SUNBURN 505 



tion of new melanin (Hamperl et al., 1939a). The failure to differentiate 

 the processes of mclanization and pigment darkening has led to confusion 

 and to the belief that primarj^ melanization is brought about by wave 

 lengths longer than those of the erythemal spectrum. 



Miescher and Minder (1939) confirmed and extended the findings of 

 Henschke and Schultze, offering an explanation in terms of the Meirowsky 

 effect. Meirowsky in 1909 and later Lignac (1923) had found that dark- 

 ening of the pigment of cadaver skin is brought about by ultraviolet radi- 

 ation or by heat, and that such darkening does not occur in the absence of 

 oxygen. Miescher and Minder pointed out that the pigment darkening 

 in living skin parallels these findings, and presented further evidence of 

 the identity of these effects. They postulated that the darkening repre- 

 sents the oxidation of melanin pigment already present in the skin in a 

 reduced leuko form. The exact nature of this leuko form was not speci- 

 fied, but P'igge (1939) has since shown that melanin may be reversibly 

 bleached in an in vitro oxidation-reduction system. The writer has sug- 

 gested (1945) that the melanoid pigment described in human skin by 

 Edwards and Duntley (1939a, b), as a result of studies of spectral reflec- 

 tion, is a leuko form of melanin, or a mixture of the oxidized and reduced 

 forms. Sharlit (1945) has vehemently attacked the findings and conclu- 

 sions of Miescher and Minder. Unfortunately, he does not give any 

 description of the ultraviolet radiation he used nor does he indicate that he 

 recognized either the difference between pigment formation and pigment 

 darkening or the spectral difference of the radiation bringing about the 

 two effects. It is to be hoped that this discrepancy in results will be 

 explained by further experimental studJ^ 



Hormones and Tanning. That hormones may influence the tanning of 

 human skin was first shown by Hamilton and Hubert (1938), who found in 

 eunuchoids that previously sunburned areas darkened in color when 

 androsterone was injected. Although other factors are concerned, includ- 

 ing circulatory changes, melanin pigment is involved in this phenomenon 

 (see Hamilton, 1948). 



Biochemical Aspects. In vitro reactions leading to the formation of 

 melanin have been applied with considerable success to melanin formation 

 in certain species, but until recently there have been objections to inter- 

 preting suntanning in terms of such reactions. There are still uncer- 

 tainties to be explained, but the resolution of some of the difficulties seems 

 to be in sight. The general subject has been recently reviewed by Lerner 

 and Fitzpatrick (1950), and there will be no attempt to do so here. Let 

 us assume, tentatively, that the melanin formation associated with suntan 

 follows the same chemical steps as the in vitro reaction, which may be 

 schematized in abbreviated form suitable to this discussion as follows: 



tyrosinase 



Tyrosine + Oa > dihydroxyphenylalanine -^ intermediates -^ 



(do pa) 



melanin. 



