ISOLATION AND COMPOSITION OF PENTOSE NUCLEIC ACIDS 375 



plished. The soluble PNA found in the supernatant fluid after sedimenta- 

 tion of the microsomes is also bound to protein.^" 



The PNA-lipoprotein complexes which constitute the particulate frac- 

 tions of animal cells may be isolated by fractional centrifugation of tissue 

 homogenate.s. Analogous procedures may be used for the isolation of some 

 animal viruses and of all plant viruses which resemble the cytoplasmic 

 particles in size, composition, and distribution in the host cell. Alterna- 

 tively, the tissue may be extracted with isotonic saline and the cytoplasmic 

 PNA-proteins isolated by isoelectric precipitation of the extracts at a pH 

 of about 5. The soluble PNA-proteins as well as the microsomes and mito- 

 chondria are obtained together in this procedure. This method can be used 

 for the isolation of PNA-proteins from the organs of animals and from 

 yeast, and for the isolation of plant viruses. Other methods, such as the 

 fractionation of tissue extracts with ammonium sulfate or sodium sulfate, 

 have been useful in special cases. 



2. Isolation of Nucleoprotein from Animal Tissues 



a. Fractional Centrifugation 



This method for the isolation of the cytoplasmic nucleoproteins is de- 

 scribed in Chapter 21. Similar procedures were used for the isolation of the 

 virus of eciuine encephalomyelitis from infected chick embryos by Taylor 

 et al}^ The virus was found to be composed of protein, phospholipid, and 

 PNA. 



6. Extraction with 0.14 ^^ Sodium Chloride Followed by Isoelectric Precipi- 

 tation 



This method takes advantage of the fact that PNA-protein but not 

 DNA-protein is readily soluble in 0.14 M sodium chloride. The PNA- 

 protein is subsequently precipitated by addition of acid to a pH of 4-5. 

 The preparation of calf thymus PNA-protein by this procedure has been 

 described by Mirsky and Pollister,^^ and that of pancreas PNA-protein by 

 Kerr and Seraidarian.'^ These preparations consisted of mixtures of particu- 

 late and soluble nucleoproteins, and were not further characterized but 

 used directly for the preparation of free PNA. 



c. Salt Fractionation 



This method has not been used extensively. It does, however, deserve 

 attention because of its recent successful application by Hamoir to the 



'" D. Szafarz, Biochim. et Biophys. Acta 6, 562 (1951). 



" A. R. Taylor, D. G. Sharp, D. Beard, and J. W. Beard, J. Infectious Diseases 72, 

 31 (1943). 



12 A. E. Mirsky and A. W. Pollister, J. Gen. Physiol. 30, 101 (1946). 



13 S. E. Kerr and K. Seraidarian, J. Biol. Chem. 180, 1203 (1949). 



