42 A TEXTBOOK OF PHYSIOLOGY 



and is supposed to have its origin from tyrosin. Animal pigments 

 such as melanin are formed from tyrosin by the action of a ferment, 

 tyrosinase. Kresol and phenol are formed out of phenyl alanin and 

 tyrosin in the colon by bacterial decomposition, and are excreted as 

 *' ethereal sulphates " in the urine. 



Tryptophan, C U H 12 N 2 2 , is Z-indol-amino-propionic acid; it con 

 tains a heterocyclic ring formed of the fusion of the benzene and 

 pyrrhol rings. Its formula is 



C.CH CH(NH,)COOH 



It is the mother-substance of indol and skatol. It is responsible for 

 the glyoxylic reaction (see later). 



Prolin is Z-a-pyrrolidin carboxylic acid, C 5 H 9 NO 2 , or graphically: 

 HjC - CH 2 



CH.COOH 



j 

 C 



NH 



It has been found in both animal and vegetable proteins. 



Oxyproline is oxy-pyrrolidin carboxylic acid, and was first obtained 

 by the hydrolysis of casein and gelatin. 



Arginin, lysin, and histidin, each contain six molecules of carbon, 

 and being of a basic nature, were formerly classified together as the 

 " hexone bases." 



Lysin is a-diamino-caproic acid; leucin is monaniino-caproic acid. 

 The formula of lysin is CH 2 (NH 2 ).(CH 2 ) 3 CH<(^^ H 



By putrefaction of lysin pentamethylendiamin (cadaverin) is produced , 

 while tctramethylendiamin (putrescin) is formed from ornithin. 



Arginin is a guanidine derivative of ornithin, which is a-S-di- 

 amino-valerianic acid, C 4 H 7 (NH 2 ) 2 COOH. It has basic properties, 

 and reacts strongly to litmus. Its formula is 



(NH)CNH.(CH2) 3 CH.COOH 



NIL, NH 2 



Histidin is not a true diarnino-acid ; it is a diazine derivative. It 

 is amino-imidazol-propionic acid, and has the formula 



