386 B. MAGASANIK 



inhibited by heparin,^^- ^^ but so far this polysaccharide has not been used 

 to protect PNA during isolation. It is claimed that the detergent sodium 

 dodecyl sulfate^^ or guanidine hydrochloride^^ will denature the enzyme, 

 and that the use of these substances for the separation of the PNA from 

 protein will protect PNA from ribonuclease. The only nucleic acid speci- 

 mens whose degradation by ribonuclease during isolation can be excluded 

 with certainty are those derived from plant viruses, as the nucleoprotein, 

 which is the form in which the PNA is separated from host components, 

 is resistant to the action of nucleolytic enzymes.^^ 



The effect of the damaging agents referred to so far, viz., alkali, acid, 

 and ribonuclease, is manifested by a change in the nucleotide composition 

 of the PNA, Other agents seem to affect only the physical properties of 

 PNA. The PNA obtained from plant viruses, whose protein moiety was 

 denatured by short heating or by dilute ethanol, are viscous and possess 

 a molecular weight of about 300,000.^7 On standing, such PNA prepara- 

 tions break down spontaneously into smaller particles. The isolation of 

 PNA preparations of similar viscosity, particle size, and instability from 

 the liver of rat, rabbit, and calf, has recently been reported.*^ By analogy 

 with the observations on DNA where viscosity and high particle weight 

 are correlated with biological activity, it is thought that such PNA prep- 

 arations are more nearly representative of native PNA than others of 

 small particle weight.''^ The preparations of PNA of high particle weight 

 lose their viscosity and rapid rate of sedimentation in solutions of high 

 sodium chloride content or upon heating.*^ The use of strong electrolyte 

 solutions and heat in the process of isolation should probably be avoided. 



In the succeeding sections the newer methods used for the isolation of 

 PNA are listed; several characteristic methods, which have served for the 

 preparation of the nucleic acids whose composition is discussed subse- 

 quently, are presented in detail. This chapter does not intend to give an 

 historical survey of the methods used for the isolation of PNA and will, 

 therefore, not describe the older methods in which the PNA was extracted 

 from tissue with dilute alkali. A description of these methods, which are 

 still used for the production of commercial yeast PNA, is found in Levene's 

 monograph. ^^ 



" J. S. Roth, Arch. Biochem. and Biophys. 44, 265 (1953). 



" N. Zollner and J. Fellig, Am. J. Physiol. 173, 223 (1953). 



« E. L. Grinnan and W. A. Mosher, J. Biol. Chem. 191, 719 (1951). 



^ It should be emphasized that this is only an assumption; it may be that the vis- 

 cous preparations are artifacts resulting from the aggregation of PNA molecules 

 in the course of isolation. 



" P. A. Levene and L. W. Bass, "Nucleic Acids." Chemical Catalog Company, New 

 York, 1931. 



