METABOLISM OF PROTEINS 587 



there in the decomposition of amino-acids. Some writers, indeed, 

 take the view that the muscles, containing as they do three-fourths 

 of the proteins of the body, and utilizing as they appear to do a 

 large proportion of the amino-acids of the food-protein, are more 

 important seats of urea formation than the liver. Yet the fact that 

 it is far easier to demonstrate this power e.g., by perfusion experi- 

 ments for the liver than for such tissues as the muscles, renders it 

 difficult to avoid the conclusion that in the preparation of an end- 

 product so important as that in which the great bulk of the nitrogen 

 leaves the body, a certain degree of specialization has been developed, 

 and that this preparation has been largely entrusted to a special 

 organ. And, while it may be true that larger amounts of amino-acids 

 are taken up and utilized by the muscles than by the liver under 

 certain conditions, this does not show that amino-groups removed 

 from the amino-acids in the muscles may not be largely transferred 

 to the liver before being changed into urea. Further, the transforma- 

 tion of amino-acids into dextrose (and glycogen) may be assumed to 

 entail a considerable absorption of amino-acids by the hepatic cells. 



Processes by which Urea is formed. In the case of only one of the 

 amino-acids derived from proteins can urea be obtained by a simple 

 process of hydrolytic cleavage. This is arginin (o-amino-5-guanidin- 

 w-valerianic acid) that is to say, normal valerianic acid, 



CH 3 .CH 2 .CH a .CH a .COOH, 



i y ft a 



in which an amino group is attached to the o carbon atdm, while 

 guanidin N rT 2 ^C.NH is attached to the S carbon atom (p. 566)- 



When arginin is hydrolysed by barium hydroxide it yields urea and 

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

 appearing in each. Thus, 



NH a 



a O > 



Arginin. 



NH a 

 .CH.C 



=O+ NH 2 .CH a .CH a .CH a .CH.COOH 



UreaT Ornithin. 



The amount of arginin, and therefore the amount of urea which can be 

 artificially obtained in this way, varies extremely with the different 

 proteins. Thus, salmin, a protamin (p. 2), 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 

 (Kossel and Dakin). 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 



