CHEMISTRY OF PURINES AND PYRIMIDINES 89 



Uracil was also isolated from the nucleic acids of wheat germ ("Tritico- 

 nuclein"*^), calf thymus and herring sperm,*'' and beef spleen,*^ although 

 it was suspected*" '^^ that the uracil might have arisen from cytosine^''^2,62 

 as a result of hydrolysis (10% H2SO4 at 150°). I^racil is now recognized as 

 a universal constituent of the nucleic acids of only the pentose (or ribose) 

 type (Chapters 4 and 11). Uracil occurs naturally in the free form in ergot*' 

 and in more complex combinations (see below). 



Until 1925, the composition of the nitrogenous constituents of the nucleic 

 acids was described only in terms of the five pyrimidines and purines dis- 

 cussed above; in that year, Johnson and Coghill reported*^ the occurrence 

 of a homologue of cytosine, o-methylcytosine (XI V),*^ among the hydro- 

 lytic products of a nucleic acid from tubercle bacilli, tuberculinic acid. 

 (Johnson had been seeking this substance in natural sources for some 

 twenty-one years.) It was concluded** that the picrate of the isolated 

 product was crystallographically identical with that of synthetic 5-methyl- 

 cytosine,** but no melting point was recorded for comparison. In 1949, an 

 examination of "5-methylcytosine" prepared by Johnson revealed*^ it to be 

 a mixture consisting mainly of cytosine. Although a careful chromatographic 

 study*'^ of the DNA from avian tubercle bacilli did not confirm the early 

 report,** a paper chromatogram of an hydrolysate of DNA from calf 

 thymus revealed the presence of a small component ("epicytosine"), the 

 properties of which led Hotchkiss*^ to the tentative view that it might be 

 5-methylc>i:osine. Wyatt*^''" has found 5-methylc>i;osine as a definite, 

 though minor, constituent of the DNA's of mammalian, fish, and insect 

 sources, and as a major pyrimidine component of wheat germ DNA. The 

 latter finding has been confirmed .'^^ It could not be detected in bacteriaP" 



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