METABOLISM, NUTRITION AND DIETETICS 503 



to produce these symptoms than are found in animals with the 

 Eck's fistula. Although the portal vein carries to the liver a 

 much greater supply of blood than the hepatic artery, ligation 

 of the latter causes a greater diminution in the ratio of the 

 amount of urea to the total nitrogen in the urine than ligation 

 of the former. This indicates that a good supply of oxygen is 

 an important factor in the formation of urea in the liver (Doyon 

 and Dufourt). But this is no proof that the process by which 

 it is formed is an oxidation. The work of the liver, like that of 

 other tissues, is no doubt deranged by lack of oxygen. 



(5) In acute yellow atrophy, and in extensive fatty degenera- 

 tion of the liver, urea may almost disappear from the urine, 

 and leucin, tyrosin, and other amino-acids may appear in it along 

 with a much larger amount of ammonia than normal. The 

 amino-acids and ammonia formed in the intestine in the digestion 

 and absorption of proteins pass unchanged through the degener- 

 ated liver, and are excreted by the kidney. 



Processes by which Urea is Formed. If it be granted, as in the 

 face of the evidence it must, that the liver plays an important part 

 in the formation of urea, we have still to ask what the materials are 

 upon which it works, in what organs they are produced before being 

 brought to the liver, and by what process they are there changed 

 into urea. To the last question it may be at once replied that we 

 know but little of the process by which urea is formed in the body. 

 In the laboratory urea can be obtained from protein either by hydro- 

 lysis or by oxidation. We have already remarked that when a 

 protein is split up by boiling with dilute acid under proper conditions, 

 very much the same decomposition products appear as in tryptic 

 digestion (p. 332). One of these, arginin (C 6 H 14 N 4 O 2 ), on further 

 hydrolytic cleavage by barium hydroxide, yields urea and ornithin 

 (diamino-valerianic acid), half of the nitrogen of the arginin appear- 

 ing in each. The amount of arginin, and therefore the amount of 

 urea which can be artificially obtained in this way, varies extremely 

 with the different proteins and protamins (p. 2). Thus, salmin, 

 a substance prepared from the milt of salmon, yields 84*3 per 

 cent, of its weight of arginin, while the casein of cow's milk 

 yields only 4-8 per cent., and gluten-fibrin, one of the proteins 

 of wheat, only 3 per cent. In the body the hydrolysis of arginin 

 to urea and ornithin is accomplished by the ferment arginase. 

 This ferment is found in the liver, and also in many other 

 organs. The urea formed in this way appears very rapidly in the 

 urine. The ornithin itself is then more slowly transformed into 

 urea. Since the ordinary food-proteins are poor in arginin, the 

 amount of urea which can possibly be formed in mammalian meta- 

 bolism by this process cannot be large, even if most of the arginin, 

 as is the case when it is fed to an animal, is transformed into urea. 

 Other amino-acids, as already mentioned, also cause an increased pro- 

 duction of urea, corresponding to their nitrogen content, when 

 administered by the mouth or subcutaneously. The same is true 

 when, instead of simple amino-acids, polypeptids, like glycvl-glycin, 

 alanyl-alanin, or leucyl-leucin, are given. 



Urea has also been artificially obtained from protein by oxidation 



