PROTEIN METABOLISM 763 



ammo-acid were formed in the body, one might expect to find it without 

 difficulty in the urine. Knoop found that the administration of either 

 phenyl-a-keto-butyric acid or phenyl-a-oxybutyric acid led to the excretion 

 of the corresponding amino-acid in the urine. Since keto-acids occur as the 

 ordinary products of the breakdown of amino-acids and also as the inter- 

 mediate products of oxidation of oxy-acids, e.g. lactic acid, it is evident that 

 the animal body can assimilate ammonia and form amino-acids, provided 

 only that it is supplied with the proper non-nitrogenous acids. These latter 

 need not be derived from proteins at all, but, like lactic acid, be a result of 

 carbohydrate metabolism. Thus, if the fitting non-nitrogenous food be given 

 (e.g. oxy-fatty acids, or carbohydrates, from which these bodies may be 

 formed), part of the nitrogen set free by protein disintegration might be 

 recombined with the formation of amino-fatty acids without giving rise to 

 urea or appearing in any way in the nitrogen balance-sheet of the body. 

 This possibility enjoins the necessity of caution in interpreting the results of 

 metabolism experiments where the nitrogen excreted is taken to represent the 

 total protein metabolism of the body. The fate of the nitrogen does not, 

 however, matter much to the energy balance-sheet of the body, since so far 

 as regards energy the residue of the protein molecule or the amino-acid 

 molecule which is left behind after the process of deamination has taken place, 

 has lost only from one-fifth to one-tenth of the total energy of the original 

 molecule. This is shown in the following Table of the heat equivalents of 

 some of the amino-acids and their corresponding fatty and oxy-acids : 



Substance 



Leucine ..... 855 



Isobutylacetic acid . . . . 837 



Alanine ...... 389 



Propionic acid ..... 367 



Lactic acid ..... 329 



Pyruvic acid . . . . .not determined 



Even in the case of the smallest molecule the loss of energy attendant 

 on simple deamination and conversion into the corresponding oxy-acid only 

 amounts to about 20 per cent. We thus come to the conclusion that the 

 urea output in the urine after a protein meal tells us nothing whatever about 

 the fate of that part of the protein which contains 80 to 90 per cent, of the 

 total energy of the protein food. So far as concerns the output of energy, the 

 exogenous protein metabolism may be regarded as practically non-nitrogen- 

 ous. The rise in the rate of excretion of urea after a protein meal was 

 regarded both by Voit and Pfliiger as a sign that the cells of the body pre- 

 ferred to use proteins for all their requirements if this substance were 

 available. We see now that the big output of urea after a protein meal 

 affords no basis for this view, but is rather a sign that the. body has no need 

 for all the nitrogen contained in its food and that this must be got rid 

 of before the really valuable part, the energy-giving part of the protein 

 molecule, is admitted into the metabolic cycle of the cells. 



The important problem in the energy metabolism of protein is thus, not 



