Influence of Copper(II) Ions on Radicals in DOPA–Melanin

Among the biopolymers, melanins are unique in many respects. The other essential biopolymers - proteins, nucleic acids, and carbohydrates - are chemically well characterized; their precursors (monomer units) and modes of connection between the monomer units are known, and sequences of their connection can be determined with well-established methodologies. In contrast, we still do not have a method to determine accurately the ratio of various units present in melanins. This is largely because of the chemical properties of melanins, such as their insolubility over a broad range of pH, the heterogeneity in their structural features, and also because of the lack of methods that can split melanin polymers into their monomer units (all other biopolymers can be hydrolysed to the corresponding monomer units). To overcome this difficulty, we developed a rapid and sensitive method for quantitatively analysing eumelanin and pheomelanin in biological samples by chemical degradation methods followed by HPLC determination. This HPLC microanalytical method for characterizing eumelanin and pheomelanin has become a useful tool for the study of melanogenesis. This review will summarize the usefulness and limitations of the various chemical and spectrophotometric methods used to analyse melanins at the biochemical, cellular, and tissue levels. Emphasis is given on the usefulness of 4-amino-3-hydroxyphenylalanine as a specific marker of pheomelanin.

Various drugs and other chemicals, such as organic amines, metals, polycyclic aromatic hydrocarbons, etc., are bound to melanin and retained in pigmented tissues for long periods. The physiological significance of the binding is not evident, but it has been suggested that the melanin protects the pigmented cells and adjacent tissues by adsorbing potentially harmful substances, which then are slowly released in nontoxic concentrations. Long-term exposure, on the other hand, may build up high levels of noxious chemicals, stored on the melanin, which ultimately may cause degeneration in the melanin-containing cells, and secondary lesions in surrounding tissues. In the eye, e.g., and in the inner ear, the pigmented cells are located close to the receptor cells, and melanin binding may be an important factor in the development of some ocular and inner ear lesions. In the brain, neuromelanin is present in nerve cells in the extrapyramidal system, and the melanin affinity of certain neurotoxic agents may be involved in the development of parkinsonism, and possibly tardive dyskinesia. In recent years, various carcinogenic compounds have been found to accumulate selectively in the pigment cells of experimental animals, and there are many indications of a connection between the melanin affinity of these agents and the induction of malignant melanoma.

Changes are induced in the electron spin resonance signal amplitude and microwave power saturation of the naturally occurring free radical in melanin by bound paramagnetic ions. The changes serve as experimental observables in competition experiments between diamagnetic and paramagnetic metal ions for melanin binding sites and between melanin and ethylenediaminetetraacetic acid for paramagnetic metal ions. Evidence is presented for the existence of several specific types of metal binding sites. The interaction of copper with free radicals leading to loss of electron spin resonance signal amplitude is magnetic in nature and not, as has been supposed, chemical.