KIDNEY AND SKIN AS EXCRETORY ORGANS. 849 



The amino-acids not so used have their NH 2 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. 825). The general nature of 

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



CH 3 CHNH 2 COOH -f H 2 O = NH 3 + CH 3 CHOHOCOH. 



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. The 

 nitrogen has never entered into the structure of the body. But on 

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

 its own tissues, urea continues to be formed, so that in part the urea 

 of the urine contains some nitrogen which has probably arisen 

 from the destruction of protein tissue. The intermediate steps in 

 this latter process are not definitely known, but possibly they may 

 be analogous to those described, that is to say, the protein may pass 

 through the stage of amino-acids and subsequently undergo de- 

 aminization. 



3. Urea arises from the arginin, formed in the cleavage of the 

 protein molecule by conversion of the contained guanidin radicle. 

 Kossel and DakinJ have demonstrated the existence of a ferment, 

 present in the liver especially, but found also in the kidney, thy- 

 mus, muscle, etc., arginase, which is capable of splitting arginin 

 into urea and ornithin. The reaction may be represented by the 

 following equation: 



NHC<^ 2 (CH 2 ) 3 CHNH 2 COOH+H 2 0=CO<^+NH 2 (CH 2 ) 3 CHNH 2 COOH 



Arginin (guanidia-diamino-valerianic acid). Urea. Diamino-valerianic acid. 



Unlike cases 1 and 2, the urea in this instance is formed from the 

 guanidin residue contained in the arginin and not from the amino- 



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

 1914. 



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

 t "Zeitschrift f. physiol. Chemie," 1904, xlii., 181. 

 54 



