382 MAHLON B. HOAGLAND 



formed in the absence of an acceptor for the activated amino acid. It was 

 further apparent that individual enzymes activated individual amino 

 acids 159 " 163 and that all amino acids were activated by the same mecha- 

 nism. 164 ' 166 Much work has now established the mechanism on a firm 

 footing. 167 " 170 Final unequivocal evidence has recently been presented by 

 Kingdon et at. who have actually isolated a tryptophan adenylate from 

 large quantities of purified tryptophan activating enzyme. 171 



There are still a number of interesting questions relating to amino acid 

 activation which are being pursued but these do not have relevance to 

 the subject of this chapter. We may, however, emphasize (or reemphasize) 

 three important aspects of the reaction which impinge on our concern 

 with the role of RNA in protein synthesis. First, there is general agreement 

 that amino acid activation — i.e., the formation of the enzyme-bound amino 

 acyl adenylate compounds — -is an enzymic reaction entirely independent 

 of RNA. This statement is based on the fact that ribonuclease does not af- 

 fect the activation of amino acids by crude or purified activating enzyme 

 preparations; and that the more highly purified activating enzymes do not 

 contain measurable RNA and are fully active in the absence of RNA. 



[It should be mentioned, however, that Ogata et al. m ~ 17i have obtained 

 evidence that ribonuclease does inhibit activation. Their maximal inhib- 

 itory effects (50%) were obtained by preincubating the "pH 5 enzyme" 

 fraction with ribonuclease, followed by reprecipitation of the enzyme at 



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