metabolism of the nucleic acids 411 



2. The PNA's of the Cytoplasm 



The distribution of PXA in the cell cytoplasm has been studied by a 

 large number of workers and the subject has been reviewed in Chapter 21. 

 It appears from these studies that the main bulk of the cytoplasmic PNA 

 is located in the microsomal fraction while lesser amounts are found in the 

 mitochondrial fraction and in the nonsedimentable material (the cell sap). 

 The composition of the PNA's from these different cytoplasmic fractions 

 has been shown to be the same,'"-'^''^ and their metabolic activities have 

 been the subject of considerable research. 



Marshak and Calvet^^ have studied the uptake of P^^ by the PXA's of 

 the large and small particles obtained by differential centrifugation of 

 homogenates of rabbit liver in dilute citric acid, and have observed that 

 at time intervals up to 12 hr. the activity of the microsomal PNA exceeded 

 that of the mitochondrial PNA. Similar results have been obtained by 

 Jeener^^ and by .Teener and Szafarz^' for the fractions isolated from buffered 

 homogenates of rat liver. With mouse embryo, on the other hand, the ac- 

 tivity of the mitochondrial PNA was found to exceed that of the micro- 

 somes. In both cases, Jeener and his co-workers have found that the ac- 

 tivity of the nonsedimentable PNA exceeds that of either of the particulate 

 fractions. A very thorough investigation of this topic has been made by 

 Barnum and Huseby,"'^* who have studied the incorporation of P^- into 

 the PNA of the cytoplasmic fractions of fasted mouse liver and mammary 

 carcinoma. These authors found no significant difference between the 

 specific activities of the PNA's from the two particulate fractions at any 

 time interval up to 24 hr. after administration of the isotope. At all times, 

 however, the activity of the PNA of the cell sap was observed to be con- 

 siderably higher than that of the particulate PNA. Reichard,^"^ working 

 with regenerating rat liver and glycine-N^^ found no difference in isotope 

 uptake by the same base in each of the cytoplasmic fractions. Hultin et al.,^°^ 

 using P^- and glycine labeled with N^* in studies on the PNA of the cyto- 

 plasmic fractions of chicken liver, found that in both sets of experiments 

 the cell sap contained more isotope than did either mitochondria or micro- 

 somes. In general, the activity of the microsomal PNA proved to be higher 

 than that of the mitochondria, although with P'^ the difference between 

 the cell sap and the granules was very much greater than that between 

 mitochondria and microsomes. Glycine-N^^ and formate-C^"* have been 

 used by Smellie et al.^^ as PNA precursors in investigations of the relative 

 metabolism of the PNA's from the mitochondria, microsomes, and cell 



102 G. W. Crosbie, R. M. S. Smellie, and J. N. Davidson, Biochem. J. 54, 286 (1953). 



1" D. Elson and E. ChargafF, Federation Proc. 10, 180 (1951). 



1"^ P. Reichard, Acta Chem. Scand. 4, 861 (1950). 



"s T. Hultin, D. B. Slautterback, and G. Wessel. Exptl. Cell Research. 2, 696 (1951). 



