284 VETERINARY PHYSIOLOGY 



constituent of protoplasm. They are present to a much 

 smaller extent in plants than in animals. While 

 carbohydrates form the chief bulk of plant tissues, the 

 greater part of the animal body, apart from bones, visible fat, 

 and water, consists of proteins. 



Plant Proteins. — The vegetable proteins belong to three 

 groups of native proteins — (a) the globtdins ; (b) the 

 glutelins, requiring a dilute alkali for their solution ; and 

 (c) the gliadins, soluble in 70 to 80 per cent, alcohol. 

 They differ from the animal proteins in the proportion of 

 amino-acids Avhich they contain. Generally they are poorer 

 in leucin, but richer in glutamic acid (C5H9NO4) and in 

 arginin (p. 17). They have a higher percentage of nitrogen 

 than those of animal origin, so that the factors 6 "2 5 which 

 is used to multiply the amount of total nitrogen present to 

 obtain the amount of protein is too high in vegetable 

 protein. The true factor varies between 5 '5 and 6 for the 

 difterent substances. 



(6) Soluble or Non-Protein Nitrogen In addition to 



proteins, feeding-stuffs contain nitrogenous substances of a 

 simple chemical structure. These are all soluble in water, 

 and the term " soluble nitrogen " is advantageously used to 

 indicate the group. 



In most cases they are early stages in the building up of 

 proteins from nitrates or ammonium salts (diagram, p. 381). 

 In other cases they are parts of protein that have undergone 

 disintegration to a soluble form, e.g. amino-acids or peptides, 

 for transportation in the iiuids of the plant. 



Amino-acids, therefore, constitute the most abundant 

 group forming usually from 50 to 70 per cent, of the whole. 

 The amides (Appendix), asparagine and glutamine, are 

 usually present to the extent of 10 to 20 per cent. The 

 Avhole group of soluble nitrogenous substances are sometimes 

 loosely termed " amides," an unfortunate term liable to 

 create confusion. 



For purposes of analysis this group is differentiated from 

 proteins by the fact that they do not coagulate on heating 

 and are not precipitated by certain reagents which pre- 

 cipitate proteins, e.g. copper hydrate. 



