THE PROTEINS 15 



however, that the structure of tyrosine was positively deter- 

 mined. Tyrosine is absent from the gelatine molecule. In 

 acute yellow atrophy of the liver and in phosphorus poisoning 

 it is claimed that tyrosine may be present as a urinary con- 

 stituent. 

 Serine /3-hydroxy-a-amino-propionic acid. 



Cramer found serine among the decomposition products of 

 sericin (silk gelatin), and it was not obtained again until 1902 

 when Fischer isolated it from various proteins as a result of 

 hydrolysis. He also definitely established its structure. 



Cystine. di-cysteine or di-/3-thio-a-amino-propionic acid. 

 HOOC.CH.NH 2 .CH 2 .S S.CH 2 .CH.NH 2 .COOH. 



Cystine has been known since 1810 having been first de- 

 scribed by Wollaston who separated it from a urinary calculus 

 and called it cystic oxide. From that period, although cystine 

 was repeatedly isolated from various organs of the body, as 

 the liver and kidney, its presence as a regular decomposition 

 product of protein was not established until 1899 when 

 K. A. H. Morner obtained it by a hydrolysis of horn. Bau- 

 mann demonstrated the relationship of cysteine to cystine and 

 thus revealed the structure of cystine. Cysteine and cystine 

 bear the same relation to one another as does a mercaptan to 

 a disulphide, thus, 



CH 2 .SH CH 2 - S - - S ' - CH 2 



CH.NH 2 CH.NH 2 * CH.NHa 



COOH COOH COOH 



cysteine cystine 



Cystine is of considerable importance in metabolism inasmuch 

 as it is the only known sulphur-containing amino acid in the 

 protein molecule. 



