398 B. MAGASANIK 



TABLE IV 

 Nucleotide Composition of PNA Isolated from Different Organs of Calf 



Prepa- Adenylic Guanylic Cytidylic Uridylic 



ration Organ acid acid acid acid Pu/Py Method" Ref. 



1 Liver 10 17.9 14.9 8.4 1.20 3 25 



2 Pancreas 10 34.5 16.8 9.5 1.69 3 25 



3 Spleen 10 19.7 17.7 8.6 1.13 3 25 



4 Thymus 10 23.8 13.9 6.5 1.65 3 25 



° See page 395. 



be discovered. Some of the observed differences are undoubtedly due to 

 differences in the method of isolation. Thus different values are reported 

 for the cytidylic acid content of calf liver PNA isolated in different labora- 

 tories (preparations 16 and 17). Of greater interest are the differences in 

 the composition of PNA preparations isolated by the same method. A 

 large number of preparations were isolated from the livers of different 

 rabbits and rats by Davidson and his collaborators.®^ The individual 

 variations in the composition of liver PNA of different rabbits (prepara- 

 tions 1-7) were found to be greater than the differences between the average 

 composition of rabbit liver PNA and rat liver PNA. The only PNA prepa- 

 ration which differs significantly from all the others in composition is that 

 isolated from a human liver (preparation 23). It seems, however, unwar- 

 ranted to attach significance to an isolated observation. 



In Table IV the composition of PNA preparations obtained from differ- 

 ent organs of the same animal are compared. Calf spleen PNA can be seen 

 to differ little from liver PNA, whereas thymus and particularly pancreas 

 PNA are considerably richer in guanylic acid. In consequence, the purine- 

 to-pyrimidine ratios of calf thymus PNA and of pancreas PNA approach 

 2.0. The high guanylic acid content of pancreas PNA has also been ob- 

 served in other laboratories. However, the preparations isolated by different 

 procedures vary greatly in composition (Table V). This variation may 

 partly be ascribed to the extent of degradation by pancreatic ribonuclease 

 which PNA undergoes during the course of the isolation procedure. The 

 influence of the action of the enzyme on the composition of the final pro- 

 duct is clearly demonstrated by comparison of preparations 3 and 4 in 

 Table V. The former was prepared in the usual manner, while the latter 

 was isolated from the tissue after removal of ribonuclease by extraction 

 with dilute acid and acetone ;^^ preparation 4 is considerably richer in 

 pyrimidines and seems to be essentially identical in composition with 

 liver PNA. 



The action of pancreatic ribonuclease on PNA preparations isolated 



