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A TEXTBOOK OF PHYSIOLOGY 



tyrosin and tryptophan. These are of great value to the body, and 

 in the liver they are probably modified into a form available for 

 the tissues. It is certain that they are not rapidly destroyed, and 

 their nitrogenous moiety excreted from the body in the form of 

 urea. If a non-nitrogenous moiety be split off, it may give rise to 

 dextrose. 



Cystin (diamino-di-thio-lactic acid) is broken down in the liver, the 

 sulphur moiety giving rise in part to taurin and in part to inorganic 

 sulphates. It is possible that the carbon-containing lactic acid portion 

 may give rise to dextrose. 



The fate of the other bodies is not sufficiently elucidated to be 

 mentioned here, but, according to this hypothesis, they also each 

 undergo their own special metabolism in the liver. The fact that 

 there occur certain inherited but very rare errors of metabolism, 

 such as alkaptonuria and cystinuria, is held to lend support to the 

 hypothesis, which may be represented diagrammatically as follows; 



Products of Protein Digestion. 



Cystin 



LIVER 



UREA UREA Dextrose UREA 



;Dex- Prepared Taurin Inorganic 

 trose for tissueSj and sulph- 



? dextrose ates 



An objection to this hypothesis is that it is not quite clear from 

 what source the body derives all the bricks necessary for the rebuilding 

 of its proteins. In these proteins, amino-acids, such as alanin and 

 glycin, are incorporated, and it is by no means clear from what source 

 such amino-acids come. 



According to the second hypothesis, the amino-acids do not pass 

 from the intestine into the blood. It is said (1) that their presence 

 there has never been conclusively demonstrated ; (2) that if the circu- 

 lation be confined to the intestinal wall and pancreas, heart, lungs, 

 and muscles of respiration, the blood contains no proteoses, peptones, 

 or amino-acids. It is therefore held that the amino-acids, during 

 their passage, are (1) either synthesized into protein, so-called 

 " plasma protein " ; or (2) deaminized, with the formation of ammonia. 



The " plasma protein " is held to be a protein so built that all the 

 cells of the body can abstract from it just the bricks they desire to 

 rebuild their own particular protein. In building up this plasma 

 protein, all the amino-acids of digestion are not used. Many are in 

 excess such, for example, as glutamic and aspartic acids, when 

 plant proteins have been ingested. These are deaminized in the 

 intestinal wall, and the ammonia thus formed passes to the liver 

 as ammonium carbonate or carbamate, there becoming converted 



