414 LIFE : OUTLINES OF GENERAL BIOLOGY 



from the sepia cuttlefish common in Mediterranean waters. But if 

 painters use "sepia" now, it is synthetic, not cuttlefish sepia. 



Of the chemistry of melanin little is known. The probability is 

 that it is not a single pure substance, but often, at least, a mixture. 

 It is very hard to purify, since it will not crystallise. It always 

 occurs in minute granules, and it almost defies solution in solvents. 

 Some biochemists have connected it with an amino-acid (i.e. next 

 door to protein) called "trj^jtophane"; and in some cases this is 

 very probable. Other biochemists believe that it is connected in 

 some way, just as haemoglobin is, and chlorophyll too, with the 

 pyrrol ring, which, we may repeat, consists of four atoms of carbon 

 linked to one atom of nitrogen, as may be represented by the symbol 



C-^ C 



I I 



But the general view in regard to melanin, a x-iew that has passed 

 beyond hypothesis, is that melanin is derived from tyrosine — an 

 important amino-acid — or from some similar substance. The general 

 interest of this will soon be apparent. 



Pure tyrosine, in a test-tube, treated with a preparation of an 

 enzyme or ferment of wide occurrence called tjTOsinase, and then 

 exposed to the air, becomes first reddish and finally black, and 

 this black pigment is, as far aS one can tell, identical with natural 

 melanin. As tyrosine and tyrosinase are both of wide distribution, 

 especially in Invertebrates, it is highly probable that natural 

 melanin is formed by this reaction. 



There is, however, a closely allied substance called by the long 

 name dioxyphcnylalanin and by the short name "Dopa", which 

 is, perhaps, the first result of the action of tyrosinase on tyrosin. 

 It blackens spontaneously, especially in contact with alkalies. 

 It has been suggested that "Dopa" is the source of the melanin of 

 Vertebrates, as in dark hair, dark feathers, and the black in the 

 lining of the body-cavity of lizards, frogs, and many fishes. It is 

 almost certain that "Dopa" plays a part in the blackening of the 

 cocoons of many insects; and according to some it also occurs in 

 some plants. 



If melanins arc derivable from tyrosine through the action of 

 a ferment, the general interest of the fact is that tyrosine is an 

 amino-acid, and that amino-acids readily arise by the breaking- 

 down of proteins which are invariably present in living matter. 

 Thus a common pigment may be interpreted as the result of a 

 disintegrative or katabolic change in proteins. 



