CHAPTER LXX 



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, we 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 

 ammo 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. 

 (Fig. 187.) 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 add- 

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

 concentration in plasma and saline alike. It has been possible in this 

 way to isolate several of the ammo acids and other ammonia-yielding 

 substances from blood. Thus, alanine and valine have been obtained as 

 crystalline salts, and histidine and creatine have been (see page 656) 

 shown to be present by their reactions. 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 ad- 

 dition of sodium carbonate, as do the diffusible substances (Rohde). 



Although amino acids can thus be separated in a pure state from cir- 

 culating blood, their concentration in a drawn specimen is too low to 

 make direct quantitative estimation possible. By the methods of Van 

 Slyke and Sorensen, already described, however, it has been shown 

 among other things that the blood always contains a certain concentra- 

 tion 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 a marked in- 

 crease, becoming doubled or more; and a similar result has been ob- 

 tained by placing pure amino acids in the small intestine. After 10 

 grams of alanine, for example, the amino nitrogen of the mesenteric 

 blood rose from 3.7 to 6.3 mg. per cent.* 



"This is a convenient way of stating per 100 c.c. of blood. 



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