PROTEINS 77 



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 87. 



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



Tryptophane, C 8 H 6 N-CH 2 -CH(NH 2 )-COOH. Recently Ellinger 

 and Flamand have shown that tryptophane possesses the following 



formula : 



C-CH 2 -CH(NH 2 )-COOH 



It is therefore $-indol-a-amino-propionic acid. Tyrophane is the 

 mother-substance ofindole, skatole, skatole acetic acid and skatole 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 mix- 

 ture 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 pro- 

 tein) 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. trytophane 

 decomposes with the evolution of gas. 



Histidine, C 3 H3N2-CH2-CH(NH 2 )-COOH. Histidine is a-amino- 

 ft-imidazol-propionic acid or &-imidazol-alanine with the following 

 structural formula: 



H NH 2 



I 

 HC -C C C COOH. 



I 



H H 

 HN N 



\/ 

 CH 



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. 



