248 Biological Chemistry. 



of diet, the chemical examination of proteins forms a 

 subject of supreme biological importance, to which much 

 attention has been devoted in recent years. A few 

 examples will suffice to illustrate this statement. It has 

 been long known that when gelatin is the sole protein 

 administered to an animal as food, " nitrogenous equili- 

 brium " cannot be maintained. This means that the 

 nitrogenous food supplied will not maintain the body 

 weight of the animal, which has consequently to draw 

 upon the protein of its tissues to maintain its condition. 

 Under these circumstances, more nitrogen is excreted by 

 the animal than is ingested. Now it is known that the 

 hydrolysis products of gelatin are deficient in certain 

 aromatic constituents, and it is probable that the protein 

 should yield these on hydrolysis in order that it may 

 serve as completely satisfactory nitrogenous foodstuff. 

 Again, it has also been shown that mice cannot be main- 

 tained in nitrogenous equilibrium when the sole source of 

 nitrogenous food is zein, the protein derived from maize. 

 It is known that tryptophan is not yielded by this protein 

 on hydrolysis, and this result indicates that a protein to 

 form a completely satisfactory foodstuff should yield some 

 of this amino-acid on hydrolysis. Furthermore, proteins 

 differ very markedly in their behaviour towards the 

 various proteoclastic ferments. The significance of this 

 statement will be made more obvious in the subsequent 

 discussion on ferment action. 



Part III (A). The Conjugated Proteins. 



THE NUCLEO-PKOTEINS. 



These substances are widely distributed in the cells of 

 the nucleus, of which they are said to form an important 

 constituent. The amount of nucleo-protein present in any 



