Synthetic analogs of purines, pyrimidines, nucleosides, and nucleotides altered in either the heterocyclic ring or the sugar moiety have numerous applications in clinical medicine. Their toxic effects reflect either inhibition of enzymes essential for nucleic acid synthesis or their incorporation into nucleic acids with resulting disruption of base-pairing. Oncologists employ 5-fluoro- or 5-iodouracil, 3-deoxyuridine, 6-thioguanine and 6-mer-captopurine, 5- or 6-azauridine, 5- or 6-azacytidine, and 8-azaguanine (Figure 33-12), which are incorporated into DNA prior to cell division. The purine analog allopurinol, used in treatment of hyperuricemia and gout, inhibits purine biosynthesis and xanthine oxidase activity. Cytarabine is used in chemotherapy of cancer. Finally, azathioprine, which is catabolized to 6-mercap-topurine, is employed during organ transplantation to suppress immunologic rejection.
HO H 5-Iodo-2'-deoxyuridine
HO OH 6-Azauridine
Figure 33-12. Selected synthetic pyrimidine and purine analogs.
Nonhydrolyzable Nucleoside Triphosphate Analogs Serve as Research Tools
Synthetic nonhydrolyzable analogs of nucleoside triphosphates (Figure 33-13) allow investigators to distinguish the effects of nucleotides due to phosphoryl transfer from effects mediated by occupancy of allosteric nucleotide-binding sites on regulated enzymes.
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