CH. XXXIX.] OKIGIN OF UREA 587 



in the urine ; they arise from the disintegration of the proteins of 

 the liver cells, but they may in part originate in the intestine, and, 

 escaping further decomposition in the degenerated liver, pass as such 

 into the urine. 



That the ammo-acids are the substances from which the liver 

 forms urea is shown by the fact that if such amino-acids as glycine, 

 leucine, arginine, etc., are administered by the mouth, or injected 

 into the blood-stream, the excretion of urea is correspondingly 

 raised. 



The transformation of arginine into urea is a subject on which we 

 have more accurate information than in the case of any other amino- 

 acids, for there is no doubt that the change, which can be brought 

 about in a test-tube, is also accomplished in the organism. If the 

 account of arginine given on p. 420 is referred to, it will be seen to 

 consist of a urea radical and a substance called ornithine. On 

 hydrolysis we therefore get urea and ornithine (diamino-valeric 

 acid), and this in the body is accomplished by a special enzyme 

 called arginase (Kossel and Dakin), which is more abundant in the 

 liver than in any other tissue. The actual yield of urea is, however, 

 greater than one would anticipate, and so it must be supposed that 

 the ornithine in its turn is broken up and urea is the result. If we 

 glance at the formula of ornithine, and compare it with that of certain 

 other amino-acids which are also undoubted urea forerunners, we 

 have the following : 



Glycine C 2 H 5 NO, 



Leucine C 6 H 13 NO. 2 



Ornithine C 5 H 12 N 2 O 2 



In all cases, the atoms of carbon are more numerous than those of 

 nitrogen. In urea (CON 2 H 4 ) the reverse is the case. The amino- 

 acids must therefore be split into simpler compounds which unite 

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

 synthetic. These simpler compounds are ammonium salts. Schroder's 

 work, which has been confirmed by subsequent investigators, proves 

 that ammonium carbonate is one of the urea precursors, if not the 

 principal one. The equation which represents the reaction is as 

 follows : 



(NH 4 ) 2 CO 3 = CON 2 H 4 + 2H 2 O. 



[Ammonium [Urea."] 



carbonate.] 



Schroder's principal experiment was this: a mixture of blood and 

 ammonium carbonate was injected into the liver by the portal vein ; 

 the blood leaving the liver by the hepatic vein was found to contain 

 urea in abundance. This does not occur when the same experi- 

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

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



