THE PLACE OF PROTEIN IN NUTRITION. 35I 



tial for growth. Casein contains both in sufficient quantity for 

 maintenance and growth ; g-hadin of wheat contains tryptophane, 

 but very httle lysine, and thus will not produce growth, though 

 it suffices for niaintenaaice ; zein of maize contains neither trypto- 

 phane nor lysine, and therefore suffices for neither growth nor 

 maintenance. On the other hand, when zein is su])plemented 

 with tryptophane, maiiitenance is secured ; upon the fm-ther 

 addition of lysine, fairly rapid growth is secured. Even normal 

 growth has been obtained with zein when further supplemented 

 with " arginine." another amiiio-acid in which it is markedly 

 deficient. 



Feeding experiments have led us to the conclusion that the 

 so-called " protein requirement " of animals is simply a require- 

 ment for definite amino-acids, and varies for different proteins 

 in strict accordance with their amino-acid make-uj). The pro- 

 tein casein is comparatively deficient in the amino-acid " cys- 

 tine," and it is found experimentaMy that the minimum 

 requirement of casein supplemented with a little cystine is lower 

 than that of pure casein. Incidentally, this iiidicates the impor- 

 tance of cystine as a protein constituent. 



In order to obtain a better knowledge of the nutritive value 

 of the proteins of feeding-stufifs, three important lines of investi- 

 gation sitogest themselves. These are : first, a quantitative supply 

 of the amino-acids of feeding-stufi:"s ; second, feeding experi- 

 ments on the substitution of protein in rations by definite mix- 

 tures of the ami'no-acids occurring in feeding-stufl:'s ; and third, 

 feeding experiments to determine the nutritive value and relative 

 efficiency of the proteins of feeding-sttiffs. 



Determinations of the amino-acid content of isolated pro- 

 teins, such as have frequently been made in the past, are not of 

 great {practical imjiortance to problems of live-stock feeding — • 

 the feeding-stufifs themselves must be studied. It is impossible 

 as yet to isolate quantitatively in a pure state the proteins of any 

 vegetable tissue ; furthermore, the nutritive value of a feed so 

 far as the nitrogen is concerned rests upon its total amino-acid 

 content derived from both proteins and pre-existing free amino- 

 acids, amides, etc., and not solely upon the amino-acids combined 

 in proteiai form. Moreover, practical animal husbandry men 

 have always to deal with the natural mixtures of proteins, or, 

 better, amino-acids as they occur in our ordinary feeding-stuffs. 

 It seems quite evident, then, that it will be necessary to deter- 

 mine the total amino-acids of feeding-stufYs directly. It is more 

 than probable ifrom recent work carried out in America that 

 the method of Van Slyke for the analysis of proteins by deter- 

 mination of the chemical groups characteristic of the amino- 

 acids can be applied with a fair degree of success to the direct 

 analysis of the nitrogen of feeding-stufifs. The method of Van 

 Slyke is incomplete, since it does not give an insight into the 

 amotmt of each amino-acid present in the protein or the feeding- 

 stufif; but it is undoubtedly the best method available at present, 

 and tliere is little doubt that the results obtained from the u^e 



