264 VINCENT G. ALLFREY 



TABLE I 



Effect of Amino Acids on ATP-Pyrophosphate Exchange by Calf Thymus 



Nuclear pH 5 Fraction 



Amino acids promoting Amino acids without 



exchange effect in this system 



L-Alanine L-Arginine* 



L-Aspartic acid Glycine* 



L-Cysteine L-Phenylalanine* 



L-Glutamic acid All D-amino acids 



L-Histidine 



L-Isoleucine 



L-Leucine 



L-Lysine 



L-Methionine 



L-Proline 



L-Serine 



L-Threonine 



L-Tryptophan 



L-Tyrosine 



L-Valine 



* Found in low concentration in 005 m KCl supernatant before precipitation 

 of pH 5 fraction. 



lyophilized calf thymus and chicken kidney tissues [21]. It was found that 

 the "non-aqueous" thymus nuclei contained slightly more activating 

 activity (in units per mg. dry weight) than the equivalent weight of whole 

 tissue. In kidney nuclei the enzyme concentration (in units per mg.) is 

 about half that of the whole tissue. In both instances, the high purity of 

 the preparations make it certain that the activating enzymes are of nuclear 

 origin and are not due to cytoplasmic contamination. 



Amino acid activating enzymes have also been detected in calf liver 

 nuclei following an isolation in dense sucrose solutions by the method of 

 Chauveau et al. [22]. These nuclei are characterized by a high degree of 

 morphological purity (but other tests have shown that the adsorption of 

 soluble cytoplasmic enzymes can occur in heavy sucrose solutions). In a 

 recent communication, Webster [23] has described the properties and 

 partial purification of an alanine-activating enzyme from pig liver nuclei. 

 Considering the variety of nuclear types successfully tested for activating 

 activity, the occurrence of amino acid activating enzymes in the cell 

 nucleus promises to be a widespread phenomenon. 



Ultracentrifugation experiments have shown that the activating 

 enzymes of the thymus nucleus tend to remain associated with the nuclear 

 ribosome fraction. As a result, they can be sedimented from neutral nuclear 

 extracts by centrifuging under conditions which bring down the ribonucleo- 



