114 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



that amido-acids are normally intermediate stages in proteid meta- 

 bolism, and glance at their formulae — 



Glycocine, C2H5NO2 Creatine, C4H9N3O2 



Leucine, CgHisNOa Arginine, CGH14N4O2 



— we see that the carbon atoms are more numerous than the nitro- 

 gen atoms. In urea, CON2H4, the reverse is the case. The amido- 

 acids must therefore be split into simpler compounds, which unite 

 with one another to form urea. Urea formation is thus in part 

 synthetic. There have been various theories advanced as to what 

 these simpler compounds are. Some have considered that cyanate, 

 others that carbamate, and others still that carbonate of ammonium 

 is formed. Schroder's work proves that ammonium carbonate is one 

 of the urea precursors, if not the principal one. The equation which 

 represents the reaction is as follows : — 



(NH4)2C03-2H20=CON2H4 



[ammonium [water] [urea] 



carbonate] 



Schroder's principal experiment was this : a mixture of defibrinated 

 blood and ammonium carbonate was injected into the Hver by the 

 portal vein ; the blood leaving the liver by the hepatic vein was found 

 to contain urea in great abundance. This does not occur when the same 

 experiment is performed with any other organ of the body, so that 

 Schroder's experiments also prove the great importance of the liver 

 in urea formation. Similar results were obtained by Nencki with 

 ammonium carbamate. 



There is, however, no necessity to suppose that the formation of 

 amido-acids is a necessary preliminary to urea formation. The con- 

 version of the leucine and arginine formed in the intestine into 

 ammonium salts and then into urea does certainly occur, but this 

 only accounts for quite an insignificant fraction of the urea in the 

 urine. If this also occurs in tissue metabolism, we ought to find 

 considerable quantities of leucine, glycocine, creatine, arginine, and 

 such substances in the blood leaving the various tissues and entering 

 the liver ; but we do not. We do, however, constantly find ammonia, 

 which, after passing into tlie blood or lymph, has united with car- 

 bonic acid to form either carbonate or carbamate of ammonium. It 

 is quite probable that the nitrogenous waste that leaves the muscles 

 and other tissues is split off from them as ammonia, and not in the 

 shape of large molecules of amido-acid which are subsequently con- 

 verted into ammonia. The experiments outside the body which most 

 closely imitate those occurring within the body are those of Drechsel, 



