THE CHEMICAL CONSTITUTION OF THE UNITS 109 



CH 2 \^ CH 3 



I 

 CH . NH 2 



^ COOH 



CH 2 SH " 



CH.NH a 



CH S 



H 2 ! 

 > CH . SH 



OOH COOH 



Thus, the work of Friedmann on the constitution of cystine, its synthesis 

 by Erlenmeyer jun. and by Fischer, definitely show that it has the 

 composition 



HOOC , CH(NH 2 ) . CH 2 S S CH 2 . CH(NH 2 ) . COOH. 



The proof that bromophenylmercapturic acid is derived from cysteine, 

 the formation of a-thiolactic acid from cystine derived either from 

 stones or proteins, and the identity of protein cystine with stone cystine 

 show that cystine is the sulphur-containing compound in the protein 

 molecule. 



