Читать книгу Plant Nucleotide Metabolism - Hiroshi Ashihara - Страница 25

Purines and pyrimidines are major chemical constituents of cells and occur primarily as components of DNA and RNA (polymerized nucleotides), and to a much lesser extent in the form of ‘free’ (aka ‘soluble’) nucleotides. Compared with nucleotides, free nucleosides and bases usually represent a very small fraction of the total purine and pyrimidine content of living cells, including plant cells. However, there are exceptions to this generalization, including the occurrence of substantial amounts caffeine (1) in plants such as coffee (Coffea arabica and Coffea robusta) and tea (Camellia sinensis), and theobromine (2) in cocoa (Theobroma cocao). Although not found in plants, the arabinosides of both thymine (3) and uracil (4) have been detected in the Caribbean sponge, Cryptotethya crypta. Guanine, in the silvery constituent of fish scales, and the excretion product, uric acid, are both free purines found in animal cells, but only occur in substantial amounts extracellularly (Henderson and Paterson 1973).