METABOLISM. 355 



the other end being attached to a large vein. The system of tubes is 

 filled with saline solution, and hirudin is injected into the animal to 

 prevent clotting of the blood ; the blood is then allowed to flow from 

 the artery through the tubes and back to the vein, a continuous circu- 

 lation of the animal's blood being thus maintained through the tubes. 

 The tubes are surrounded by normal saline solution at the body 

 temperature, and, as the blood flows through them, its diffusible con- 

 stituents, including sugar and amino-acids, pass through the collodion 

 wall into the saline solution and can be subsequently examined. Since 

 amino-acids diffuse into the salt solution, they must have been previously 

 present in the circulating blood 



Deamination. Immediately after their absorption amino-acids 

 undergo a change, which is called deamination, and which consists 

 in the removal of the ammo-group, and its replacement by an oxygen 

 or hydroxyl radicle. A simple illustration of this change is represented 

 in the following equation : 



CH 3 . CH . NH 2 . COOH + H 2 = CH 3 . CHOH . COOH + NH 3 . 



Alanine. Lactic acid. 



The amino-acids are thus converted into oxy- or keto-acids, which 

 on reduction become ordinary fatty acids. This change is probably 

 brought about by an enzyme, and takes place partly in the walls of the 

 intestine itself and partly in the liver. Its occurrence in the intestinal 

 wall is shown by the fact that the amount of ammonia present in the 

 portal blood is increased after a protein meal ; that it occurs in the 

 liver is proved by the observation that when amino-acids are added to 

 pounded liver substance under aseptic conditions, the amount of ammonia 

 rapidly increases. The extent to which this change in the amino-acids 

 takes place is not known, but a fraction of these acids enters the general 

 circulation without undergoing deamination. The removal of the 

 amino-group does not appreciably diminish the calorie value of amino- 

 acids or their usefulness as a source of energy to the body. Thus the 

 calorie value of 1 gram molecule of alanine is 389 ; if its amino group 

 is replaced by hydrogen, the calorie value of the propionic acid thus 

 formed is 367, the difference being comparatively trivial. 



FORMATION OF UREA. 



The ammonia set free by the deamination of amino-acids in the 

 intestinal wall is carried in the portal circulation to the liver, and, 

 together with that similarly formed in the liver itself, is converted into 

 urea. If we regard the ammonia set free as entering^ into combination 



