258 URINARY MELANIN. 



It is very probable that several dark-coloured pigments such as the uromelanins 

 of Plosz and Thudichum obtained by the action of acids on urinary pigments or 

 chromogens are allied to if not identical with these humus substances. 



5. Urinary melanin *. 



Certain tumours are not infrequently observed which from their 

 extremely dark pigmentation are spoken of as 'melanotic,' the 

 colouring substance being known as melanin 2 . The urine of pa- 

 tients suffering from these tumours is either dark brown or 

 black when voided or speedily assumes this colour after brief exposure 

 to the air or by the action of nitric acid or other oxidising agents, the 

 pigment to which the colour is due being apparently identical with that 

 present in the tumour. This action of oxidising agents indicates that 

 here also, as in the case of other urinary pigments, there is primarily 

 some chromogenic forerunner (melanogen) of the actual pigment. 

 This chromogen may be partially precipitated from the urine by baryta 

 water and completely by normal lead acetate. When the latter pre- 

 cipitate is suspended in water and decomposed by sulphuretted hydro- 

 gen, it yields a colourless solution which when evaporated to dryness 

 leaves a dark amorphous residue insoluble in water, ether, cold 

 alcohol, acetic acid and dilute mineral acids. The fully formed pigment 

 may, like its chromogenic forerunner, be partially precipitated by 

 baryta water, the remainder being precipitable by the subsequent 

 addition of normal lead acetate. The baryta precipitate contains the 

 larger amount of the pigment, and from it the colouring matter may be 

 more easily obtained than from the precipitate with the lead salt, since 

 the latter carries down other urinary pigments at the same time. The 

 isolation of the urinary melanin in a pure form from the baryta 

 compound admits of no suitably concise description; it must suffice 

 here to state that an impure product is obtained by decomposing the 

 compound with sodium carbonate assisted by gentle warmth and pre- 

 cipitating the pigment from the resulting solution by a slight excess of 

 sulphuric acid. The product when purified is partly insoluble, partly 

 soluble in acetic acid of 50 75 p. c. Of these portions the former 

 when dried is a brownish-black amorphous powder, insoluble in either 

 water, alcohol, ether, chloroform or dilute (mineral) acids, but readily 

 soluble in alkalis. The latter was obtained in too small amounts to 

 admit of complete investigation. On analysis the pigment was found 



1 Morner, Zt. f. physiol. Chem. Bde. xi. (1887), S. 66, xn. (1888), S. 229. Gives 

 list of literature to date. See also Zeller, Langenbeck's Arch. Bd. xxix. (1884), S. 

 2, and later Brandl u. Pfeiffer, Zeitsch.f. Biol. Bd. xxvi. (1890), S. 348. 



2 The name melanin is more usually applied as a generic title for the dark 

 brown or black pigments such as occur in the hair, epidermis, retinal epithelium, 

 choroid, &c. 



