794 PHYSIOLOGY 



these experiments showed was that the proteoses and peptones disappeared, i. e. were 

 converted into something which did not give the biuret test. The discovery of the 

 ferment erepsin by Cohnheim led this observer to repeat the experiments of Hofmeister 

 and Neumeister with a view to testing the conclusions drawn by these physiologists. 

 Cohnheim found that, although it was perfectly true that proteose and peptone disap- 

 peared when intestinal mucous membrane and peptone were placed together in the 

 presence of either blood or of Ringer's fluid, this disappearance was due, not to a regener- 

 ation of coagulable protein, but to the fact that the erepsin of the mucous membrane 

 carried the process of hydrolysis a step further, converting the proteoses and peptones 

 into the ultimate crystalline products of protein hydrolysis. Similar observations were 

 made by Kutscher and Seemann, who showed that at any time after a protein meal these 

 end-products, especially leucine, tyrosine, lysine, and arginine, were to be found in the 

 contents of the small intestine. A repetition of Salvioli's experiment by Cathcart and 

 Leathes deprived this also of much of its significance. It was found that the artificial 

 circulation, although sufficient to maintain the activity of the muscular wall of the 

 intestine, as evidenced by the peristaltic movements, was insufficient to keep the mucous 

 membrane alive. After one hour's experiment the loop contained a mass of epithelial 

 cells mixed with the products of the action of erepsin on the introduced peptone solution. 

 In no case was there any diminution in the amount of uncoagulable nitrogen, i. e. there 

 was no formation of coagulable protein, while the processes of absorption had been 

 brought by the desquamation entirely to a standstill. 



All the evidence shows that protein, however introduced, whether as 

 coagulated protein or as albumose and peptone, undergoes complete 

 hydrolysis either in the gut or in the wall of the gut before entering the 

 blood stream. It should thus be possible to feed an animal on a diet in 

 which the necessary protein had been replaced by the corresponding amount 

 of ultimate products of protein hydrolysis, i. e. by a mixture which would 

 give no biuret reaction. 



Such a possibility was formerly negatived on theoretical grounds by Kiihne and by 

 Bunge. It was thought by these observers either that the animal body lacked the 

 power of synthesis of proteins from these crystalline products (hydration products), or 

 that any complete hydration occurring in the intestine would involve such a loss of 

 energy to the body as to be unteleological. Neither of these theoretical objections is 

 justified in fact. We know from the researches of Fischer and others that, although the 

 different proteins in our food present a marvellous qualitative similitude, in that all of 

 them yield on hydrolysis the same kinds of amino-acids, there are great differences in 

 the relative amounts of these amino-acids contained in different proteins. Thus in 

 gelatin, glycine is contained in considerable quantities, but is absent in many of the other 

 proteins. Caseinogen is distinguished by the large amount of leucine that it yields, 

 while gliadin, the chief protein of wheat flour, contains very large amounts of glutamic 

 acid. It is difficult to imagine how, for instance, muscle protein could be formed from 

 wheat protein, a process continually occurring in the growing animal, unless we assumed 

 that the protein molecule is first entirely taken to pieces, and that its constituent mole- 

 cules are then selected by the growing cells of the body and built up in the order and 

 proportions which are characteristic of muscle protein. Moreover, when we measure 

 the amount of energy change involved in the hydrolysis of the proteins, we find it is 

 relatively small. There is not a loss of 5 per cent, of the total energy available i. e. i he 

 heat of combustion of the products of pancreatic digestion would differ from that of 

 the original protein submitted to digestion by less than f> per (cut. The energy of the 

 protein as evolved in the body lies, not in the coupling of the amino-acids with one 

 another, or indeed in the coupling of the nitrogen to the carbon but, like that of the 

 other foodstuffs, in the carbon itself, and is derived from the combust ion of the carbon 

 of the molecule under the influence of the oxidising processes of the body into carbon 

 dioxide. 



