DIGESTION AND ABSORPTION OF PROTEIN 5 



particular substances when the organism is in a pathological condition. 

 The products excreted in the urine in cases of alkaptonuria and cysti- 

 nuria, 1 or in such a disease as acute yellow atrophy of the liver, have 

 been examined, both after normal feeding and after the addition of 

 different amino acids, etc., to the food. The course of metabolism has 

 also been studied in pathological conditions artificially produced, as, for 

 example, after phosphorus poisoning. 



The discussion of the question of metabolism is opened with a 

 short preliminary rsum6 of the present position as regards the course 

 of gastro-intestinal digestion. This is inserted, as it is impossible to 

 obtain a proper understanding of the subsequent fate of the protein 

 without some knowledge of the degree of distintegration which it 

 undergoes in the digestive tract. 



Gastric Digestion. 



Within the last decade a very considerable advance has been made 

 in our knowledge of the course of digestion of protein. Actual diges- 

 tion does not commence until the protein material has reached the 

 stomach, where it is subjected to the action of gastric juice pepsin 

 and hydrochloric acid. A great deal of modern work has been done 

 on the extent and degree of digestion which takes place here. It may 

 be assumed that under normal conditions the digestion only proceeds 

 as far as the peptone stage, but that all protein does not of necessity 

 reach this stage of degradation. Lavroff (241), Langstein (240), and 

 others, however, have clearly demonstrated that if the gastric digestion 

 be allowed to go on long enough, in vitro at least, the breakdown of 

 protein can be carried on to the formation of abiuret 2 products. This 

 degree of digestion need not be considered here, for even under the 

 most favourable conditions in vitro very many weeks are required. 



It would seem, from many different observations, that proteoses 

 form by far the largest part of the products of peptic digestion. The 

 amount of proteose formed depends to some extent on the nature of 

 the protein which has undergone digestion. The following table from 

 an article by London shows this very clearly : 



1 In the present monograph this side of the question will not be specifically considered. 

 For further information Dr. Garrod's valuable book on " Inborn Errors of Metabolism " 

 may be consulted. 



a i.e. products of digestion which no longer give the biuret reaction. 



