i HUMIN-SUBSTANCES 89 



2. Tyrosin, which, according to v. Fiirth and Schneider 1 and 



Ducceschi, 2 is converted by ferments and other oxidising media 

 into dark-coloured substances (see index under Tyrosinase). 



3. Lysin, according to Hart. 3 



4. Tryptophane, according to Nencki, 4 Hopkins, and Cole. 5 



To put it shortly, Samuely holds that normal pigments are formed 

 by the indol-, pyrrol-, pyridin-, and tyrosin-radicals of the albumin 

 molecule. In this connection Ellinger's formula of tryptophane (p. 53) 

 would explain the tendency to a closure of the pyridin ring in trypto- 

 phane, and the assumption of a pyridin-nucleus in the albumin-mole- 

 cule 6 become therefore unnecessary, as this assumption is based on the 

 fact that melanoidins on being reduced give pyridin (Samuely). 7 



Samuely also observed a more or less abundant formation of humin 

 on subjecting carbohydrates along with amino-acids or other nitrogenous 

 bodies to the action of boiling hydrochloric acid. Glucose and tyrosin 

 together give an especially large -amount of humin. Schmiedeberg 8 

 noticed further that nucleic acid, which contains a carbohydrate along 

 with xanthiri-bases, gives rise to melanoidin. Similarly, egg-white is 

 very apt to form humin because it contains a large amount of carbo- 

 hydrate. Antipeptone, analogously, does not form melanin, 9 because it 

 ^s deficient in glucosamin, tryptophane, and tyrosin. 

 . werV\ formation of melanoidin depends on oxidation, for Samuely 

 ^chp^nat access of oxygen is as necessary as in the case of 'tyro- 

 sinase,' 10 and this explains why v. Fiirth obtained xanthomelanin. 

 (See p. 94, under 'Disintegration with Nitric Acid.') 



Hart 3 has pointed out that the formation of humin introduces a 

 considerable uncertainty, when lysin and ammonia have to be estimated, 

 as humin is formed to a much greater extent when sulphuric acid alone 

 is used than when sodium chloride is added as well : in the latter case 

 much more lysin and ammonia are obtained. Differences between the 

 action of hydrochloric and of sulphuric acid were also observed by 

 Hoppe-Seyler. Langstein explains the absence of glucosamin amongst 

 the dissociation-products of egg-white resulting from the action of 

 strong hydrochloric acid, by assuming that the glucosamin unites with 



1 0. v. Fiirth and H. Schneider, Hofmeister's Beitrage, 1. 229 (1901). 



2 V. Ducceschi, cited from Samuely, ibid. 2. 355 (1902). 



3 E. Hart, Zeitschr. f. physiol. Ghem. 33. 347 (1901). 



4 M. Nencki, Ber. d. deutsch. chem. Ges. 28. I. 560 (1895). 



5 F. G. Hopkins and S. W. Cole, Journ. of Physiology, 27- 418 (1901). 



6 F. Hofmeister, Ergebnisse der Physiol. 1. 768 (1902). 



7 Samuely, Hofmeister's Beitrage, 2. 355 (1902). 



8 0. Schmiedeberg, Arch.f. experiment. Path. u. Pharmakol. 43. 57 (1899). 



9 F. Miiller, Zeitschr. f. physiol. Chem. 38. 279 (1903). 

 10 M. Gonnermann, Pfluger's Arch. 82. 289 (1900). 



