294 IV. THE PRINCIPLES OF LIFE PHENOMENA 



lated primitive organisms can sometimes be cultivated in media 

 containing blood plasrri, but never in media containing merely amino 

 acids or other simple protein components. The cultivation can only 

 te achieved when the organisms or assimilase masses acquire the 

 dimension of bacteria. 



Liver slices are able to incorporate an;iino acids in vitro, but the 

 ability is remarkably reduced by the mechanical grinding. According 

 to Winnick (123) rat liver slices are 4 to 41 times as active as the 

 homogenate in the uptake of methionine and 3 times as active in the 

 uptake of glycine, indicating that synthetic ability is markedly re- 

 duced by the decomposition of the cell mass. In recent years numer- 

 ous reports on the uptake of labelled amino acids not only by tissue 

 slices but also homogenates have appeared (124), and considerable 

 evidence has accumulated indicating that this uptake of amino acids 

 represents a synthesis of peptide bonds (125). 



It was found by Brunish and Luck (126) that the protein fraction, 

 containing nucleic acids, in a liver homogenate, can incorporate the 

 greatest amount of phenylalanine. On studying the uptake of radio- 

 active alanine itt vitro into the proteins of rat liver homogenate, 

 Siekevitz (127) concluded that when the alanine was incubated with 

 the homogenate, the highest specific activity in the homogenate protein 

 was found in the microsome fraction. Significant incorporation did 

 not occur when either isolated microsomes or mitochondria were in- 

 cubated with radioactive alanine, but when mitochondrial and micro- 

 somal fractions were incubated together in the presence of the alanine 

 incorporation occurred. 



This finding suggests that the dimension of the assimilase is im- 

 portant not only for the reason that the dimension may determine 

 the polymerization degree but also for the reason that the great 

 dimension may provide the space for the association of a number of 

 particles having a variety of properties, because it is conceivable that, 

 in order to secure the energy supply for the synthesis of proteins, 

 that is, for the uptake of amino acids, a variety of particles may be 

 needed to cooperate for the common purpose, whereby each of the 

 particles may play each specific part. A single virus-like particle 

 may thus be unable to synthesize the protein even if the size of the 

 particle is considerably great ; only when numerous different element- 

 ary-bodies are united, protein synthesis becomes possible. 



In the recent work on the incorporation of labelled amino acid into 

 proteins in cell-free systems, Zamecknik and Keller (127 a) have shown 

 that, in addition to a microsome-rich fraction, a soluble non-dialysable 

 fraction and also an ATP generating system are necessary for the 

 process. Thus it has been clearly indicated that a single virus-like 



