

SPECIAL METABOLISMS 425 



into urea. It may be assumed in this case that the non-nitrogenous 

 moiety formed as the result of deaminization is taken to the liver, 

 being there converted into dextrose and, finally, in part to glycogen 

 (see next page). 



In support of this view, it has been shown that a dog may be fed 

 on digested protein (amino -acids), and kept in nitrogen equilibrium, 

 even when its portal blood is short-circuited from the liver by an 

 Eck's fistula (see p. 448). The blood coming from the intestine during 

 periods of digestion is stated to be demoiistrably richer in protein of 

 a globulin nature. 



This view may be regarded as a modification of older ones concern- 

 ing protein metabolism. According to these, serum albumin and 

 serum globulin were formed in the intestinal wall, and passed into the 

 portal blood. The views differed as to the subsequent fate of these 

 proteins. According to one view, all protein was subsequently built 

 up into living protoplasm before it was destroyed; according to the 

 other, some of the protein, called " tissue protein," was built up into 

 protoplasm ; the remainder was not so built up, but served as a source 

 of energy the so-called " circulating protein." The speed with which 

 an increased intake of nitrogen appears in the urine as urea is against 

 the view that such nitrogen has been built up into protoplasm. On 

 the other hand, when we consider the rapidity with which protoplasm 

 grows e.g., yeast multiplying we cannot put such a possibility out 

 of court. As to whether any of the protein formed in the intestinal 

 mucous membrane acts solely as circulating protein, there is at 

 present no evidence. 



Much more work is required upon this intricate and difficult 

 subject. Up till now, there can be said to be no established theory 

 of protein metabolism, only two tentative hypotheses, of which the 

 second, given above, appears the more probable namely, that a 

 special plasma protein is synthesized in the intestinal mucous mem- 

 brane, containing all the necessary bricks for the rebuilding of the tissue 

 proteins. Excess of digested protein is deaminized in the intestinal 

 mucous membrane, and the ammonia thus formed is short-circuited 

 from the body as urea. 



Protein, or the amino-acids split from it, increases the rate of 

 metabolism and heat-production stimulating the cells of the body to 

 activity, having a specific dynamic action greatev than carbohydrate 

 and much greater than fat. This stimulating action is traced to oxy- 

 or keto-acids, the non-nitrogenous moiety of protein metabolism. 



