THE CYTOPLASM 245 



late structures.^^ Subsequent experiments, in which improved procedures 

 were used for the fractionation of Uver homogenates, have demonstrated 

 that about 50 % of the PNA of hver is associated with the isolated micro- 

 somes (Table VIII). This fraction is the only one in which the concentration 

 of PNA exceeds that in the original tissue.^^'**'^''""'^ 



On a number of occasions, the question has arisen as to whether the 

 PNA of the nuclear, mitochondrial, and soluble fractions might be a re- 

 flection of contamination by microsomes. That cell nuclei contain PNA 

 has been firmly estabhshed, however, through several independent lines 

 of evidence. Thus, in vivo studies of the rate of incorporation of isotopes 

 have demonstrated that the turnover of nuclear PNA is much more rapid 

 than that of the PNA of any other cell fraction.^^'^^'^o* Furthermore, there 

 are striking differences in the nucleotide composition of nuclear and cyto- 

 plasmic PNA.^''^'-^^ This question is discussed further in Chapters 26 and 28. 



The significance of the presence of PNA in the mitochondrial fraction of 

 liver is still open to some question. Attempts to remove the PNA by re- 

 peated sedimentation of mitochondria have been unsuccessful,^^ however, 

 and further fractionation both of mitochondria^'^ and of homogenates^^ has 

 failed to provide evidence that the PNA of the fraction is associated with 

 extraneous elements. Equivocal results have been obtained in comparisons 

 of the composition of mitochondrial PNA with the composition of the PNA 

 of other fractions. Crosbie el al?'^^ have found essentially no differences, 

 whereas Elson and Chargaff,^^^ in a preliminary report, have noted dissimi- 

 larities in nucleotide content. The low concentration of PNA in mitochon- 

 dria and the possibility of contamination of the preparations by other 

 PNA-containing structures have led the latter investigators to express 

 some reservations as to the significance of their results. Smelhe et al.^* 

 have reported essentially no differences in the rate of incorporation of P'^ in 

 mitochondrial and microsomal PNA obtained from normal or regenerating 

 liver. Khesin,2<'8 on the other hand, has found that the rate of incorporation 

 in normal liver is slower for mitochondrial than for microsomal PNA but 

 that in regenerating liver the mitochondrial PNA shows a more rapid 

 turnover than does the PNA of any other cytoplasmic fraction. 



An appreciable amount of PNA of liver is present in the soluble fraction 

 (Table VIII). That this "soluble" PNA is different from the PNA of the 

 other liver fractions is indicated by the fact that its rate of turnover is 

 second only to that of the nuclear PNA (cf. footnotes 63, 64, 208). The 



206 A. Marshak and F. Calvet, J. Cellular Comp. Physiol. 34, 451 (1949). 



2»« G. W. Crosbie, R. M. S. Smellie, and J. N. Davidson, Biocheiyi. J. 54, 287 (1953). 



2" D. Elson and E. Chargaff in "Phosphorus Metabolism" (McElroy and Glass, eds.), 



p. 329. Johns Hopkins Press, Baltimore, 1952. 

 ^os R. V. Khesin, Biokhimiya 17, 664 (1952). 



