750 PHYSIOLOGY 



THE FATE OF THE AMINO- ACIDS AFTER ABSORPTION BY THE 

 INTESTINAL EPITHELIUM. During a condition of starvation the normal 

 protein requirements of the body, or rather of the active tissues, are met 

 at the expense of the less active tissues. The protein characteristic of any 

 tissue can be taken down, removed to another part of the body, and built 

 up into the protein characteristic of some other active tissue. It is difficult 

 to conceive that such a transference and transformation could occur in any 

 other way than by a more or less thorough disintegration of the protein 

 molecule at one place and its synthesis at the other, and we know from the 

 researches of Hedin and others that every tissue contains intracellular 

 ferments which are capable of effecting the disintegration of the protein 

 molecule, and are responsible for the autolytic degeneration of tissues after 

 death. If therefore the normal interchange of protein between the tissues 

 is accomplished, as we know it to be in plants, by the disintegration of the 

 proteins into their constituent amino-acids and their subsequent reintegra- 

 tion, there is no a 'priori reason to believe that the blood carries the proteins 

 from the alimentary canal to the tissues in any other form than that of 

 amino-acids. The experimental proof of this conclusion was hardly possible 

 before the invention of a reliable method for the detection of small 

 quantities of amino-acids in the blood and tissues. This is rendered possible 

 by van Slyke's method, in which after the separation of coagulable proteins 

 by alcohol the amino-acids are determined by measuring the nitrogen evolved 

 on addition of nitrous acid. Van Slyke has shown that the blood always 

 contains a certain amount of amino-acids even during fasting. After a pro- 

 tein meal there is a considerable increase in the amount of amino-acids. 

 Thus the blood of fasting animals contains from 3-1 to 54 milligrams amino- 

 acid nitrogen per 100 c.c. Blood taken after food contains 8-6 to 10-2 

 milligrams ammo-acid nitrogen per 100 c.c. of blood. The question of the 

 fate of amino-acids thus absorbed from the intestine to the blood is decided 

 by an estimation of the amino-acid content of the different tissues after 

 the injection of amino-acids into the blood. Van Slyke has found that 

 after the injection of amino-acids only a certain proportion is excreted 

 with the urine, and that the rest of the amino-acids rapidly disappeared 

 from the blood and are taken up by the tissues without undergoing any 

 immediate chemical change, though in the case of certain tissues, such 

 as the muscles, a definite saturation point exists which sets the limit to the 

 amount of amino-acids that can be absorbed. On the other hand the 

 capacity of the internal organs, and especially of the liver, for the absorption 

 of amino-acid is much greater. 



It is worthy of note, however, that the absorption of amino-acids by the 

 tissues from the blood is never complete, i.e. the amino-acids of the blood 

 must be in a state of equilibrium with those of the tissues, although the con- 

 centration in the latter may be much greater than in the former. If several 

 hours be allowed to elapse after the injection of amino-acids before the 

 analysis of the tissue is undertaken, it is found that the amino-acid nitrogen 

 content of the liver may have returned to normal although the concentration 



