PROTEIN METABOLISM 



857 



attempts have been made to explain the method by which urea may 

 be derived from the amino-acids. 



F. Hofmeister succeeded in preparing urea by oxidising amino- 

 acids with potassium permanganate in the presence of ammonia and 

 ammonium sulphate. He assumes that urea is formed by an oxida- 

 tion synthesis. The first step would be the formation, by oxidation 



24 Hours 



FIG. 351. The hourly variation in the excretion of nitrogen after a meal. 

 The meal was given at 0. The thick line represents the average 

 absorption of the food from the alimentary canal. The thin-lined 



curves represent the N. excretion (1) after a meal of 1000 grm. meat ; 



(2) after 500 grm. meat and 150 grm. fat ; (3) after a meal of 500 grm. 



meat ; (4) and (5) both represent the excretion in a fasting animal. 



(From TIGERSTEDT after FEDER.) 



of protein or amino-acids, of the group CONH 2 , and this at the moment 

 of formation would combine with the NH 2 left over in the oxidation 



of the ammonia to form urea, 



According to Drechsel and Nencki, the immediate precursor of the 

 urea is probably ammonium carbamate, which loses a molecule of 

 water, thus : 



C0< 



H0 = 



NH 



Schroder, on the other hand, suggested that the urea was formed 

 from ammonium carbonate by the loss of two molecules of water. 



NH 2 



/ 



ONH 4 



- 2H 2 = 



