DIGESTION AND ABSORPTION OF PROTEIN 1 1 



after absorption, could be used for rebuilding purposes, and which there- 

 fore might be regarded as a stage towards protein regeneration. They 

 held that their view was strengthened by the fact that the increase of 

 the excretion of nitrogen after the administration of leucine did not cor- 

 respond with the amount of leucine fed, and further that an analogy 

 existed in plant physiology where it was demonstrated that the de- 

 composition products of protein, namely asparagine, leucine and tyro- 

 sine could be regenerated into protein when carbohydrate was also 

 present. They suggested that a similar combination might occur in 

 the liver of animals. Kutscher and Seemann (236) as the result of their 

 experiments stated that they considered the above hypothesis very 

 plausible, and concluded that the appearance of leucine and tyrosine, 

 which they found, was the normal condition, and that these crystalline 

 substances must be looked on as constituents, which, after absorption, 

 would be utilized for the formation of tissue protein. They were unable 

 to prove, however, that the hypothesis put forward by Salkowski and 

 Leu be was correct, as a series of experiments in which they performed 

 a variety of Eck's fistula (cutting the liver out of the circulation), using 

 a glass cannula for connexion between the portal vein and the vena 

 cava, did not give decisive results. They further ligatured off the 

 lower part of the vena cava, kidney vessels, carotids and subclavian, so 

 that the blood circulated only through the intestine, heart and lungs. 

 They were unable to detect amino acids in the blood even at the 

 height of digestion. This question of the presence of digestion pro- 

 ducts in the blood will be dealt with more fully later, when the fate of 

 the absorbed material is discussed (see p. 16). Bunge in his textbook 

 argued on teleological grounds against the conversion of any consider- 

 able amount of protein into amino acids. He held that if any such 

 conversion took place it must be small, as the dissipation of chemical 

 energy firstly in the decomposition and secondly in the necessary build- 

 ing up processes would be considerable and quite contrary to nature. 

 These teleological arguments of Bunge can now be shown to be false, 

 as the loss of energy in the conversion of protein into digestion products 

 is remarkably small, as Rubner's calorimetric estimation of Loewi's 

 digestion products proved. Even in plants, in the process of utilization 

 of the stored protein, the formation of crystalline decomposition pro- 

 ducts must take place before resynthesis is possible. 



It would appear, then, that the material absorbed is taken up for 

 the most part in the form of abiuret products, of which the greater part 

 consists of simple amino acids. 



