102 



KNOWLEDGE 



April, 1915. 



melanotic tumours, and tyrosinase alone in tin- 

 skin of young "i t"' taJ rabbits.* 



I he discovery of this wide distribution of tyro- 

 sinases resulted in the formulation of the now 

 commonly accepted theory thai the pigments of 

 the hair, skin, and scale: are in a greal many 

 produced by the action of an oxydase upon 

 some chromogenic substance. It is possible that 

 this chromogenic substance, which is of an aromatic 

 nature, musl be spUt off from the protein mole- 

 cule by a proteolytic enzyme before it can be 

 oxidised. 



Further investigations have suggested that an 

 oxydase is of a dual nature, its two constituents 

 being a peroxide and a peroxydase. The peroxide 

 rnbines with the peroxydase (B) to form 

 an unstable compound (AJ'.O.,). This immediately 

 breaks down to yield : 



(i) An atom of " active " oxygen, O. 

 (ii) The peroxydase, B. 

 (iii) The compound A.,0. 



The chromogen is oxidised to the pigment by 

 the " active. " oxygen, whilst the compound A 2 

 is reconverted by atmospheric oxygen to the 

 peroxide A 2 0.,, so that the cycle can be repeated 

 indefinitely. 



The isolation of the melanins and the determina- 

 tion of the constitution of the melanin molecule 

 has presented a problem of extraordinary difficulty 

 to the organic chemist, and even now a satisfactory 

 solution has not been obtained. The reasons for 

 this are chiefly the difficulty of obtaining sufficient 

 material, or of separating it from the keratin of the 

 skin and hair which usually surrounds it, and the 

 lack of strong enough reagents to dissolve the 

 keratin substance without altering the melanin 

 molecule. Attempts have been made to obtain 

 melanin from many sources, such as the skin of 

 negroes, human and horse hair, sarcomatous livers, 

 and the choroid coat of eyes ; but except in the last 

 two cases the melanin has been profoundly altered 

 in the process of extraction. Lately, however, 

 in America, a pigment has been obtained by the 

 long-continued action of very dilute alkalies upon 

 black wool in a form which evidently resembles more 

 closely the naturally occurring pigment than any 

 described hitherto. In this form it dissolves, not 

 only in alkali, but also in dilute acid, and behaves 



in many other ways like a protein substance, or 

 a substance which is chemically combined with a 

 protein. 



The investigation oi the natural pigments was 

 originated by the discovery of the black artificial 

 humins which occur as a cleavage residue during 

 the acid hydrolysis oi proteins. Recently another 

 humin has been studied, namely, the product 

 of the action of tyrosinase upon tyrosine, and 

 tyrosine itself has been oxidised by strong hydro- 

 chloric acid to a black substance. These artificial 

 products seem to bear a fairly close relationship 

 to the natural melanins, though it is difficult to 

 determine their constitution accurately. In spite 

 of the fact that tyrosine so easily yields black 

 products, it is not necessarily the mother-substance 

 of the natural pigments, as has so often been 

 suggested, for there is no reason why tryptophane, 

 or some other cyclic compound, should not function 

 equally well. 



It may be seen from what has been said that the 

 black pigments have received the exclusive atten- 

 tion of investigators, and that the other hair 

 pigments, of which yellow would seem to promise 

 most opportunity for research, have been entirely 

 neglected, f It might be thought that this yellow 

 substance is connected with the well-known 

 urinary pigment, urochrome, or else with those 

 other body ^pigments called lipochromes, which 

 give to fat butter and blood serum their yellow 

 colour, and which have lately been shown to be 

 identical in many cases with the crystalline vege- 

 table pigments, carotin and xanthophyll. It does 

 not, however, seem to resemble either of these, and 

 indeed it is difficult to know whether a true yellow 

 pigment really exists, or whether the effect is 

 produced by the density with which the black 

 pigment has been deposited. For by suitably 

 diluting a solution of melanin, which is deep brown 

 by transmitted light and black by reflected light, 

 it is possible to obtain solutions of various shades 

 of brown, or even, on extreme dilution, a bright 

 yellow. The black, chocolate, and yellow pig- 

 ments show very similar solubilities in both acid 

 and alkali ; and, moreover, when equal weights 

 of these three pigments, dissolved in equal volumes 

 of dilute alkali, are compared by means of a colori- 

 meter the difference in the depths of the colours 

 is insignificant. 



* It seems probable that only the peroxydase constituent of the tyrosinase can be extracted in this case, since the 

 addition of hydrogen-peroxide is necessary for the proper oxidation and development of the colour in vitro. 



j The beautiful yellow pigment of our common brimstone butterfly has been shown to be a derivative of uric acid, 

 while the white pigment of cabbage butterflies is nothing else but uric acid itself. The uric-acid pigments are 

 confined to the Pieridae, and are not present in the wings of the Heliconiidae, which are mimicked so closely by the 

 Pieridae (genus Leptalis). 



