486 DIGESTION. 



As has been indicated (Chapter III) only those pepticles are split 

 hydrolytically by enzymes which are formed from the ammo-acids 

 occurring in nature. It has also been shown that with this limited 

 action of trypsin or the pancreatic juice only a certain number of pep- 

 tides, di-, tri-, as well as tetrapeptides are split, while it is without action 

 upon a number of others. The structure of these plays an important 

 role, as, for example, alanyl-glycine, CH 3 .CH(NH 2 ).CO.NH.CH 2 .COOH, 

 is split, while its isomer glycl-alanine, NH 2 .CH 2 .CO.NH.CH(CH 3 ).COOH, 

 is, on the contrary, not split. The nature of the amino-acids existing 

 in the peptide is also of importance. Those dipeptides which contain 

 alanine as acyl for example, alanylglycine, alanyl-alanine, and alanyl- 

 leucine A are readily hydrolyzed, while several dipeptides in which 

 a-aminobutyric acid or leucine functionates as acyl are very resistant. 

 The number of amino-acid groups is also of importance, as, for example, 

 triglycyl-glycine is not split, while tetraglycyl-glycine is. In those 

 peptides which are racemic bodies the hydrolysis takes place asym- 

 metrically, so that (see Chapter III) only one-half of the racemic body 

 is attacked, and those active amino-acids result as products which are 

 contained in the natural protein bodies. 



The behavior of the polypeptides with trypsin, or nearly related 

 enzymes, is of the very greatest interest and in many regards very 

 important. Thus in the polypeptides we have a means of determining 

 the kind of enzyme, whether it belongs to the pepsin, trypsin, or erepsin 

 group. We know of no polypeptide which is split by pepsin; trypsin 

 splits only certain polypeptides, but not others, while the erepsin on the 

 contrary seems to split all polypeptides, which are composed of amino- 

 acids, occurring in nature. By the aid of the polypeptide reaction ABDSR- 

 HALDEN and co-workers have also been able to show that the trypsin-like 

 enzyme, occurring in the blood plasma, is not identical with trypsin 

 because it does not attack glycyl-Z-tyrosine, which is split by trypsin. 



By observations upon the optical behavior of a solution of a certain 

 optically active polypeptide on the addition of enzyme we can follow the 

 decomposition step by step, as many polypeptides have a stronger rota- 

 tion than their cleavage products, or as a change in the direction of the 

 rotation occurs during decomposition . In this manner we can determine 

 the enzyme hydrolysis in a certain time and in this way the interesting 

 investigations of ABDERHALDEN and GIGON l have shown that the hydrolysis 

 of glycyl-Z-tyrosine with yeast press-fluid is retarded by the addition ot 

 optically active amino-acids occurring in nature, while the optical anti- 

 podes of the same amino-acids is without action or at least have only 

 a very weak retarding action. An analogous condition in digestion 



1 Zeitschr. f. physiol., Chem. 53. 



