412 R. M. S. SMELLIE 



sap. They found that incorporation of both isotopes into the PNA of the 

 cell sap was appreciably greater than into either of the particulate fractions, 

 the difference between which was very small. The renewal of the pyrimidine 

 bases in the PNA from the mitochondria and residual cytoplasm of rat 

 liver homogenates has been studied with the aid of orotic acid.*° Here again, 

 at various time intervals, the mitochondria exhibited lower activities than 

 the residual cytoplasm, which was presumably composed of both micro- 

 somes and cell sap. Observations by Smellie et al.^ provide results for the 

 relative activities of the PNA's from the cytoplasmic fractions of normal, 

 regenerating, and weanling rat liver as well as from normal, maternal, and 

 fetal rabbit liver. In every instance the uptake of P^- was greatest in the 

 PNA from the cell sap, while in normal, regenerating, or weanling rat liver 

 the specific activities of the PNA's from the mitochondria were equal to 

 or slightly higher than those of the PNA's from the microsomes. The same 

 pattern was observed with normal and maternal rabbit liver, but in fetal 

 rabbit liver the isotope content of the microsomal PNA was considerably 

 higher than that of the mitochondrial PNA. 



Tyner et al.,^^ in an important contribution, have examined the specific 

 activities of the nucleotides and bases derived from the PNA of rat liver 

 and Flexner-Jobling tumor nuclei and cytoplasmic fractions after the 

 simultaneous administration of P^^ and glycine-2-d They found that at 

 2 hr. in both liver and tumor, as well as in the livers from tumor-bearing 

 animals, the uptake of P^- and C* by the PNA of the cell sap was appre- 

 ciably higher than in either of the particulate fractions. The PNA of the 

 mitochondria invariably showed lower activity than that of the microsomes 

 at short time intervals (2 to 5 hr.), but at longer time intervals this dif- 

 ference became very small. Tyner et al.^^ also found that the specific ac- 

 tivity of the phosphorus in the PNA nucleotides was appreciably higher 

 than that of the carbon in the PNA bases. 



Recently, Wikramanayake et al.^°^ have followed up their previous 

 studies*^ on the effect of different levels of energy intake on liver PNA 

 metabolism with similar work on the PNA of the different parts of the liver 

 cell. With animals maintained on a protein-free diet, an increase in the 

 level of energy intake brought about an increase in the uptake of P'^ by 

 the PNA of the liver cell. During the first 2 hr. after administration of the 

 isotope, this change was found to be accounted for by an increased uptake 

 by the PNA of the nucleus. At later time intervals (4 and 18 hr.), all sub- 

 cellular fractions exhibited the higher incorporation of labeled phosphorus 

 into the PNA when the level of energy intake was raised. This effect was 

 less marked on the PNA of the cell sap than on that of the nucleus or of 

 the particulate fractions. 



106 -p. W. Wikramanayake, F. C. Heagy, and H. N. Munro, Biochem. et Biophys. Ada 

 11,566 (1953). 



