io8 THE CHEMICAL CONSTITUTION OF THE PROTE INS 



phenylcysteine, which, on acetylation, was converted into bromophenyl- 

 mercapturic acid : 



It was first observed by Suter, in 1895, that a-thiolactic acid was 

 formed by the hydrolysis of proteins, and it was constantly obtained by 

 Friedmann. It was always regarded as a secondary product, but its 

 formation from cystine could not be explained, as cystine is a -thiopro- 

 pionic acid. 



In 1904 Morner found that pyruvic acid was a constant product of 

 hydrolysis of proteins, and that this compound gave a-thiolactic acid 

 with hydrogen sulphide. Its formation was thus explained, but it was 

 curious that hair, which is very rich in sulphur, gave less pyruvic acid 

 than horn, which is less rich, and thatcaseinogen, which contains very 

 little sulphur, also gave it. Morner therefore supposed that there was 

 another sulphur-containing compound in the protein molecule, which 

 supposition was strengthened by Neuberg and Mayer's statement that 

 stone cystine differed from protein cystine in many of its physical pro- 

 perties. Morner's subsequent work on the decomposition of stone 

 cystine, when he obtained a-thiolactic acid, ammonia and alanine, 

 helped to support this idea ; he regarded the alanine as formed from 

 cystine and the a-thiolactic acid from the isomeric a-thio-/3-amino- 

 propionic acid, both of which he supposed were present in the stone 

 cystine in equal quantities. 



Fische^ and Suzuki soon afterwards showed that Neuberg and 

 Mayer's stone cystine contained tyrosine, and that its different beha- 

 viour to protein cystine was due to the presence of this compound. 

 Rothera and also Abderhalden could find no difference between stone 

 cystine and protein cystine, and further, Gabriel's synthesis of iso- 

 cysteine or a-thio-y3-aminopropionic acid and isocystine, which had quite 

 different properties to cystine, though the two were much alike in many 

 of their reactions, proved that stone cystine and protein cystine must 

 be identical substances. Finally, it has been shown by Friedmann that 

 a-thiolactic acid, ammonia and alanine can be obtained from protein 

 cystine, which decomposition may take place, according to Gabriel, in 

 the following way : 



