CHEMICAL PATHWAYS 107 



activation enzyme to activate the amino acid and to transfer it to RNA 

 remained strictly proportional in the course of purification (Lipmann et al., 

 1959). Studies by HoUey (1957), HoIIey and Goldstein (1959) are also 

 relevant to the mechanism of the transfer. The purified alanine activation 

 enzyme which catalyses the pyrophosphate ATP exchange in the presence 

 of alanine, catalyses also the exchange of i^C-AMP with ATP in the 

 presence of both alanine and soluble RNA. This last exchange is com- 

 pletely abolished by ribonuclease. This fact strongly suggests that the 

 reaction between enzyme bound alanine adenylate and soluble RNA is 

 direct. Mager and Lipmann (1958) provided evidence for the reversibility 

 of the amino acid transfer to s-RNA in Tetrahymena preparations, by 

 showing that reversion is obtained by the addition of pyrophosphate and 

 AMP. Inhibition of the pyrophosphate exchange by s-RNA also supports 

 a direct interaction between s-RNA and the activation enzyme (Goldstein 

 and Holley, 1960). The aminoacyl group alone is transferred to soluble 

 RNA, not the adenylic acid residue, which is liberated (Wong and Mol- 

 dave, 1960). 



One must conclude that activation of the amino acid with formation of 

 the enzyme-bound aminoacyl adenylate, and the transfer of the aminoacyl 

 residue to the RNA are both brought about by the activation enzyme. The 

 donor specificity of the activation enzymes must be such that they can 

 distinguish between the twenty diflferent acceptor RNAs and deliver the 

 activated acyl group to the right one only. It has been briefly reported that 

 two activation enzymes both specific for alanine were isolated from pig 

 liver: the one from the cytoplasm, the other from nuclei (Weber, 1960). 

 The cytoplasmic enzyme is said to transfer alanine to cytoplasmic RNA 

 from muscle and liver, but not to nuclear RNA or yeast RNA. There are 

 indications that a certain degree of species specificity might also exist at this 

 level. The activation enzymes of guinea pig will transfer to animal s-RNA, 

 but E. colt RNA is a bad acceptor (Allen et al, 1960). 



What happens to the activated amino acids which are bound to their 

 specific soluble RNA? They can be transferred to the ribosomes where they 

 first appear as polypeptides. This was shown first by Hoagland et al. (1957, 

 1958) with rat liver homogenates. More recently Lacks and Gros (1959) 

 established that the amino acids bound to soluble RNA in E. coli are very 

 rapidly renewed and that any loss of labelled amino acid from the soluble 

 RNA is accompanied by a gain of i^C in the proteins. 



Extremely little is known about this transfer at present, except that 

 guanosine triphosphate (GTP) is required at this stage, possibly also ATP 

 (Hoagland et al, 1957, 1958; Webster, 1959; Ogata et al, 1960). Some 

 other factor, which is especially abundant in regenerating liver, also 

 favours the passage of amino acids from soluble RNA to microsomal 

 protein (Rendi, 1959: Grossi and Moldave, 1959, 1960; Nathans and 



