410 



R. M. S. SMELLIE 



nucleotides in the nucleus or cytoplasm although adenylic acid usually 

 exhibits the highest and guanylic acid the lowest specific activity. 



Other workers have used labeled precursors of the purine and pyrimidine 

 bases in studies on the metabolism of nuclear and cytoplasmic PNA. 

 Potter et al}^^ and Hurlbert and Potter^" have examined the uptake of 

 orotic acid-6-C^'* into the PNA of the nucleus and cytoplasm of rat liver 

 from rats bearing Flexner-Jobling tumors (Fig. 3). They found that the 

 uptake of C^* by the pyrimidines was much more rapid in nuclear PNA than 

 in cytoplasmic PNA. Similar observations have been recorded by Ander- 

 son and Aqvist^^ using orotic acid-N^^. (see Table V Chapter 25) 



Payne et al}^ and Smellie et al.^^ using formate-C'^ have also observed 

 greater assimilation of the isotope into the PNA of the nucleus than into 



4 8 12 20 HOURS 91 



Fig. 3. T6e specific activities of the nuclear PNA, mitochondrial PNA, and super- 

 natant PNA of rat liver at various times after the administration of orotic acid-6- 



Q14 90A01 ; 



that of thp^cytoplasm (see Table V Chapter 25). In both nucleus and cyto- 

 plasm, adenylic acid was more heavily labeled than guanylic acid. 



Glycine labeled with N^* ^^ or C'^ ^® has also been used to demonstrate 

 that the uptake of isotope by nuclear PNA was greater in a given time than 

 by cytoplasmic PNA. The difference between the PNA's from the two 

 sources was more striking with glycine-2-C^'' than with glycine-N^*. 



All these observations leave no doubt as to the independent nature of 

 the PNA of the nucleus and its considerable metabolic importance. While 

 the proportion of the total PNA of the cell which resides in the nucleus is 

 small, the fact that it exhibits such a relatively high metabolic activity, 

 as evidenced by isotope experiments, means that the total turnover of 

 nuclear PNA in a given time is not very much smaller than the total turn- 

 over of cytoplasmic PNA in the same time. The full significance of this 

 metabolically active PNA remains something of a mystery; several sug- 

 gestions have been made to account for the observed phenomenon but 

 none of these is completely satisfactory. 



lo' V. R. Potter, R. O. Recknagel, and R. B. Hurlbert, Federation Proc. 10, 646 (1951). 



