CHAPTER LXVIII 



THE METABOLISM OF PROTEIN (Cont'd) 



AMINO ACIDS IN THE BLOOD AND TISSUES 



In the Blood. — Furnished with the general facts concerning the chem- 

 istry of proteins, Ave may now proceed to consider the more precise 

 knowledge recently acquired concerning the history of this substance 

 in the animal economy. Although no one has succeeded in separating 

 amino acids in pure condition from drawn blood even during the height 

 of digestion, it has nevertheless been possible to do so from circulating 

 blood by a method of dialysis, known as vividiffusion, elaborated by 

 Abel 33 and his pupils. The method consists in connecting a long tube 

 of collodion with, the two ends of a cut artery in an anesthetized animal. 

 The tube, coiled many times, is then immersed in a solution containing 

 approximately the same salt content as the blood plasma of the animal. 

 The diffusible constituents of the blood plasma dialyze into the saline 

 solution; or any one of them may be prevented from dialyzing by adding 

 that particular substance to the saline in such amounts as will make its 

 concentration in plasma and saline alike. In some ways, it will be seen, 

 the apparatus may be considered as an artificial kidney. Its possible 

 clinical application for the purpose of removing poisons from the blood 

 is under investigation. It has been possible in this way to isolate several 

 of the amino acids and other ammonia-yielding substances from blood. 

 Thus, alanine and valine have been obtained as crystalline salts, and 

 histidinc and creatine (see page 622) shown to be present by their reac- 

 tions. All of the amino substances, however, do not dialyze, and these 

 exceptions are further characterized by the fact that they do not readily 

 give up their ammonia on the addition of sodium carbonate, as do the 

 diffusible substances (Rohde). Although amino acids can thus be sepa- 

 rated in a pure state from circulating blood, their concentration in a 

 drawn specimen is too low to make direct quantitative estimation possible. 

 My the methods of Van Slyke and Sorensen, already described, however, 

 it has been shown among other things that the blood always contains a 

 certain concentration of amino acids ; thus, in that of fasting animals from 

 3 to 5 mg. per 100 c.c. of blood are usually found present. During the 

 absorption of a protein meal, the amino content of the blood undergoes 



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