THE NUCLEIC ACID CONTENT OF TISSUES AND CELLS 3 



to the source of the nucleic acids. ^^ In estimations of the sugar components, 

 the sugars of the purine nucleotides are mainly responsible for the intensity 

 of coloration. The ultraviolet absorbing properties of a nucleic acid also 

 depend to some extent on the proportions of purine and pyrimidines. Which- 

 ever of these two methods is employed, the procedure is simplified if the 

 results are expressed in terms of the P content of the standard PNA or 

 DNA. The average amount of P in PNA is 9.4 % as compared with the 

 9.9 % for DXA.i 



Until 1945, the nucleic acids were estimated largely as total nucleic acid 

 phosphorus (NAP)" or on the basis of their purine content.^*- ^^ Because 

 PNA and DNA differ in their location within the cell and in their physio- 

 logical beha\dor, information about the total nucleic acid content is in most 

 cases not very illuminating. 



When PNA is determined by the color reaction of its pentose with or- 

 cinoP" and DNA by the reaction of deoxypentose with diphenylamine,-^ sub- 

 stantial errors may arise if there is incomplete separation of the nucleic 

 acids from interfering tissue constituents, such as glucose, glycogen, muco- 

 polysaccharides, proteins, or amino acids. Da\ddson and Waymouth^- and 

 von Euler and Hahn^* tried to avoid such sources of error by extracting the 

 nucleic acids from tissue residues with sodium chloride and dilute alkaline 

 solutions and precipitating them as their lanthanum salts. But this pro- 

 cedure, apart from being tedious, is subject to errors^ and has been super- 

 seded by more convenient methods. 



The two most commonly employed are those of Schmidt and Thann- 

 hauser^ and of Schneider.^ The more recent method of Ogur and Rosen^* 

 appears to be of more limited application. The various fractions obtained 

 in these procedures are shown in Table I. All three methods involve a pre- 

 liminary extraction of acid-soluble compounds (fractions I and XIII) and 

 Hpids (II, XI, and XII). 



1. Procedure of Schmidt and Thannhauser (1945) 



In the Schmidt and Thannhauser^ method the PNA is separated from 

 DNA and most of the tissue protein, after overnight incubation at 37° with 

 normal alkali. Acidification of the alkaline digest III wdth trichloroacetic 



'* E. Chargaff, Chapter 10, and B. Magasanik, Chapter 11 in this book. 

 " I. Berenblum, E. Chain, and N. G. Heatley, Biochem. J. 33, 68 (1939). 

 i« N. Alders, Biochem. Z. 18, 400 (1927). 



15 T. B. Robertson and M. C. Dawbarn, Australian J. Exptl. Biol. Med. Set. 6, 261 

 (1929). 



20 W. Mejbaum, Z. physiol. Chem. 258, 117 (1939). 



21 Z. Dische, Mikrochemie 8, 4 (1930). 



" J. N. Davidson and C. Waymouth, Biochem. J. 38, 39 (1944). 

 " H. von Euler and L. Hahn, Svensk Kern. Tidskr. 58, 251 (1946). 

 " M. Ogur and G. Rosen, Arch. Biochem. 25, 262 (1950). 



