675 



shield the nitrogenous constituents from utilization for energy. These ex- 

 periments also demonstrate the very considerable amounts of acid chiefly 

 lactic and acetic which appear concomitantly with the utilization of the 

 carbohydrate for energy. In this respect, the sugar-protein cultures con- 

 trast strikingly with the purely protein cultures, which become more or 

 less alkaline, due to the gradual accumulation of basic, nitrogenous waste 

 products arising from the combustion of the nitrogenous constituents of 

 the non-saccharine media. The nitrogenous waste products arising from 

 the utilization of protein for structural requirements and structural re- 

 placements, although relatively small in amount, were also clearly indi- 

 cated in these quantitative analytical studies. 



A word of explanation of the analogy between the metabolic waste 

 products of man and of bacteria will be required to indicate the parallelism 

 between human [multicellular] nitrogenous metabolism and bacterial 

 [unicellular] metabolism. 



It will be remembered that the principal end product of the physio- 

 logical metabolism of the proteins of the food and the tissues in man is 

 excreted through the kidneys into the urine as urea. Urea is derived, in 

 the last analysis, largely or chiefly from the deamination of amino acids : 

 the ammonia liberated is changed, principally in the liver, to urea. 



Ammonia has no energy value and whenever amino acids [protein or 

 protein derivatives] are used in the body for energy, for transformation 

 into glucose, or glycerin, or for storage as glycogen or fats, the ammonia 

 is discarded and changed to urea, unless a deficit of alkali leads to its 

 combination with acids that must be excreted through the kidneys* The 

 excretion of urea is markedly increased when a purely protein diet is 

 provided, and it is greatly reduced when the energy requirements of the 

 body are provided for by a carbohydrate regimen, supplying, however, 

 sufficient protein for structural and replacement needs. 



This urea may be regarded, therefore, as of exogenous and of en- 

 dogenous origin (Folin), the former being influenced largely by an 

 excess of protein above the structural requirements, the latter more spe- 

 cifically associated with structural changes in the tissues and organs. The 

 exogenous urea is greatly influenced by the nature of the diet, being in- 

 creased when the energy requirement of the body is obtained chiefly by 

 the oxidization of proteins and reduced when the energy needs are de- 

 rived largely from dietary carbohydrate and fat. The endogenous urea 

 is less variable under proper dietary conditions. 



Similarly, bacteria deaminize amino acids prior to their utilization 

 of the remainder of the amino acid molecule for energy. Also, a small 

 amount of ammonia is apparently produced from the utilization of some 

 nitrogenqus substance for the structural needs of the bacterial cell. Bac- 

 teria have no livers; therefore, so far as is known, they do not excrete 

 urea (Kendall and Walker). Ammonia, which has an analogous origin 



