FIGURE 1. Metabolism of FU. DPD = dihydropyrimidine dehydrogenase; FUH₂ = dihydrofluorouracil; FUPA = -fluoro-ß-ureido-propanoic acid; FBAL = -fluoro-ß-alanine; BAL = ß-alanine; F^– = fluoride ion; FdUrd = fluorodeoxyuridine; FUrd = fluorouridine; FdUMP, FdUDP, and FdUTP = fluorodeoxyuridine mono-, di-, and triphosphate, respectively; FUMP, FUDP, and FUTP = fluorouridine mono-, di-, and triphosphate, respectively; TS, thymidylate synthase; F-DNA and F-RNA = fluorinated DNA and RNA, respectively; CH₂FH₄ = 5,10-methylene tetrahydrofolate. (Note that "F" in CH₂FH₄ symbolizes folate, not fluorine (6). Chemical structures of FU and its metabolites can be found in (4) and (7).)