90 THE PROTEIN SUBSTANCES. 



(diamino-valeric acid) by the imide binding. This imide binding 

 is not, like the a-amino-acids, broken by trypsin ; but rather by an 

 enzyme, arginase, discovered by KOSSEL and DAKiN. 1 



If the proteins are considered as consisting chiefly of peptide4ike 

 complexes consisting of amino-acids united and containing also several 

 NH 2 groups at the ends, it is readily understood that the proteins are 

 amphoteric electrolytes, like the amino-acids, which form salts with bases 

 as well as with acids and undergo hydrolytic dissociation. As we also 

 accept the theory that the protein molecule contains a large number of 

 COOH as well as NH 2 groups, it follows that the proteins may be poly- 

 basic acids as well as polyacidic bases. The different proteins act in 

 this regard somewhat differently, thus the protamines are strongly basic 

 while casein behaves strikingly acid, and others take a certain mean 

 position. It is unfortunately impossible to base a classification of the 

 proteins upon this behavior, as well as upon chemical constitution. 

 The general properties, such as solubility and precipitation properties, 

 are too uncertain to aid us, and especially as in the investigations of 

 proteins we, as a rule, cannot decide whether we are dealing with a pure 

 or with a contaminated substance, namely, with mixtures. Experience 

 has shown that the solubility and precipitation property of the proteins 

 is strongly influenced by the presence of other bodies, and under such 

 circumstances a proper classification, as demanded by science, is 

 impossible. On the other hand, a classification is important, and as the 

 ones used up to the present time were based upon the solubilities and 

 precipitation properties, we give the following schematic summary of the 

 chief groups of protein bodies: 



I. Simple Proteins. 

 A. TRUE ALBUMINOUS BODIES OR PROTEIDS 



f Ser albumin, 



Albumins I , . . 



L Lactaloumin, and others. 



,-M u ,. f Fibrinogen, 



Globulins { . , * 



[ Serglobuhns, and others. 



Phosphoproteins, (Nucleoalbu- J Ovovitellin, 



mins) | Casein, and others. 



Coagulated proteins. 



Histones. 



Protamines. 



Zeitschr. f. physiol. Chem., 41. 



