264 Biological Chemistry. 



have many properties in common with proteoses and 

 peptones obtained by the digestion of natural proteins. 



The general properties of the proteins may be sum- 

 marized in the following way : Although they are sub- 

 stances which vary very greatly in physical properties, 

 they all yield on hydrolysis mixtures of amino-acids and 

 belong to a group of products which have been designated 

 " polypeptides." The individual proteins differ from one 

 another in the number and character of the amino-acids 

 which they yield on hydrolysis. Methods have been 

 elaborated for the quantitative estimation of such products, 

 which have been briefly described. A complete quantita- 

 tive estimation requires large amounts of material, but it 

 is possible, when working with relatively small amounts, 

 to determine the amount of ammonia, monoamino-acids, 

 and diamino-acids yielded by any given protein. A 

 method has also been described by means of which the 

 amounts of arginine, histidine, and lysine (the diamino- 

 acids) yielded by hydrolysis of a protein can be ascer- 

 tained with a fair degree of accuracy when only a small 

 amount of substance is available for investigation. By 

 analytical methods of this description, it is possible to 

 distinguish between proteins of various origin. It must be 

 remembered, however, that there are various difficulties in 

 determining whether a protein is a pure chemical entity. 

 It possesses no definite melting point, and other ordinary 

 criteria of purity are also wanting. The methods of investi- 

 gation are, therefore, more uncertain itian those usually 

 applied to crystalline organic substances, and the results 

 obtained are in many cases only approximately correct. 



Proteins occur naturally in combination with other 

 groups which are not of polypeptide clmracter. The most 

 important of such combinations are the nucleo-proteins, 

 which are proteins combined with nucleic acid. There 



