50 THE PHYSIOLOGY OF PROTEIN METABOLISM 



These results show that the loss is inconsiderable, even as low as 3 per 

 cent. What most workers lose sight of in considering this question 

 is the fact, that the actual demand for nitrogen by the body is not 

 high, and that the fundamental use of nitrogenous food stuffs is to repair 

 protein tissue waste and not to supply energy in the form of a com- 

 pound of a fatty acid and ammonia. Protein must be regarded simply 

 as a suitable and convenient compound for the introduction of a certain 

 amount of organized nitrogen into the tissues. Too much stress is laid 

 on the quantity of protein introduced. Protein is of importance to the 

 tissues not because of any inherent virtue in itself, but merely because 

 it contains within its molecule certain compounds of nitrogen more 

 or less ready for building purposes. 



Lower forms of life can build up the amino acids for their protein 

 formation from ammonium salts or nitrates present in their nutritive 

 mixtures, although they utilize the amino acids and flourish amazingly 

 on them if given the opportunity. Abderhalden and Rona (12) found 

 that Aspergillus Niger on a potassium nitrate medium could form its 

 protein which contained glycine, alanine, leucine, glutamic and aspartic 

 acids. Plants work on the same principle, but are even more adapt- 

 able, as certain members can utilize the atmospheric nitrogen when 

 required, a power which animals never possess, as Oppenheimer (308) 

 and Krogh (231) have so clearly shown. 



This deaminizing activity is fundamental in its nature, probably it 

 is essential for synthetic activity. The idea that such a splitting of the 

 protein molecule into a nitrogenous part and a non-nitrogenous part 

 takes place is by no means a new one. Voit many years ago accounted 

 for the comparatively rapid output of nitrogen and the comparative slow 

 output of carbon after a protein meal on the grounds that soon after 

 absorption there was a splitting of the protein molecule into a nitrogen 

 rich part which was rapidly dealt with, and a nitrogen poor part which 

 was more slowly utilized. 



