76 PHYSIOLOGICAL CHEMISTRY 



26. Cystine is very slightly soluble in water but its salts, with both 

 bases and acids, are readily soluble in water. It is levo-rotatory. 



It was formerly claimed that cystine occurred in two forms, i.e., 

 stone-cystine and protein-cystine, and that these two forms are distinct 

 in their properties. This view is incorrect. 



For the preparation of cystine from wool or hair see page 86. 



For a discussion of cystine sediments in urine see Chapter XXV. 



Tryptophane, C 8 H 6 N-CH2-CH(NH2)-COOH Recently Ellinger 

 and Flamand have shown that tryptophane possesses the following 

 formula: 



v _ GCH 2 -CH(NH 2 >COOH 



CH 

 NH 



It is therefore (3-indolyl-a-amino-propionic acid. Tryptophane is the 

 mother-substance oj indole, skatole-, skatolyl acetic acid and skatolyl 

 carboxylic acid, all of which are formed as secondary decomposition 

 products of proteins (see Chapter XIII on Putrefaction Products). 

 Its presence in protein substances may be shown by means of the 

 Hopkins-Cole reaction (see page 98). It may be detected in a tryptic 

 digestion mixture through its property of giving a violet color reaction 

 with bromine water. 1 Tryptophane is yielded by nearly all proteins, 

 but has been shown to be entirely absent from zein, the prolamin (alcohol- 

 soluble protein) of maize, and also from gelatin. 



According to Osborne and Mendel, 2 tryptophane is present in maxi- 

 mum amount in lactalbumin. Upon being heated to 285C. trypto- 

 phane decomposes with the evolution of gas. 



Histidine, CsHs^-CHs-CHtN^-COOH.-- Histidine is a-amino-p- 

 imidazolyl-propionic acid or p-imidazolyl-alanine with the following 

 structural formula: 



H NH 2 



I I 

 HC- -C C C COOH. 



I I 

 H H 

 HN N 



\/ 1 . 



CH f 



The histidine obtained from proteins is levo-rotatory. It has been 

 obtained from all the proteins thus far examined, the majority of them 

 yielding about 2.5 per cent of the amino acid. However^ about n per 



1 Kurajeff: Zeit. physiol. Chem., 36, 501, 1898-99. 



2 Osborne and Mendel: Jour. Biol. Chem., 20, 357, 1915. v 



