CHEMICAL PATHWAYS 109 



tein synthesis is founded were tracer experiments in which incorporation 

 of amino acids into crude protein material was studied. But Campbell et al. 

 (1960) and Ogata et al. (1960) obtained evidence for the validity of this 

 scheme for labelling of a specific protein, serumalbumin, in a liver homo- 

 genate. The requirements are a source of ATP, the pH 5 fraction and GTP; 

 ribonuclease treatment of the pH 5 fraction which contains the soluble 

 RNA inhibits the incorporation into serumalbumin. Finally, in living 

 E. coll, chasing experiments in which non-labelled amino acids where 

 added after a period of incorporation of labelled ones showed that the 

 amino acids bound to soluble RNA behave as if they were intermediates on 

 the way of protein synthesis (Lacks and Gros, 1960). These results which 

 were obtained with intact exponentially growing bacteria indicate that the 

 pathway under consideration is indeed operative in bacteria as well as in 

 animal tissues. 



However, other observations indicate that we do not know the whole 

 story yet, even for the activation steps perhaps. In the experiments just 

 mentioned with living E. colt for instance, it would seem that the rate of 

 renewal of the amino acids bound to soluble RNA can account only for 

 part of the total incorporation into proteins in exponentially growing bacteria, 

 as if an alternative pathway contributed to the incorporation. Silkworm 

 fibroin contains 42 per cent glycine and 28 per cent alanine. However, the 

 activation enzymes for tryptophan and for tyrosine are much more active 

 than the one for glycine (Heller et al., 1959). Although the lability of the 

 activation enzymes makes a comparison of absolute activities difficult, and 

 its significance questionable, these observations nevertheless again raise 

 the question of the existence of alternative pathways. 



C. OTHER FACTORS INVOLVED IN PROTEIN 

 SYNTHESIS 



(a) The incorporation enzymes. Beljanski and Ochoa (1958) isolated 

 from bacteria a protein fraction which is required for the incorporation in a 

 cell free system from Alkaligenes foecalis, in the absence of amino acid 

 activation enzymes (Beljanski, 1960). This fraction 'replaces' the pH 5 

 fraction in the incorporation of leucine into proteins of rat liver microsomes 

 (in this sense that it causes a very strong stimulation of incorporation, as 

 the activation enzymes would do). This 'incorporation enzyme', however, 

 does not catalyse the amino acid promoted exchange of pyrophosphate 

 with ATP. Instead, this purified fraction causes the exchange of each 

 i4C-labelled nucleoside diphosphate (ADP, GDP, UDP, CDP) with the 

 corresponding triphosphate. Differential thermal inactivation of the ex- 

 change of the four pairs of nucleotides points to the existence of four 



