ISOLATION AND COMPOSITION OF DEOXYPENTOSE NUCLEIC ACIDS 347 



about one-quarter of the cytosine is replaced by 5-methylcytosine.^^''-^'^^^ 

 The latter has been recognized by Wyatt*-^ '^'^ '^^^ as occurring in traces in 

 several deoxypentose nucleic acids of animal origin (compare also reports 

 of other investigators'"* •"'•^^''), after an older claim of its being a constitu- 

 ent of the nucleic acid of tubercle bacilli^^* could not be confirmed.'"' The 

 chemistry of these purines and pyrimidines is discussed in Chapter 3 and 

 that of the corresponding nucleosides and nucleotides in Chapter 4. Certain 

 bacteria and bacteriophages appear to be able to incorporate, from the 

 medium, unusual pyrimidines, such as 5-bromouracil and 5-iodouracil, in 

 their deoxypentose nucleic acids in replacement of part of the thymine i^"'^^** 

 but these substances can hardly qualify as normal constituents. 



3. Unidentified Constituents 



There is little sense in attempting a catalogue of our ignorance. As will 

 appear later, most of the nitrogen and phosphorus of those deoxypentose 

 nucleic acids that have been investigated in detail has been accounted for; 

 and the remaining 1 to 3 % may well be attributed to analytical imperfec- 

 tions. But only very few nucleic acids have been studied at all, and even 

 they may contain as yet unrecognized nucleotide satellites. What must be 

 avoided at the present stage is the grand sweep of induction that extracts 

 general laws from single observations on ill-defined, and sometimes not 

 even specified, preparations (often known as "commercial sperm nucleic 

 acid"). 



It is not impossible that a careful search for novel nucleosides or nucleo- 

 tides by means of ion-exchange chromatography (see Chapter 6) would be 

 more rewarding than the separation of the free nitrogenous constituents by 

 chromatography on filter paper (see Chapter 7). Studies of this type have 

 been carried out on a few deoxypentose nucleic acids ;^^^-^''^ they have 

 yielded no unexpected results. There exists, however, a body of analytical 

 observations concerning differences in the hydrolysis behavior of different 

 nucleic acids that may provide some clues.'""'* '^'''"'■^^' The most in- 

 teresting ob.servation, perhaps, concerns the behavior of the adenine nucleo- 

 tides of the deoxypentose nucleic acid of wheat germ."'* As judged from the 

 extent of its liberation by various hydrolyzing agents, adenine seems to 

 occur in two types of linkage. The predominant type is broken with equal 



"" G. R. Wyatt, Nature 166, 237 (1950). 



"6 G. R. Wyatt, Biochem. J. 48, 581 (1951). 



"6 T. B. Johnson and R. D. Coghill, J. Am. Chem. Soc. 47, 2838 (1925). 



"7 F. Weygand, A. Wacker, and H. Dellweg, Z. Nalurforsch. 7b, 19 (1952). 



"» D. B. Dunn and J. D. Smith, Biochem. J., 58, X (1954). 



"9 E. Volkin, J. X. Khym, and W. E. Cohn, J. Am. Chem. Soc. 73, 1533 (1951). 



"« R. L. Sinsheimer and J. F. Koerner, J. Biol. Chem. 198, 293 (1952). 



"' R. O. Hurst, A. M. Marko, and G. C. Butler, J. Biol. Chem. 204, 847 (1953). 



