44 RIBONUCLEIC ACIDS AND PROTEIN SYNTHESIS 



Strongly indicate that, in living cells, RNA integrity is essential for 

 protein synthesis. The probable biochemical mechanisms of the 

 latter will now be studied briefly. 



7. BIOCHEMICAL MECHANISMS OF PROTEIN SYNTHESIS 



A number of substances are required for protein synthesis. The 

 presence of an amino acid pool is obviously necessary and it is 

 clear from the foregoing that ATP, a soluble enzyme fraction and 

 RNA are all indispensable for amino acid incorporation into 

 the proteins of relatively simple systems, isolated microsomes, for 

 instance. In the following discussion the theory which is now 

 generally accepted and which involves all these substances (amino 

 acids, ATP, soluble enzymes and RNA) will be briefly presented. 

 We shall first consider the role of ATP and the soluble enzymes, 

 and then that of RNA. 



a. ATP and the soluble enzymes fraction of Hoagland 



The necessity of an energy supply for peptide synthesis has been 

 repeatedly emphasized by Borsook (1950, 1953, 1956a, b), Lipmann 

 (1949), Chantrenne (1951) and others. Borsook's (1950) experi- 

 ments have conclusively demonstrated that incorporation of labeled 

 amino acids into proteins requires energy. The process is stopped, or 

 markedly reduced, by anaerobiosis or addition of cyanide, azide 

 dinitrophenol, etc. In the homogenate system of Siekevitz (1952) 

 this uptake of tagged amino acids into proteins is more closely linked 

 to phosphorylation than to oxidation. The evidence for the neces- 

 sity of ATP as an energy source in amino acid incorporation into 

 proteins is still stronger in the experiments performed by Zamecnik 

 and Keller (1954) with isolated microsomes. As we have seen, incor- 

 poration proceeds provided that microsomes, a non-dialyzable 

 soluble fraction and an ATP-generating system are present. This 

 system is mainly localized in the mitochondria in the normal living 

 cell. The mitochondria therefore probably play an important part 

 in protein synthesis. But it is an indirect one, for their main function 

 is to generate the energy-rich bonds of ATP. In the simplified 



