294 DIETlCnc Dia-'ICIENCV. 



(b) A deficiency which is not attrilnitable either to the 

 protein fed or to possible lack of suitable mineral com- 

 ponents, since the simi)le substitution of lard by butter 

 sufficed to correct the deficiency. 



Deficiency in the Kind or Quality of Protein. — C"arl X'oit, 

 Panum and Oerum, and others, long ago recognised that a 

 marked difference existed between gelatine and other proteins, 

 and had shown that gelatine was incapable of functioning as sole 

 protein in a diet, although it could to a large extent replace or 

 " spare " the catabolism of other |)roteins. Munk, for exam])le, 

 found that with dogs on a mixed diet not (|uite four-fifths of the 

 total protein in the ration could be re])laced by gelatine. 



To understand such facts, it is necessary to consider the 

 constitution of the ])rotein molecule. As already mentioned, the 

 proteins, however complex in themselves, may be broadlv re- 

 garded as compounds built up mainly of comparatively simple 

 amino-acids. Up to the present time at least i8 different amino- 

 acids have been definitely determined as protein units. These 

 are: glycine, alanine, valine, leucine, isoleucine. phenylalinine, 

 tyrosine, serine, cystine, asjmrtic acid, glutamic acid, arginine, 

 lysine, caseinic acid, histidine, proline, oxyproline, and trypto- 

 phane. 



Any given protein may contain all of these or few of them. 

 Thus nearly all are to be found in the proteins of blood, whereas 

 silk fibroin, the simplest protein known, is su]:)posed to contain 

 only three — glycine, alanine, and tyrosine. The simplest protein 

 molecule, however, contains a ver\- large number of amino-acid 

 units, even if they be few in kind. In some ])roteins one parti- 

 cular amino-acid may preponderate very largely over all others. 

 Thus, for instance, the salmine of fish s]:)erm contains over 80 

 per cent, of arginine, and is therefore built up chiefly of this one 

 amino-acid, although others, such as valine, serine, and ])roline, 

 are also present. The proteins of leguminous seeds, such as 

 beans and peas, most nearly approach muscle protein in composi- 

 tion, and this is jM^obably the explanation of their high value as 

 food-stuffs. 



To return to the gelatine feeding experiments. Investiga- 

 tion of this particular protein has shown that three important 

 amino-acids are missing — tyrosine, cystine, and tryptophane. Tf 

 these three units be isolated from other sources and added to a 

 diet containing gelatine as sole protein, it is now found that life 

 and tissue-equilibriiuu can be maintainetl — for short periods, at 

 least. Thus Kaufmann brought himself into nitrogenous equili- 

 brium on a diet in which caseinogen was the protein contained, 

 and then found that if he re])laced the caseinogen nitrogen by 

 nitrogen from gelatine 93 per cent., tyrosine 4 i)er cent., cvstine 

 2 per cent., and tryptophane i ])er cent., the mixtiu-e ])racticallv 

 sufficed to prevent loss of body protein. 



The experiments of TTo]ikins and W'illcock' in IQ07 with 

 sein, one of the proteins contained in maize, yielded evidence 



