374 CHEMICAL DYNAMICS 



amino-acid radicals in a polypeptid is of great importance in 

 determining the susceptibility of the polypeptids to hydrolyis 

 by trypsin, but also the order in which the amino-acid groups 

 are combined and the nature of these groups themselves, apart 

 altogether from their optical configuration. Thus alanyl-glycin, 

 CH 3 .CH 2 NH 2 .CO.NH.CH 2 .COOH, is readily hydrolysed; while 

 the isomeric glycyl-alanin, NH 2 .CH 2 .CO.NH.CH.CH 3 .COOH, is 

 not. When alanin is the acyl radical, as in alanyl-glycin, alanyl- 

 alanin and alanyl-leucin, hydrolysis occurs, but it does not when 

 leucin, valin or aminobutyric acid are the acyl radicals (for in- 

 stance, leucyl-alanin, leucyl-glycin and leucyl-prolin). 



The number of amino-acid groups within the molecule is also 

 of great importance. Thus tetraglycyl-glycin is hydrolysed in 

 the presence of trypsin, while glycyl-glycin, diglycyl-glycin and 

 triglycyl-glycin are not; other things being equal it is evident 

 that mere length of the peptid chain per se confers upon it greater 

 susceptibility to attack by this enzyme. 



It will be observed, on surveying the above results, that a 

 number of polypeptids are not hydrolysable by pure trypsin 

 which nevertheless are built up out of naturally occurring radicals. 

 Particularly resistant to hydrolysis are those peptids which con- 

 tain a preponderance of glycin, phenylalanin or pyrrolidin- 

 carboxylic acid.* In the light of the theory presented above, 

 therefore, the question forces itself upon us, how can such peptid 

 groups come to form, as they do, constituent parts of the mole- 

 cules of many proteins? Obviously, if they cannot be digested 

 by trypsin and our theory is valid then they cannot have been 

 introduced into the protein molecule through the agency of trypsin ; 

 they must have been introduced through the agency of some 

 other enzyme or enzymes. 



In this connection the very significant fact will be observed 

 that whereas Fischer and Bergell succeeded in hydrolysing leucyl- 

 alanin through the agency of pancreas-extract, Fischer and Abder- 

 halden were unable to secure the hydrolysis of this substance 

 through the agency of pure trypsin. This pointed to the exist- 

 ence in the extract of enzymes other than trypsin which, unlike 



* Emil Fischer and E. Abderhalden (10). The connection between these 

 results and the theory of Kuhne that the proteins are built up of hemi- and 

 anti-groups, respectively, hydrolysable and non-hydrolysable by pepsin and 

 trypsin is sufficiently evident to require no further comment here. 



