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TRACE ELEMENTS AND SKIN PIGMENTATION

Of the many natural pigments displayed by human skin, melanin, if not always the most prominent, is perhaps the most significant. Its formation in the skin has been well documented over the last 25 years, yet, because of the difficulties in isolating it in a pure state, its chemical structure and composition remain largely unknown (Duchon, Fitzpatrick & Seiji, 1968). Likewise, there is little known about the interactions between melanin and that group of elements, found in the body in a concentration of less than 100 p.p.m., which are becoming increasingly known as 'trace elements'. In animals, efforts to achieve a better understanding of these interactions seem to have been prompted by the economic value attached to wool, fur and other coating materials. The best known example is that of copper and its role in wool pigmentation. The loss of pigment found in copper deficiency states in sheep has been attributed to a breakdown in the earlier stages of melanin synthesis, which are catalyzed by the copper-dependent enzyme tyrosinase (Underwood, 1971). On the other hand, colour changes of hair found in children with a protein deficient state were not related to their copper nutritional status (Gopalan, Reddy & Mohan, 1963). Of the fifty or more trace elements known to exist in the body, zinc offers the most interesting prospect in its relation with melanin. Very high zinc levels are found in heavily melanised structures, like the choroid of the eye (Underwood, 1971), pigmented moles (Molokhia & Portnoy, 1973) and melanosomes (Seiji et al., 1963). The relatively minute mass of melanin present in these tissues makes it safe to conclude that this pigment must be extremely rich in zinc. However, the nature of the association between zinc and melanin remains unexplored and the significance of such a high zinc content in pigmented tissues is at present unknown. It has been suggested that melanin may be acting as a potent chelating agent, binding trace metals like zinc (Lorincz, 1954). In vitro studies, however, have shown that melanin behaves more like a weak cation exchanger (Bruenger, Stover & Atherton, 1967). The possibility that zinc may be catalyzing some of the later steps of melanin synthesis can not be ruled out. On the other hand, zinc atoms may be acting as a link between melanin and the protein component in a manner similar to that suggested for iron in the iron pigment of red hair (Flesch, 1968). Hyperpigmented skin lesions have been reported in patients with abnormalities of trace element metabolism, such as Wilson's disease (Leu et al., 1970) and haemochromatosis (Althausen et al., 1951). Although high serum copper levels are found in a number of hyperpigmentary conditions, like xeroderma pigmentosum (El-Hefnawi et al., 1962) and melanosis accompanying pregnancy or the intake of oral contraceptives, many other conditions with hypercupraemia are not associated with pigmentary changes. In a recent study of vitiligo (Molokhia & Portnoy, 1973), there was no evidence of copper deficiency in either skin or serum, while serum zinc levels were significantly reduced. However, any speculation on the role of trace elements in vitiligo would have to take into account the structural defect which underlies the absence of melanin.