Melanin biological effects

Cells have substantial chemical defenses against the UV photoproducts produced in seawater and intracellular fluids. Many organisms have antioxidants (e.g., carotenoids, ascorbate, tocopherols, anthocyanins, and tridentatols) that quench photo-oxidative reactions.64-67 Cells also have enzymes (e.g., catalase and superoxide dismutase) that can counteract the oxidative nature of peroxides and other radicals.26 Some compounds, such as the UV-absorbing pigment melanin, can act as both optical filter and antioxidant.68 The MAA mycosporine-glycine (Figure 15.3) functions in a similar dual capacity.69 The role of UV-mediated reactions in seawater relative to biological effects is an important current area of study. 

The distinguishing feature of tyrosinase is that it catalyzes the oxidation of monohydric phenols, like tyrosine, to the dihydric form and dihydric phenols, like DOPA and catechol, to the corresponding quinones. The striking biological effects of this enzyme arise from quinones which polymerize to produce the darkening of various plants on injury and melanin in mammals. The relative oxidation rates of several dihydric phenols by tyrosinase are given in Table III. 

Spin-trapping experiments connected with biological superoxide production have also been used to examine the biochemistry of anti-tumour antibiotics bleomycin (Sugiura and Kikuchi, 1978) and mitomycin C (Lown et al., 1978), as well as the effect of iron concentration on xanthine oxidase reactions (Buettner et al., 1978), and the photochemistry of melanins (Felix et al., 1978). 

Skin depigmentation effect. Extract of the dried seed, administered externally to adults at a dose of 5%, was active. Skin-lightening cosmetics contained extract of Coffea arabica seeds (containing chlorogenic acid) as melanin-formation inhibitors. The extract has been incorporated into cosmetics for skin-aging prevention or into hair preparations for hair protection. Biological activity reported has been patented . 

The deoxygenation of a polyphenolic melanin precursor can be effected biologically and the role of NADPH in this has been studied (ref.58)... 

Synthetic melanins are obtained by biomimetic oxidation reactions using known precursors. So far, four different methods for melanin synthesis have been reported, i.e. in vitro enzymatic, autooxidative, electrochemical, and photochemical methods. Of these, the first two have been generally used, for large scale preparations of the pigment polymers and have been reviewed elsewhere (70, 211). The latter two methods which are discussed here, have been used effectively to understand the mechanism of the melanization process in biological systems. 

Melanin plays key roles to protect internal tissues against the harmful effects of uv radiation in human beings. The most common form of biological melanin is a complex polymer of either or both of 5,6-indolequinone and 5,6-dihydroxyindole carboxylic acid. CPMAS NMR and FT-IR spectroscopy have been carried out of this and related systems. 

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