44 THE PHYSIOLOGY OF PROTEIN METABOLISM 



where protein sparing action has been exhibited there must apparently 

 have been a certain amount of conversion (see Embden and Schmitz' 

 work, p. 119) or transformation of ammonium salts into amino acids 

 as, unlike the amides, these salts could not bring about sparing action 

 by contributing to the energy needs of the body. 



The Fate of Amino Acids. 



So far consideration has been given to feeding experiments with 

 digestion products of proteins mixtures of amino acids, simple and 

 compound, known and unknown. The point now to be discussed is 

 the fate of the amino acids introduced into the body either singly or 

 in groups, and either by the mouth or parenterally. It would appear, 

 that amino acids are utilized when the active form natural to the body 

 is fed or injected, whilst the other form is excreted unchanged. Like- 

 wise when a racemic amino acid is given, the natural form is burned 

 and the abnormal one appears unchanged in the urine. Levene and 

 Meyer (255), for instance, found after feeding an animal with natural 

 alanine that all the nitrogen appeared in the urine within twenty-four 

 hours as urea, 90 per cent, of it being excreted within nine hours. On 

 the other hand the optical antipode to the natural alanine (1. alanine) 

 was only partially converted into urea (68 per cent.) ; the rest appeared 

 in the urine as such. Natural 1. leucine was more slowly broken down, 

 about 54 per cent, appearing as urea in the first twenty-four hours, 

 the remainder in the following twenty-four hours. Natural 1. phenyl- 

 alanine was all converted into urea, but the operation as in the case 

 of leucine was slow ; d. phenylalanine was only converted into urea 

 to the extent of 31 per cent. Again the nitrogen of aspartic acid 

 was removed to the extent of 86'6 per cent, in twelve hours, while 

 the optical antipode to the natural 1. aspartic acid was only excreted 

 in the form of urea to the extent of 3 1 '6 per cent. Arginine nitrogen 

 was found to be excreted as urea to the extent of 97 per cent. 

 Thompson (395) also found in the case of arginine that the nitrogen 

 was largely excreted as urea. Differences in the fate of injected 

 polypeptides have been noted mainly by Abderhalden and his co- 

 workers. Abderhalden and Bergell (7), for instance, found that 

 although glycine when injected subcutaneously into the rabbit was 

 burned completely, yet when the dipeptide glycylglycine was injected 

 glycine appeared in the urine, but on the other hand if glycyl-1. 



