246 GEORGE H. HOGEBOOM AND WALTER C. SCHNEIDER 



nucleotide composition of the PNA of the soluble fraction is essentially the 

 same as that of the PNA of other cytoplasmic fractions.^"^-"'' 



Although it is evident that a considerable amount of information is 

 available concerning the intracellular distribution of PNA and the com- 

 position of the PNA of various cell fractions, the significance of these re- 

 sults remains uncertain in the absence of definitive data relating to the 

 metabolic function of PNA. The fact that certain viruses are similar to 

 microsomes in their high PNA content (and general physical properties) 

 apparently initiated the suggestion that the latter particles may be the 

 center of protein synthesis within the cell. Although this hypothesis has 

 received some experimental support/"** the analogy on which it was based 

 does not seem to hold. Thus in certain tissues other than liver (e.g., adult 

 kidney^*^' and spleen-"^ and embryonic tissues^^**), PNA is associated mainly 

 with fractions that are nonmicrosomal in sedimentation characteristics. 

 Furthermore, studies of PNA turnover demonstrate merely that micro- 

 somal PNA is renewed relatively slowly^^ ■-"* and thus fail to reveal any 

 clue as to its metabolic role. The recent experiments of Binkley,^'^ indicating 

 that a preparation of protein-free ribose polynucleotide can catalyze an en- 

 zymic reaction, offer the greatest promise toward an eventual understanding 

 of the function of the PNA of cell structures. 



"Ds M. L. Peterraann and E. J. Mason, Proc. Soc. Exptl. Biol. Med. 69, 542 (1948). 



210 J. Brachet and R. Jeener, Enzymologia 11, 196 (1944). 



211 F. Binkley, Exptl. Cell Research Suppl. 2, 145 (1952). 



