278 



M. G. KRITSMAN AND A. S. KONIKOVA 



By using myosin in which different amino acids were labelled it was found 

 that, as well as the process of incorporation of amino acids, which has just been 

 mentioned, there was also elimination of amino acids. 



The formation and breakdown of the protein of crystaUinc carboxypeptidase 

 has been observed by Wood & Roberts [2] ; a similar phenomenon has also been 

 observed in experiments on trypsin. 



This is evidence for the reversibility of the process of transfer of free amino 

 acids into the structure of simple proteins. 



This, together with the facts discussed earlier, give us reason to consider 

 that the metabohsm of protein, i.e. its formation and breakdown, can take place 

 without the participation of biological structures and is a manifestation of the 

 chemical properties of simple proteins, and that these processes are in many 

 ways similar to the process of protein metabolism which takes place in com- 

 plicated biological systems. 



20r 







Fig 2. Relation between the amount of free amino acids taken up by an isolated 



protein and their concentration in the surrounding medium. Concentration of 



free amino acids in mg/ml. 



I Serum protein. II Myosin. 

 I. Glycine. 2. Methionine. 3. Tyrosine. 4. Cysteine. 



We took, as our unit of the amount of amino acid incorporated in a protein, the 



amount incorporated when the free amino acid was present to the extent of 0-06 



mg/ml. in the medium. 



Having fotmd that free structural imits of protein can be transferred from 

 the surrounding medium and incorporated into the structure of isolated proteins, 

 we considered it necessary to find out whether this process of protein metabolism 

 provides a continual possibility of variability in the composition of protein mole- 

 cules or whether it merely leads to the re-introduction of components of the 

 same sort, as occurs in the exchange reaction between barium carbonate and the 

 CO2 of the air. 



To clear up this point we undertook investigations which showed that the 

 incorporation of structural units from the surrounding medium into proteins 



