Pyrimidine (Metadiazine)s are heterocyclic, six-membered, nitrogen-containing carbon ring structures, with uracil, cytosine and thymine being the basal structures of ribose-containing nucleosides (uridine, cytidine, and thymidine respectively), or deoxyribose-containing deoxynucleosides, and their corresponding ribonucleotides or deoxyribonucleotides. Pyrimidines serve essential functions in human metabolism as ribonucleotide bases in RNA (uracil and cytosine), and as deoxyribonucleotide bases in DNA (cytosine and thymine), and are linked by phosphodiester bridges to purine nucleotides in double-stranded DNA, in both the nucleus and the mitochondria. Pyrimidine activated sugars are also involved in polysaccharide and phospholipid synthesis, glucuronidation in detoxification processes, glycosylation of proteins and lipids and in the recently identified novel endothelium-derived vasoactive dinucleotides.