesis can be estimated using neutral paper filters, such as the 3MM Whatman filters so far employed. The main advantages of this sort of paper are the very low blank values obtained in the absence of tyrosinase and its greater mechanical resistance in the successive washing steps. It is shown that the sensitivity of the assay can be enhanced by the addition of 1 mM Ni(II) to the incubation mixture or of NaOH to stop the enzymatic reaction and allow the incorporation of indolic intermediates into the polymer. Furthermore, the accuracy is also enhanced by the proposed modifications, since all reactions from dopaquinone are standardized, and the assay becomes only dependent on the tyrosinase activity. On the other hand, phaeomelanosynthesis cannot be estimated using neutral paper because of the slow rate of polymerization of the intermediates and the poor absorption of thiol-dopa conjugates to this kind of paper. It is shown that synthesis of this type of melanin can be estimated in the presence of glutathione by means of a cationic filter paper and by washing the excess of the radioactive substrate with distilled water instead of acidic media. Thus, the assay may be adapted to measure eu- or phaeomelanosynthetic activity by introducing slight modifications. This assay must be used with caution if detergent-solubilized tyrosinase is used, because detergents strongly inhibit melanin absorption to paper filters.