88 THE PROTEIN SUBSTANCES. 



Z-leucine. On combination with diglycylglycine the tetrapeptide /-leucyl- 

 diglycyl glycine is produced. WALDEN'S reversion does not take place 

 with all amino-acids ; other methods can also be used to obtain the 

 optical antipodes, such as the preparation of the alkaloidal salts of the 

 benzoyl or formyl combinations of the racemic amino-acids. 



A comparison of the artificially prepared polypeptides with the pro- 

 teins, and especially with the cleavage products of these last, the so-called 

 proteoses and peptones, is of great interest in several respects, especially 

 in connection with certain reactions. For instance there are several" 

 polypeptides which give the biuret reaction which is characteristic of 

 the proteins in general, and also several (polypeptides containing tyrosine), 

 which give MILLON'S reaction (see further on). The above-mentioned 

 octodecapeptide is precipitated by phosphotungstic acid, tannin and 

 ammonium sulphate; we also know tri- and pentapeptides containing 

 tyrosine, which are very similar in properties to the proteoses. 



The behavior of the polypeptides with proteolytic enzymes is of 

 especially great interest. As far as known no artificial polypeptide 

 is split by gastric juice. On the contrary there are many, as shown by 

 FISCHER and especially by ABDERHALDEN and his collaborators, that 

 are split by pancreatic juice or intestinal juice or by yeast-press juice, 

 and by many enzymes occurring in animal tissues and fluids. The 

 constitution of the polypeptides is of the greatest importance in this 

 connection. Thus, for example, FISCHER and ABDERHALDEN, 1 have found 

 with experiments with dog pancreatic juice that d-alanyl-d-alanine 

 and Z-leucyl-Z-leucine are split, but not d-alanyl-Z-alanine or Z-leucyl- 

 d-leucine. It sterns that only those polypeptides which are constructed 

 from amino-acids occurring in nature are hydrolyzable by enzymes. 

 If a racemic dipeptide is in part hydrolyzable then it is composed in part 

 of amino-acids occurring in nature. Only that part is split off and the 

 cleavage takes place asymmetrically. If for example, we take the two 

 racemic dipeptides from alanine and leucine mentioned on page 86, 

 we find that of the two racemic bodies only one (d-alanyl-Z-leucine + 1- 

 alanyl-d-leucine) , which contains the combination d-alanyl-Z-leucine, i.e., 

 the two optical forms found in nature, is in part hydrolyzed while the 

 other (d-alanyl-d-leucine -f Z-alanyl-Z-leucine) is not attacked. The hydrol- 

 ysis of the first racemic body is in this case referable, it seems, to the 

 combination d-alanyl-/-leucine. 



We will refer again, in Chapter IX, to this important and interesting 

 enzymotic cleavage of the polypeptides.' It is sufficient here to call atten- 



1 Zeitschr. f. physiol. Chem., 51. The numerous works of Abderhalden and collab- 

 orators cannot be here especially cited, but they may be found in Zeitschr. f. physioL 

 Chem. Bdd., 48, 49, 51, 52, 53, 54, 55, -56, 57. 



