174 S. S. COHEN 



Davie et al. (1956) have isolated a relatively pure, soluble, specific, trypto- 

 pliaii-activatiiig enzyme from the pancreas. Schweet (1957) has obtained a 

 tyrosine-specific enzyme from pancreas. 



By means of the former enzjone it has been possible to show that some 

 tryptophan analogs, such as azatryptophan and tryptazan, are activated, 

 incorporated into protein, and then block growth (Sharon and Lipmann, 

 1957). Other analogs, such as 5-methyl tryptophan, inhibit the enzyme by 

 competing with tryptophan at the activation step and appear to block 

 growth in this manner. 



In the studies of Novelli and DeMoss (1957), several activating enzymes 

 for amino acids were also found in E. coli extracts. These workers have found 

 in a wide variety of bacterial and animal extracts that only eight amino acids 

 are activated in a comparable PP exchange. The others were presumed to be 

 activated in as yet undetermined transacylation reactions. However, Nisman 

 el al. (1957) have recently observed that lysis of protoplasts, a milder method 

 of cell rupture, permits the demonstration of activating systems for many 

 more amino acids. 



4. Intermediate Reactions 



Since the amino acid activation and binding are affected by a soluble 

 enzyme and the deposition of the amino acid occurs most actively at the 

 ribonucleoprotein centers of the microsomal fraction, the nature of the 

 transfer from the enzyme-AMP-amino acid complex is posed. Guanosine 

 di- or triphosphate is essential in some as yet undetermined fashion to this 

 transfer (Keller and Zamecnik, 1956; Littlefield and Keller, 1957). Liver 

 microsomes derived from vitamin Bja-deficient rats are ineffective in the 

 incorporation of amino acids in these systems, but become effective on 

 supplementation with exogenous vitamin B.., thereby implicating this 

 substance as a cofactor as well (Wagle et al., 1957). 



Most recently it has been found that a low molecular fraction of RNA 

 present in the supernatant fraction reacts with the complex and incorporates 

 the amino acid (labeled leucine) as a soluble RNA-amino acid compound. 

 (Hoagland et al., 1957b). This reaction is sensitive to ribonuclease. In the 

 presence of GTP a deproteinized RNA-amino acid derivative can transfer 

 its bound amino acid to the microsomal fraction. This is the first direct 

 demonstration of a role for polynucleotides in protein synthesis. The course 

 of these reactions may be represented as shown in formula (XXV). 



amino acid 



/ ' GTP 

 E -j- soluble RNA > RNA-amino acid ■ > microsomes-amino acid 



\ 

 AMP 



(XXV) 



