394 B. MAGASANIK 



12% and 8%, respectively. More important than the actual values is the 

 atomic N/P ratio. A nucleic acid composed of equimolar amounts of the 

 four nucleotides would have an N/P ratio of 3.75. Preponderance of purines 

 over pyrimidines and of cytosine over uracil results in an increased N/P 

 ratio. PNA preparations from animal sources, particularly from pancreas, 

 have N/P ratios as. high as 4.3. N/P ratios higher than this are indicative 

 of contamination of the preparation with other nitrogenous substances, 

 generally with protein. 



The amount of inorganic phosphorus obtained after hydrolysis of PNA 

 with dilute acid is a measure of the purine content of the preparation, for 

 only purine-nucleoside-bound phosphate is hydrolyzed under these condi- 

 tions. Similarly, the estimation of pentose by the orcinol reaction is a 

 measure of purine-bound pentose. Pyrimidine nucleotides are not hydro- 

 lyzed under the conditions of this test, and consequently, the pyrimidine- 

 bound pentose does not react with orcinol. [Cf. Dische, Chapter 9.] 



It is usual to measure the amount of protein and DNA in the PNA 

 preparations by specific methods, such as the biuret test, and the diphenyl- 

 amine reaction, respectively. The PNA preparations which are obtained 

 by the isolation methods presented above are usually free from protein, 

 and do not contain more than 5% DNA. In general, contamination with 

 DNA is more effectively excluded when the nuclei are removed prior to 

 the isolation of PNA. The presence of DNA does not interfere with the 

 estimation of pentose nucleotides by paper chromatography or ionophore- 

 sis. [Cf. Chapters 7 and 8.] 



The colorimetric estimation of pentoses by the orcinol method does not 

 differentiate between ribose and other pentoses. However, the nature of 

 the sugar component may be determined after hydrolysis by isolation and 

 conversion into derivatives, or by paper chromatography in various sol- 

 vents. By these methods the pentose components of PNA preparations 

 isolated from mammalian organs,^ '^^'^^ from tobacco mosaic virus, ^^^ from 

 cucumber virus CV3," and from C. perfringens,^^ have been unequivocally 

 identified as D-ribose. At present, pentose nucleic acids isolated from these 

 sources may safely be called ribonucleic acids. 



VI. The Nucleotide Composition of PNA 



1. General Considerations 



The analysis of purified DNA preparations isolated from a great variety 

 of cells has led to the conclusion that (a) the deoxypentose nucleic acids 

 of different species of organisms have different nucleotide compositions, 

 (b) those of different organs of the same species have identical composi- 

 tions, and (c) the ratios of adenine to thymine and guanine to cytosine 



" R. Markham and J. D. Smith, Biochem. J. 46, 513 (1950). 



" D. L. MacDonald and C. A. Knight, /. Biol. Chem. 202, 45 (1953). 



