PROTEINS 171 



different physiological functions which the proteins have to per- 

 form in the animal body: thus, tryptophane is absolutely essential 

 to the maintenance of life, but does not promote growth; lysine, 

 on the other hand, definitely promotes growth, so that animals 

 which have been maintained without any increase in weight for 

 many months immediately begin to grow when furnished with a 

 diet in which lysine is a constituent; while arginine seems to be 

 definitely associated with the reproductive function; and cystine, 

 with the growth of hair, feathers, etc. It is not known whether 

 there is any similar relation of amino-acids to the functions of 

 different proteins in plant metabolism. 



The separation of the individual amino-acids from the mix- 

 ture which results from the hydrolysis of any given protein is a 

 long and tedious process and, at best, yields only moderately 

 satisfactory results. For that reason, it has recently been almost 

 entirely abandoned in favor of the separation devised by Van 

 Slyke, which divides the total nitrogenous matter in the mixture 

 resulting from the hydrolysis of a protein into the following groups ; 

 ammonia N, humin (or melanin) N, cystine N, arginine N, histidine 

 N, lysine N, amino N of the filtrate, and non-amino N of the fil- 

 trate. These groups can be conveniently and fairly accurately 

 separated out of the hydrolysis mixture, by means of various 

 precipitating agents, and the quantity of N in the several precip- 

 itates determined by the usual Kjeldahl method. The actual 

 process for these separations need not be discussed here, as it is 

 given in detail in all standard text-books dealing with the methods 

 of biochemical analysis. The distribution of the nitrogen in any 

 given protein into these various groups is characteristic for that 

 particular protein, and the process serves both as a means of 

 identification of individual proteins and a method for tracing their 

 changes through various vital, or biochemical, transformations. 



GENERAL PROPERTIES OF THE PROTEINS 



Individual proteins differ slightly in their characteristics, 

 but in general they are all alike in the following physical and chem- 

 ical properties.* 



* Since the proteins are essentially colloidal in nature, many of the terms 

 used in the discussions of their properties, and these properties themselves, 

 will be better understood after the chapter dealing with the colloidal condi- 



