METABOLISM 203 



either used for constructing other compounds, or burned as fuel, 

 as are the fats and carbohydrates. 



The composition of the proteins of the tissues is remarkably 

 constant, and quite independent of the nature of the food pro- 

 tein. A horse was bled to remove much blood protein. This 

 the body was obliged to replace. The horse was fed at the time 

 on gliadin, a grain protein containing a large percentage of 

 glutamic acid. The blood proteins contain less than a quarter 

 as much glutamic acid as does gliadin, but they were regenerated, 

 and showed no variation from their normal composition. 



The proteins of each individual tissue, and of each different 

 animal undoubtedly are specific, that is those from different 

 sources differ slightly in composition. It has been suggested that 

 possibly they are the substances which transmit species charac- 

 teristics, since they vary in different animals, whereas most of 

 the other body compounds, such as salts, carbohydrates enzymes, 

 nucleic acids, etc., are practically identical in different animals. 

 This is in the realm of speculation, however. 



Carbohydrate Metabolism. A field of interest second only 

 to that of protein metabolism is the metabolism of carbohydrates. 

 Carbohydrates make up a large and important part of our food, 

 and although they play a much less conspicuous role as constitu- 

 ents of body tissue, they are excellent fuels, and furnish the body 

 with a considerable proportion of the material burned for the 

 maintenance of body temperature and the performance of me- 

 chanical work. 



The various carbohydrates of the food such as starches, dex- 

 trins, disaccharides, etc., are reduced to monosaccharides by the 

 digestive enzymes, and as such are absorbed and pass into the 

 blood stream. What is their further fate? The monosaccha- 

 rides have been shown to be present in the blood stream as such, 

 and not combined or united with any other substance, at least 

 in more than the most unstable union. Dialysis of blood against 

 dextrose solutions of various strengths has shown that the blood 

 sugar evidently is free. 



The blood from the intestine is gathered into the portal vein 



