THE NITROGEN COMPOUNDS 63 



82. Important properties of the proteins. The pre- 

 vious description of the various groups of proteins cannot 

 be understood to its fullest extent except by those who 

 have a good knowledge of the fundamentals of organic 

 chemistry. Nevertheless, the facts given serve to impress 

 the important chemical and physical properties which 

 these bodies possess, and point to the necessity of study- 

 ing them individually in their relation to foods and nutri- 

 tion. It is not rational to speak of protein as if the 

 term represents an individual entity, but the members 

 of this general class of compounds must be considered 

 by sub-classes at least, in discussing their place in 

 nutrition. 



A fact of importance is the varying constitution of 

 the protein molecule, and consequently the possible 

 variation in the nutritive function of the individual 

 proteins. 



83. The unlike constitution of the various proteins. 

 We have already seen that certain proteins are particu- 

 larized in part by containing phosphorus, others sulfur, 

 and others iron. A phosphorus-bearing protein may have, 

 and undoubtedly does have, a nutritive function that 

 cannot be exercised by an albumin not carrying 

 phosphorus. 



84. Cleavage products of the proteins. It is well 

 known that when proteins are submitted to the action 

 of acids, alkalies, and certain ferments (enzyms) they 

 break up into simpler compounds which we speak of as 

 cleavage products, chiefly amino-acids which are some- 

 times designated as the building-stones of the proteins. 

 It is very significant that the kinds, and especially the 

 proportions, of these products differ greatly with different 

 proteins. For instance, the purin bases, which certainly 



