KIDNEY AND SKIN AS EXCRETORY ORGANS. 845 



*^^^ NH2 — H2O = CO^. j^.jj^. 



Ammonia salts may arise similarly in the tissues of the body, 

 since the cells contain intracellular enzymes capable of causing 

 livfln)lytic cleavage of the protein molecule. So far as ammonia is 

 produced in this way, it may be converted to urea by the action of 

 the liver and possibly by a similar action in other tissues. 



2. Urea arises from the monamino-acids by a process of deamin- 

 ization, whereby the NH2 group is converted to ammonia and 

 then probably to urea. In the digestion of protein in the alimen- 

 tary canal amino-acids are formed in quantity. The current belief 

 in physiology is that these amino-acids are carried in the blood to 

 the various tissues and are there resynthesized in part to form the 

 characteristic protein of the tissue. The tissues pick and choose the 

 several amino-acids necessary for their reconstruction or growth. 

 The amino-acids not so used have their NHo group removed by 

 the process of deaminization, leaving behind an organic acid or 

 oxyacid which is of further use in nutrition, for example, as a 

 source for the production of sugar (p. 821). The general nature of 

 the process is represented by the conversion of alanin to lactic acid. 



CH3CHNH2COOH + H2O = NH3 + CH3CHOHOCOH. 



There is evidence to show that this process of deaminization,* as 

 well as the further conversion of the ammonia to urea, takes place 

 in the liver, although it is quite possible that both functions may 

 be exhibited by other tissues as well. On this view we can under- 

 stand why the amount of urea eliminated in the urine rises and falls 

 with the amount of protein taken as food.f On a large protein 

 diet the amount of nitrogenous material supplied is in excess of the 

 amount needed for tissue construction. It may be supposed that 

 the excess nitrogen is promptly removed and excreted as urea 

 according to the process described above. It seems probable that 

 the larger part of the urea actually found in the urine under normal 

 conditions comes in this way rather directly from the food so far 

 as its nitrogen is concerned. The ammonia formed in this way 

 might be toxic to the body, but, as we have seen, it is readily 

 neutralized by the CO2 to form ammonium carbonate, and this, in 

 turn, is easily dehydrated to form urea. On the other hand, it may 

 be assumed that this som-ce of ammonia furnishes the body with a 

 means for safeguarding its reaction under those abnormal condi- 

 tions in which there is an overproduction of fixed organic acids. 

 But on the lowest protein diet or in starvation, when the body is 



* Van Slyke and Meyer, "Journal of Biological Chemistry," 16, 187, et ntq , 

 1914. 



t Folin, "American Journal of Physiology," 13, 117, 1905. 

 t "Zeitschrift f. physiol. Chemie," 1904, xUi., ISl. 



