394 PATHOLOGICAL PIGMENTATION 



three factors : First, it is extremely difficult to obtain melanin 

 in a pure condition ; second, the process of purification requires 

 the action of strong acids and alkalies, which undoubtedly 

 modify the composition of the melanin ; thirdly, melanin is 

 probably not a single substance of definite composition, but 

 includes several related but different bodies. The values found 

 vary for carbon from 48.95 to 60.02 per cent.; for hydrogen 

 from 3.05 to 7.57 per cent.; for nitrogen, 8.1 to 13.77 per cent. 

 Hofmeister gives, as a characteristic of melanins, that their 

 elementary molecular composition is always nearly in the pro- 

 portions N:H:C=1:5:5. 



A particularly prominent constituent of melanin is sulphur, 

 which has been found in as high proportions as 10 per cent, in 

 melanin from sarcomas, and even 12 per cent, in sepia from the 

 squid ; in melanin from hair the sulphur is usually about 2-4 

 per cent.; but in choroid melanin, and in some other forms, sul- 

 phur seems to be absent. The proportions of sulphur obtained 

 from the same specimen purified by different methods show 

 wide variations, and hence v. Fiirth considers that neither the 

 sulphur nor the iron are indispensable constituents of the mel- 

 anin. Probably the melanin molecule contains atom-complexes 

 that have a tendency to bind certain sulphur and iron com- 

 pounds (e. g., cystin or hematin derivatives). 



There is much reason to believe that the melanin is derived 

 from certain groups of the proteid molecule that seem readily to 

 form colored compounds. The aromatic compounds of the pro- 

 teid molecule, such as tyrosin, phenylalanin, and tryptophan, 

 readily condense with elimination of water and absorption of 

 oxygen, to produce dark -colored substances. When proteids are 

 heated in strong hydrochloric acid, we obtain a dark-brown 

 material, which closely resembles the melanins both in element- 

 ary composition and in general properties, so that it is referred 

 to as " artificial melanin " or " melanoid substance." These 

 substances, like the natural melanins, when decomposed by 

 fusing with caustic potash, yield skatol, indol, and pyrrol deriv- 

 atives, which are undoubtedly derived from the tyrosin and 

 tryptophan of the proteid molecule. Therefore, it seems prob- 

 able that both the melanoid substances and the true melanins 

 are formed from the chromogen groups of the proteid molecule 

 through processes of condensation, elimination of water, and 

 the taking up of oxygen. 



Tyrosinase. In the sepia sacs of the cuttle-fish, in meal- 

 worms which form a melanin-like pigment, and in plants that 

 produce the black Japanese lacquer, have been found oxidizing 



