Pfizer Handbook of Microbial Metabolites 508 



m. PYRIMIDINES 



Pyrimidines are fundamental components of living 

 cells. They have long been recognized as constituents of 

 nucleic acids, and more recently other functions have 

 been discovered. 



Microorganisms are rather rich in nucleoproteins. 

 Yeast, which has been a common experimental source, 

 contains about 4 percent of its dry weight in nucleic 

 acids, and bacteria up to 15 percent. Bacteriophages are 

 largely nucleoprotein, and certain plant viruses entirely. 

 By contrast, thymus gland, one of the richer animal tissue 

 sources, contains only about 3 percent. 



The protein moieties often are relatively low in molecu- 

 lar weight, some of them qualifying as large peptides, and 

 they generally seem to be rich in basic amino acids. The 

 total nucleoprotein molecular weights, however, are very 

 high — often running to many millions. The complexity 

 of the nucleic acid moiety varies wdth the complexity of 

 the species. Since the DNA carries the genetic informa- 

 tion, it might be expected to be more complex and higher 

 in molecular weight for the human species than, for ex- 

 ample, in a simple plant virus. 



Two types of nucleic acids have been distinguished, 

 both widely distributed. Ribose nucleic acid (RNA) and 

 deoxyribose nucleic acid (DNA) have been mentioned 

 earlier in connection with their roles in protein synthesis 

 and genetics. 



Neither of these substances has been obtained entirely 

 pure, but newer techniques such as electrophoresis and 

 paper chromatography have permitted refinements. The 

 important purine and pyrimidine components of RNA are 

 adenine, "guanine, cytosine and uracil. In DNA thymine 

 takes the place of uracil, and 5-methylcytosine is a minor 



NH 



V Hn"T\ N^l HN 



H HoN H O H O H 



Adenine Guanine Cytosine Uracil 



(6-amino- (2-amino-6- (2-oxy-6-amino- (2,6-dioxy- 



purine) oxypurine) pyrimidine) pyrimidine) 



component in some species. 



