596 METABOLISM 



and the nitrogen equilibrium, i Page 571.) It has been shown that suc- 

 cess in maintaining nutritional efficiency depends partly on the nature 

 of the process used for digesting the protein, and partly on the presence 

 or absence of carbohydrate in the digestion mixture. It was found 

 that acid hydrolytic products failed to maintain equilibrium, and it was 

 believed that this "was owing to the fact that the acid had more completely 

 disrupted the protein molecule, and had left no polypeptides, which, it 

 was imagined, remained intad during enzyme action and were essential 

 for proper protein metabolism. This view has now been considerably 

 altered, since it has been shown that the acid actually destroys certain 

 amino acids which the enzyme leaves intact. The amino acid particu- 

 larly concerned is tryptophane. Thus, when animals were fed with three 

 diets, consisting of (1) fully digested casein, (2) fully digested casein 

 from which the tryptophane had been removed, and (3) fully digested 

 casein from which the tryptophane had been removed and then the 

 proper amount of pure tryptophane added, it was found that nitrogen 

 equilibrium could not be maintained on the second diet, which contained 

 no tryptophane, whereas it was maintained on the first and third diets. 

 That this explanation is correct is further supported by the fact that, 

 if the protein is only partly digested by acid — that is, not digested 

 enough so as to break up all the tryptophane — it can efficiently maintain 

 nitrogen equilibrium. 



Regarding the necessity for carbohydrates, it is possible that under 

 certain conditions these may be produced from the protein itself. At 

 least, it has been possible for Abderhalden, who has done a large share 

 of this work, to maintain an animal in nitrogen equilibrium with a diet 

 of digestion products and fat containing no carbohydrate. 



These results obtained in different classes of animals have also been 

 confirmed for the human subject. A boy suffering from a stricture of the 

 esophagus, when fed by rectum for fifteen days with digestion products 

 resulting from the action of trypsin and erepsin on flesh, gave evidence 

 of nitrogen retention. 



Concerning the second type of evidence, many investigators attempted 

 to separate the amino acids themselves from the blood, particularly dur- 

 ing the digestion of a large amount of protein, but the results were at 

 first entirely negative because of the lack of methods that were suffi- 

 ciently delicate to make it possible to detect the slight increase that 

 could be expected even when a maximum absorption of nitrogen had 

 occurred. The very large flow of blood through the portal vein causes 

 such extensive dilution of any substances added to it that the concentra- 

 tion of the substance in an isolated sample of the blood can be only 

 trivial. 



