26 THE PHYSIOLOGY OF PROTEIN METABOLISM 



Does all the protein absorbed in the form of digestion products be- 

 come converted into coagulable protein, or does the intestinal mucous 

 membrane exert a certain selective action ? The experiments of Ab- 

 derhalden, London and Oppler (45) in which they showed that after 

 feeding with gliadin, as the digested material descended the digestive 

 tract tyrosine disappeared from the intestinal contents but glutamic 

 acid increased in amount, certainly point to some such selective activity. 

 In support of this work was that of London (264) who was unable to 

 show that any marked accumulation of glutamic acid took place in the 

 intestinal wall, even at the height of digestion, after gliadin feeding. 



Further, how is the fact to be explained that following protein di- 

 gestion and absorption there is a very great and rapid rise in the out- 

 put of nitrogen in the urine, mostly in the form of urea, if immediate 

 resynthesis take place ? There is also the observation of Nencki, Pavloff 

 and Zaleski (298) that, as the result of protein digestion, there is a 

 marked rise in the amount of ammonia present in the portal blood. 

 This rise in the ammonia content of the portal blood is supported by 

 and supports the observed increase in the excretion of urea in the 

 urine as the conversion of ammonia into urea certainly takes place 

 for the most part in the liver. Of course one must admit that this 

 deaminization presents also certain difficulties in the digest product 

 absorption hypothesis. Does a selection of appropriate amino acids or 

 amino acid groups which are allowed to pass on into the tissues un- 

 changed take place or is this deaminization a mere protective mechan- 

 ism a certain concentration of amino compounds being permitted to 

 pass but any excess immediately undergoing deaminization, just as in the 

 case of carbohydrate a certain limited amount is fixed by the liver (and 

 the tissues) any excess being excreted in the urine ? In favour of the 

 selective action are certain observations of Lang (238) who found that 

 in vitro some tissues deaminized certain amino acids more readily than 

 others (see page 52). If this be true for the living organism then 

 one might regard the intestinal wall and the liver as successive layers 

 of a highly protective filter which exerted a selective action on the 

 material which they let through. 



The balance of evidence seems to me to be in favour of the hypo- 

 thesis that the synthesis of protein in the body is a function of each 

 individual cell, and is not confined to one set of cells (those of the 

 intestine). Further that the material which is utilized in this syn- 

 thesis is not circulating in the fluids which bathe the tissues as a 

 " whole " or " neutral " protein but in the form of amino acids or groups 

 of these the products of the hydrolytic decomposition of protein. 



