GENERAL METHODS HISTORY OF PROTEIN FOOD. 895 



serves for purposes of energy-supply, just as the non-nitrogenous 

 foodstuffs might do. We may suppose that it is the excess amino- 

 acids not used in tissue construction that undergo this fate. 



A third possibility in regard to the fate of the amino-acids, or rather the 

 absorbed products of protein digestion, has been discussed more or less at 

 times in physiology. In addition to the portion that is applied to the repair 

 or construction of tissue, it has been held that another portion may be syn- 

 thesized to a form of non-organized or non-living protein held as a sort of 

 storage supply in the liquids or tissues of the body. It has been supposed 

 that this material is called upon first in fasting or starvation, and that it 

 constitutes an easily used form of protein food of limited amount which is 

 designated as circulating or storage protein. Our knowledge in regard to this 

 material is quite mdefiinite at present. Certainlv none of the known proteins 

 of blood or lymph seem to discharge this function. 



It was supposed formerly that the process of deaminization 

 of the excess of the amino-bodies is accomphshed mainly in the 

 liver, but this view has been brought into question.* That the 

 process takes place in the body is beyond doubt, and it is likely 

 that the liver shares in this activity even if it is not the chief tissue 

 concerned. That the body is able to build up its own protein 

 from a mixture of amino-acids, such as is produced in a complete 

 digestive hydrolysis of protein, has been demonstrated beyond 

 doubt by feeding experiments in which the nitrogenous food was 

 all supplied in the form of such a mixture. The first experiments 

 of this kind were made by Loewi.f He fed dogs on a diet con- 

 sisting of protein which had been submitted previously to a pro- 

 longed pancreatic digestion until it was completely hydrolyzed, 

 together with sufficient fats and carbohydrates. On this diet the 

 animal was maintained in nitrogen-equiUbrium. This experiment 

 has been verified and extended by others on man as well as upon 

 dogs, and indeed Abderhalden and Rona report that they have 

 been able to keep a dog not only in nitrogen-equilibrium but with 

 a plus balance of nitrogen when fed on the split products of meat 

 alone, without addition of fats or carbohydrates. This last 

 experiment would indicate that the amino-bodies not only give to 

 the body material from which it can reconstruct its own protein, 

 but they furnish also a usable source of energy for the body 

 needs, a result which we can understand on the hypothesis men- 

 tioned above, according to which the amino-acids, after removal 

 of the NH2 group, furnish an organic acid residue capable of further 

 oxidation or of synthesis to fats or carbohydrates. 



We may accept as a clear result of modern investigation that the body is 

 capable of building up its protein from such relatively simple substances as 

 the amino-acids, in fact the evidence goes to show that normally it is from these 



* See Fiske and Summer, "Journal of Biological Chemistry," 18, 285, 1914. 

 t For the literature consult Liithje, "Ergebnisse der Physiologie," vii, 

 1933, or Abderhalden, "Synthese der Zellbausteine," etc., 1912. 



