Moran R G, Mulkins M, Heidelberger C
Proc Natl Acad Sci U S A. 1979 Nov;76(11):5924-8. doi: 10.1073/pnas.76.11.5924.
Methotrexate (MTX) inhibition of the growth of mouse or human leukemia cells in culture was partially prevented by either thymidine (dThd) or hypoxanthine. 5-Fluoro-2'-deoxyuridine (FdUrd) also decreased the growth-inhibitory potency of MTX in the presence of small concentrations of 5-formyltetrahydrofolate (citrovorum factor) and sufficient exogenous dThd to support the synthesis of thymidylate nucleotides by salvage mechanisms. In addition, citrovorum factor-induced reversal of MTX was several orders of magnitude more efficient in the presence of both FdUrd and dThd than in the presence of dThd alone or in the absence of both nucleosides. Likewise, the presence of FdUrd (3 microM) and dThd (5.6 microM) completely prevented the lethality of 0.3 mM MTX to L1210 cells in culture medium supplemented with micromolar concentrations of citrovorum factor. We propose that this protection against the cytotoxic effects of MTX by dThd, hypoxanthine, and FdUrd have a common biochemical mechanism--namely, inhibition of the de novo synthesis of thymidylate by either a direct [FdUrd; inhibition of thymidylate synthetase (thymidylate synthase; 5,10-methylenetetrahydrofolate:dUMP C-methyl-transferase, EC 2.1.1.45)] or indirect (dThd and hypoxanthine; feedback inhibition by anabolites on ribonucleotide reductase and deoxycytidylate deaminase) effect. The resultant decreased rate of loss of reduced folates due to de novo thymidylate synthesis would allow a higher degree of inhibition of dihydrofolate reductase to be endured without damage to the cell.
在培养过程中，胸苷（dThd）或次黄嘌呤可部分阻止甲氨蝶呤（MTX）对小鼠或人白血病细胞生长的抑制作用。在存在低浓度的5-甲酰四氢叶酸（亚叶酸）和足够的外源性dThd以通过补救机制支持胸苷酸核苷酸合成的情况下，5-氟-2'-脱氧尿苷（FdUrd）也会降低MTX的生长抑制效力。此外，在同时存在FdUrd和dThd的情况下，亚叶酸诱导的MTX逆转比单独存在dThd或不存在这两种核苷时效率高几个数量级。同样，在添加微摩尔浓度亚叶酸的培养基中，FdUrd（3 microM）和dThd（5.6 microM）的存在完全阻止了0.3 mM MTX对L1210细胞的致死作用。我们提出，dThd、次黄嘌呤和FdUrd对MTX细胞毒性作用的这种保护具有共同的生化机制，即通过直接作用[FdUrd；抑制胸苷酸合成酶（胸苷酸合成酶；5,10-亚甲基四氢叶酸：dUMP C-甲基转移酶，EC 2.1.1.45）]或间接作用（dThd和次黄嘌呤；合成代谢产物对核糖核苷酸还原酶和脱氧胞苷酸脱氨酶的反馈抑制）抑制胸苷酸的从头合成。由于胸苷酸从头合成导致的还原型叶酸损失率降低，将允许在不损害细胞的情况下承受更高程度的二氢叶酸还原酶抑制。
