')()(') UADIATIOX ItlOLOOV 



The amino acid tyrosine is oxidized to dihydroxyphenylalaniiie, com- 

 monly known as "dopa," l)y llio action of the enzyme tyrosinase, a copper 

 protein comj^lex. Molecnlai' ()\y}>;en takes part in the (irst step in vitro 

 and ajiain in later stei)s. There seems no objection to applyinj^ such a 

 scheme to melanin formation in suntanninji;, hut difficulty arises when 

 further attempt is made to represent the melanin formation, let alone the 

 whole process of suntanning, in terms of a photochemical forwarding of 

 this in vitro reaction scheme. 



In 1933 Frankenburger found that exposure of solutions of tyrosine to 

 ultraviolet radiation resulted in the formation of a brown pigment, pre- 

 sumably melanin, but very large doses of radiation were re(iuired. Later 

 Arnow (1937) found that ultraviolet radiation could bring about oxidation 

 of tyrosine to dopa, but again the amount of radiation recjuired was much 

 greater than would be needed for the in vivo production of suntan. 

 Rothman showed in 1942 that this reaction may be accelerated by the 

 presence of small amounts of ferrous salts and that when these are present 

 the quantities of energy retiuired are nearer to those effective in sun- 

 tanning. The possibility must be considered that ultraviolet radiation 

 acts in vivo to bring about the oxidation of tyrosine to dopa in a manner 

 parallel to the in vitro reactions. After this, the succeeding steps may 

 presumably take place without the intervention of ultraviolet. There 

 seems one objection to all these schemes in that the in vitro production of 

 melanin, and specifically the initial step, are aerobic, whereas in vivo the 

 reduction of the molecular oxygen .supplied by occlusion of the circulation 

 to the skin during the exposure to ultraviolet radiation has no effect on the 

 subsequent melanization (Blum et a/., 1935; Henschke and Schultze, 

 1939a; Blum, 1941b). It may be objected that by such methods the 

 partial pressure of oxygen in the skin is not reduced sufficiently to inhibit 

 the reaction. Sharlit (1945), as a result of his studies on cadaver skin, 

 suggested that oxygen is obtainable from the cytochrome system. It can 

 only be pointed out that the same method of removing oxygen inhibits 

 pigment darkening and photosensitized oxidations. This point need not 

 be emphasized, however, since there are other difficulties to consider. 



Rothman et al. (1946) have proposed a scheme which is not based 

 directly on the in vitro reactions and so may avoid the above objection. 

 They found for human skin, as had Ginsberg (1944) for the guinea pig, 

 that there is present a factor which inhibits melanin formation in vitro. 

 This they identified tentatively as water-extractable sulfhydryl com- 

 pounds. They suggest that ultraviolet radiation and other injurious 

 agents cause destruction of these compounds with the result that the 

 formation of melanin can proceed. Fitzpatrick et al. (1949, 1950) include 

 Rothman's scheme to account for the melanin formation of suntan. 

 They also include the oxidation of tyrosine to dopa by direct photo- 

 chemical effect, and add another factor, the acceleration of this reaction 



