METABOLISM 197 



in connection with increased activity, and not as the result of 

 taking an excessive supply of this material. 



Various modes of decomposition are possible in the destruc- 

 tion of amino acids. The amino group might be removed, leaving 

 a fatty acid, or oxidation might result in the formation of an 

 oxyacid or a keto acid ; thus from alanine, propionic acid, lactic 

 acid, or pyruvic acid would result. 



CH 3 CH 3 CH 3 CH 3 



I I I I 



CHNH 2 CH 2 CHOH = 



I I I I 



COOH COOH COOH COOH 



Alanine Propionic Acid Lactic Acid Pyruvic Acid 



Enzymes are known which can produce any one of these types of 

 reaction. These products might then be further oxidized to C0 2 

 and H 2 0, or they -or their derivatives could be utilized in the 

 construction of other substances needed by the tissues. Pyruvic 

 acid and the corresponding aldehyde undoubtedly play an im- 

 portant part in the destruction of more than one amino acid, 

 and the resynthesis of other substances. The property of de- 

 aminizing amino acids undoubtedly is possessed by all cells. 



Amount of Protein Required. Nitrogen Balance. Since 

 protein is indispensable, it is of interest to know how much 

 protein is needed by the body. Protein is an expensive article 

 of diet. It would be of economic advantage to eat no more pro- 

 tein than necessary for the maximum efficiency of the body. 

 Excess protein is not stored for future use, but is broken down, 

 and its nitrogen excreted mainly as urea within a comparatively 

 short period. Ordinarily an adult is in a condition known as 

 nitrogen equilibrium. That is, he excretes just the amount of 

 nitrogen that he takes in his food. His body is neither storing 

 up nor depleting its store of nitrogenous material or protein. 

 Such a condition of exact balance does not always exist, how- 

 ever. In a growing child or in an adult during recovery from 

 a wasting disease, or even during periods of ''training" or un- 

 wonted exercise, the body may lay on protein tissue, muscle for 



