OH O 



PIGMENTS AND COLOURS 



OH O 



— C 2 H 5 



481 



—OH 



CHOH— CH 3 

 -OH 



OH O 



ECHINOCHROME A 



OH O 



SPINOCHROME A 



Indoles. These include indigoid pigments and melanins, basically sub- 

 stances containing a phenopyrrol nucleus. Indigoid pigments are produced 

 by certain gastropods, and include indigo and the purple dye dibromindigo, 

 secreted by Murex, Mitra and Nucella. This substance is the dyestuff 

 Tyrian purple which was used so extensively by the ancients. It is produced 

 by a special hypobranchial gland, and stored as the colourless leuco com- 

 pound. When secreted and exposed to sunlight, it becomes transformed 

 into the purple pigment dibromindigo (purpurin). No physiological role 

 has been established for indigoid pigments in animals, and they may well 

 be excretory products (6, 58, 80). 



In general the dark or black pigments of animals belong to the group 

 of melanins, which have a colour range from yellow to jet black. They 

 tend to be resistant substances, insoluble in water and the usual solvents, 

 but dissolved by strong alkalis. Chemically they are indole derivatives of 

 relatively high molecular weight, originating usually from tyrosine through 

 the agency of tyrosinase. The melanins which occur in different animals 

 in reality comprise a group of diverse substances, differing in degree of 

 oxidation and complexity. The reactions may be summarized — 



Tyrosinase Tyrosinase 

 Tyrosine ^DOPA — — ->Hallachrome - 



+o 



Intermediates 



+o 



+ 



Melanins 



+o 



The intermediate compound DOPA. is 3,4-dihydroxyphenylalanine 

 which has been found in certain animals. Hallachrome, a red intermediate 

 stage in the oxidation of tyrosine, occurs naturally in the polychaete 

 Halla parthenopeia, where it may be involved in cellular respiration. Brown 

 and black melanin pigments are found in sponges (Chondrosid), coelenter- 

 ates (dark varieties of Metridium senile, coloured areas of the jellyfish 

 Pelagia noctiluca), annelids (photoreceptors), arthropods (cuticle, 

 chromatophores, photoreceptors), molluscs (siphons, valves of Mytilus), 

 echinoderms (integument of echinoids, ophiuroids and holothurians) and 

 in vertebrates. Characteristically, they have an intracellular location in 

 epithelial and mesenchymal cells, or in special melanophores (the role of 

 these cells in colour-responses is described in the next chapter). Less often 

 they are extracellular, e.g. in the mesogloea of Pelagia and in the exo- 



M.A. — 16 



