RELATION" TO ENERGY SUPPLY. 



85 



governing the protein metabolism can be reached without taking 

 into consideration the energy relations. 



Ordinarily, the nonnitrogenous nutrients of the feed constitute 

 the principal source of this energy. The proteins, however, or at 

 least the cleavage products of their digestion or transformation, 

 readily undergo a process of deamidization by which their nitrogen 

 is split off and excreted, leaving a nonnitrogenous residue wmich is 

 available as a source of energy. It is evident, then, that the rela- 

 tive abundance or scarcity of the supply of nonnitrogenous nutrients 

 to the cells of the body may profoundly modify the extent and 

 character of the protein metabolism and consequently the magnitude 

 of the protein requirement. 



One instance of this effect is the so-called premortal rise of the 

 protein katabolism of the fasting animal when the store of body 

 fat is reduced below a certain level. (Compare pp. 12-13.) Here 

 the relative deficiency of fuel material in the circulation causes an 

 increased breaking down of the cell protein, presumably by hydro- 

 lytic cleavage and subsequent deamidization, its nitrogen being 

 gotten rid of as urea, etc., and the nonnitrogenous residue serving as 

 a source of energy in place of the lacking fat. 



A precisely similar thing occurs when the nonnitrogenous nutri- 

 ents in the feed are relatively deficient and is especially striking in 

 their entire absence. It was pointed out on pages 75-78 that the 

 protein katabolism during fasting is at least an approximate measure 

 of the minimum protein requirement of the body, and that if this 

 amount, or perhaps even less, be supplied in the feed, along with 

 an abundance of nonnitrogenous material, the stock of protein in 

 the body may be maintained. But if the experiment be made of 

 supplying the minimum of protein without nonnitrogenous matter 

 a very different result is obtained. 



Tlins in one such experiment by E. Voit and Korkunoff, 1 a fasting dog 

 excreted about 4 grams of nitrogen per day, equivalent, of course, to a daily 

 loss of about 24 grams of body protein, while in addition to this it must 

 have been oxidizing considerable body fat. When, however, it was fed slightly 

 more than 24 grams of protein 2 (4.1 grams nitrogen), with no other feed, 

 its nitrogen excretion jumped to 5.56 grams per day, so that it was still losing 

 daily 1.46 grams of nitrogen, equivalent to 8.76 grams of -protein. Instead of 

 the entire amount of protein in the feed being applied to make good the losses 

 of protein tissue, over one-third of it was katabolized, its nitrogen appearing 

 in the urine and its nonnitrogenous residue doubtless being used as fuel 

 material. Protein rather more than equal to the 8.76 grams lost was then 

 added to the ration, but again the protein katabolism increased and the 

 body failed to maintain its stock of protein, and it was not until protein equal 

 to about three times the fasting katabolism was fed that equilibrium was 

 reached. The details of the experiments are shown in the following table, 



1 Zeitschrift fur Biologie, vol. 32, p. 67. 



2 In the form of lean meat from which the extractives had been removed by treatment 

 with water. 



