690 PHYSIOLOGY 



off from the protein of the food, partly from the disintegrated proteins, 

 which have formed part of the living fabric of the cells. 



Folin has brought forward a number of facts which point not only to a 

 twofold origin of the nitrogen of the urine but also to a qualitative difference 

 in the two orders of protein metabolism. Whereas in the urine of man on 

 a normal diet the urea nitrogen forms 85 per cent, or more of the total 

 nitrogen, a reduction of the protein ration to the minimum necessary to 

 meet the nutritional requirements of the body causes not only an absolute 

 diminution of the urea but a large relative diminution when compared with 

 the other constituents of the urine, such as creatinin. He concludes there- 

 fore that the nitrogenous end-products of nutritional metabolism are different 

 from those of the energy metabolism. There is also a difference in the 

 time-relations of the two orders of metabolism. Whereas the nitrogen, 

 which furnishes no energy to the body, is rapidly eliminated when protein 

 is being utilised for the supply of energy to the body, the occurrence of 

 increased tissue waste causes a rise of nitrogenous excretion, which comes 

 on slowly, often after the lapse of a day, and may last two or three days. 

 The process of protoplasmic disintegration appears therefore to occur in a 

 series of stages, which occupy a considerable time and end in the production 

 of substances qualitatively distinct from that substance, urea, which is the 

 almost exclusive nitrogenous end-product of the energy metabolism of 

 protein. 



ACTION OF THE PRODUCTS OF PROTEIN DIGESTION 



Proteoses, Peptones and Amino-acids. In the digestion of the 

 naturally occurring proteins, the first products of hydration consist of a 

 mixture of substances known as proteoses and peptones. In the further 

 processes of digestion, under the influence of the ferments of the pancreas 

 and small intestine, these substances are converted into the amino-acids 

 which we have learnt to regard as the proximate constituents of the protein 

 molecule. Many experiments have been performed in order to determine 

 the nutritive value of these digestive products. In nearly all cases it has 

 been found that the meat in the diet of an animal can be replaced by a 

 corresponding quantity of the products of digestion of the same meat 

 without interfering with the nitrogenous equilibrium of the animal, and 

 Loewi and others have shown that the same result may be attained by 

 feeding an animal on the products of pancreatic digestion of protein, i. e. a 

 mixture consisting almost entirely of amino-acids. Since the proteins of 

 the body differ in their composition from the majority of the proteins of 

 the food, it is evident that each food-protein molecule has to be entirely 

 disintegrated and reconstructed before it can take its place in the body fabric ; 

 and it is therefore only natural that, so far as metabolism is concerned, the 

 results should be identical whether we feed the animal with the ordinary 

 food-protein or with the products of its metabolism. This latter mode of 

 feeding cannot however be regarded as presenting any advantages. Under 

 normal circumstances the food molecules are broken down by degrees. Their 



