72 PHYSIOLOGICAL CHEMISTRY. 



other protein test. The aromatic groups present in tyrosine, phenyl- 

 alanine and tryptophane cause proteins to yield a positive xantho- 

 proteic reaction. In severe cases of typhoid fever and smallpox, 



FIG. 23. 



TYROSINE. 



in acute yellow atrophy of the liver, and in acute phosphorus poison- 

 ing, tyrosine has been found in the urine. Tyrosine crystals are 

 shown in Fig. 23, above. 



Cystine, C 6 H 12 O 4 N 2 S 2 . Friedmann has recently shown cystine 

 to be the disulphide of a-amino-ft-thiolactic acid 1 and to possess the 

 following structural formula: 



CH 2 -S-S-CH 2 

 CH-NH 2 CH-NH 2 

 COOH COOH. 



Cystine is the principal sulphur-containing body obtained from 

 the decomposition of protein substances. It is obtained in greatest 

 amount as a decomposition product of such keratin-containing tis- 

 sues as horn, hoof and hair. Cystine occurs in small amount in 

 normal urine and is greatly increased in quantity under certain 

 pathological conditions. It crystallizes in thin, colorless hexagonal 

 plates which are shown in Fig. 24, p. 73. Cystine is very slightly 

 soluble in water but its salts, with both bases and acids, are readily 

 soluble in water. It is Isevo-rotatory. 



1 Also called a-diamino-/3-dithio-dilactylic acid. 



