OCCURRENCE OF PEPTIDS 15 



very large number of them are hydrolysed by the proteolytic 

 enzymes, pepsin, trypsin, etc., and in some cases, at all events, 

 it is certain that the hydrolysis takes place in stages, as it does 

 with the proteins and peptones (1). Further discussion of the 

 mode of hydrolysis of the polypeptids by enzymes will be found 

 in the chapters dealing with the chemical dynamics of protein 

 systems. 



5. The Occurrence of Peptids among the Products of Protein 

 Hydrolysis. It is highly probable that many of the bodies 

 which are known to the biochemist as "proteoses" " peptones, " 

 etc., will turn out to be identical with some of the polypeptids 

 already synthesized; this identity has, however, not yet been 

 proven. But several of the simpler peptids^and one tetrapeptid 

 have already been isolated from protein digests. These are to 

 be regarded as products of incomplete hydrolysis, intermediate 

 between the higher complexes and the simple amino-acids which 

 result from their complete decomposition. 



The presence of a dipeptid amongst the products of the hy- 

 drolysis of a protein of silk, silk-fibroin, was detected by Fischer 

 and Bergell in 1902 (16). Later, Fischer and Abderhalden (9) 

 showed that its anhydride, which they isolated, was the diketo- 

 piperazine of glycyl-alanin, and that it could not possibly have 

 arisen, by synthesis from glycin and alanin, during the process 

 of its isolation. At the same time they isolated another dipep- 

 tid from among the products of the incomplete hydrolysis of 

 silk-fibroin, namely glycyl-1-tyrosin, while from among the prod- 

 ucts of the incomplete hydrolysis of elastin glycyl-1-leucin was 

 obtained. Later, Fischer and Abderhalden (22), by partial 

 hydrolysis of the silk-fibroin, obtained a peptone-like substance, 

 precipitable by phosphotungstic acid, easily soluble in water, 

 insoluble in alcohol, precipitable from its aqueous solution by 

 saturation with ammonium sulphate or sodium chloride, which 

 proved to be a tetrapeptid, yielding, on hydrolysis, two molecules 

 of glycin, one of alanin and one of tyrosin. Its molecular weight, 

 determined by the cryoscopic method, was 350. The synthetic 

 pentapeptid, 1-leucyl-triglycyl-l-tyrosin, possesses very similar 

 properties, so that the peptones are not necessarily exceedingly 

 complex substances, nor is excessive complexity necessary in 

 order that substances of this type may be precipitable from their 

 aqueous solutions by saturation with ammonium sulphate. 



