Chemistry of Coat-Colour in Animals. 



37 



Since the relationship of these substances to each other is still doubtful, it 

 seems preferable to follow Gortuer* in calling all the artificial black bodies 

 hum ins, and to reserve the name melanin for the dark pigments occurring in 

 the living organism. 



The origin of the pigments remained in great obscurity until Landoltf 

 suggested that melanin might be formed by a process of oxidation. But it 

 was Bertrand's* discovery among plants of tyrosinase, an oxidase capable of 

 oxidising tyrosine to a humin, that led to the present theory. Bertrand's 

 discovery was confirmed by Biedermann§ and others who found that tyrosinase 

 occurred among many plants and animals. 



This wide distribution of tyrosinase and its obvious connection with 

 pigment production gave rise to the current hypothesis of the mechanism of 

 melanin formation, namely, that melanins are due to the action of an oxidase 

 upon a colourless chromogen. This possibly takes place in two stages : (1) 

 the formation of a cyclic compound which is split from the protein molecule 

 by some autolytic ferment, and (2) the oxidation of the cyclic compound to a 

 melanin by means of a specific ferment. 



According to the theory of Bach and Chodat, oxidases are of a dual 

 nature, their constituents being a peroxide (A2O2) and a peroxidase (P). 

 The peroxide combines with the peroxidase to form an unstable compound 

 (A 2 POo). This immediately breaks down to yield 



(1) An atom of " active " oxygen 0. 



(2) The peroxidase P. 



(3) The compound A 2 0. 



The colourless chromogen is oxidised to a pigment by the " active " 

 oxygen, whilst the compound A 2 is reconverted by the atmospheric oxygen 

 to the peroxide A2O2, so that the cycle can be repeated indefinitely. 



All attempts to ascertain the constitution of the melanin molecule have 

 proved fruitless, mainly owing to the difficulty of isolating the pigment. The 

 qualitative analyses do not agree in the main features and little or nothing 

 can be deduced from the elementary analyses obtained. The mother substance 

 of melanin has generally been supposed to be tyrosine, and Gessard|| has 

 shown that in one case at least tyrosine as well as tyrosinase was present in 

 the tissues surrounding the pigment. There are, however, a number of other 



* Goitner, ' Jour. Biol. Chemistry,' vol. 8, No. 4, p. 341 (1910). 

 t Landolt, ' Zeitschrift fur Physiol. Cheni., : vol. 28, p. 192 (1899). 



X Bertrand, ' Paris, C. P.. Acad. Sci.,' vol. 122, p. 1215 (1896), and vol. 123, p. 463 

 (1696) ; ' Annales Institut Pasteur,' vol. 22, No. 5, p. 381 (1908). 



§ Biederuianu, ' Pflueger's Archiv fur gesaiumte Physiol.,' vol. 72, p. 132 (1898). 

 || Gessard, 'Paris, C.E. Acad. Sci.,' vol. 136, p. 1086 (1903). 



