BIOCHEMICAL MECHANISMS 45 



homogenate systems mitochondria can be dispensed with provided 

 that ATP is produced or present. As a matter of fact, the mere 

 presence of ATP (and guanosine diphosphate (GDP) or guanosine 

 triphosphate (GTP), which probably play a similar role, according 

 to Littlefield et ah, 1955, and Keller and Zamecnik, 1956) is enough 

 to obtain full incorporation activity with the small granules of 

 ascites tumor cells, in the presence of the soluble factor. This is due 

 to the fact that these small granules have little or no adenosine tri- 

 phosphatase activity, so that ATP is entirely available for the in- 

 corporation reaction. Addition of enzymatic systems which utilize 

 ATP, and therefore compete with the microsomes for ATP in the 

 homogenates, results in a decrease in the incorporation of the 

 amino acids into the proteins (Siekevitz, 1952; Titova and Shapot, 

 1955). Conversely, the ATP content of the cells increases some- 

 what when protein synthesis is inhibited. This is the case, as we have 

 seen, for onion roots (Brachet, 1954) or amoebae (Skreb-Guilcher, 

 1955) treated with ribonuclease. The same is also true for enucleate 

 fragments of amoebae (Brachet, 1955a). This increase in the ATP 

 content of the cells in which protein anabolism is restricted is 

 apparently a consequence of the unemployment of the high energy 

 phosphate bonds. 



Recent work by Hoagland (1955) and by De Moss and Novelli 

 (1955) has shown that a soluble, non-dialyzable factor is required 

 for the incorporation of amino acids into proteins, besides ATP and 

 RNA. According to Hoagland (1955), this soluble fraction con- 

 tains enzymes which catalyze the exchange of radioactive pyro- 

 phosphate and ATP. Addition of an amino acid, or better, a mix- 

 ture of amino acids, increases the speed of the exchange reaction 

 two to three times. The following scheme has been proposed by 

 Hoagland (1955) in order to explain the activation of amino acids 

 (AA): 



1. El J |_ + ATP = £1 JAMP — PP|_ 



in which E^ is the activation site of the amino acid on the enzyme. It 

 would bind ATP in such a way as to make the AMP — PP bond 

 more labile. 



References p. 50/54 



