Anguera Montserrat C, Field Martha S, Perry Cheryll, Ghandour Haifa, Chiang En-Pei, Selhub Jacob, Shane Barry, Stover Patrick J
Graduate Field of Biochemistry, Molecular and Cellular Biology, Cornell University, Ithaca, New York 14853, USA.
J Biol Chem. 2006 Jul 7;281(27):18335-42. doi: 10.1074/jbc.M510623200. Epub 2006 Apr 20.
10-Formyltetrahydrofolate dehydrogenase (FDH) catalyzes the NADP(+)-dependent conversion of 10-formyltetrahydrofolate to CO(2) and tetrahydrofolate (THF) and is an abundant high affinity folate-binding protein. Although several activities have been ascribed to FDH, its metabolic role in folate-mediated one-carbon metabolism is not well understood. FDH has been proposed to: 1) inhibit purine biosynthesis by depleting 10-formyl-THF pools, 2) maintain cellular folate concentrations by sequestering THF, 3) deplete the supply of folate-activated one-carbon units, and 4) stimulate the generation of THF-activated one-carbon unit synthesis by channeling folate cofactors to other folate-dependent enzymes. The metabolic functions of FDH were investigated in neuroblastoma, which do not contain detectable levels of FDH. Both low and high FDH expression reduced total cellular folate concentrations by 60%, elevated rates of folate catabolism, and depleted cellular 5-methyl-THF and S-adenosylmethionine levels. Low FDH expression increased the formyl-THF/THF ratio nearly 10-fold, whereas THF accounted for nearly 50% of total folate in neuroblastoma with high FDH expression. FDH expression did not affect the enrichment of exogenous formate into methionine, serine, or purines and did not suppress de novo purine nucleotide biosynthesis. We conclude that low FDH expression facilitates the incorporation of one-carbon units into the one-carbon pool, whereas high levels of FDH expression deplete the folate-activated one-carbon pool by catalyzing the conversion of 10-formyl-THF to THF. Furthermore, FDH does not increase cellular folate concentrations by sequestering THF in neuroblastoma nor does it inhibit or regulate de novo purine biosynthesis. FDH expression does deplete cellular 5-methyl-THF and S-adenosylmethionine levels indicating that FDH impairs the folate-dependent homocysteine remethylation cycle.
10-甲酰四氢叶酸脱氢酶（FDH）催化10-甲酰四氢叶酸依赖NADP（+）转化为CO（2）和四氢叶酸（THF），是一种丰富的高亲和力叶酸结合蛋白。尽管FDH具有多种活性，但其在叶酸介导的一碳代谢中的代谢作用尚未完全明确。有人提出FDH具有以下作用：1）通过消耗10-甲酰-THF库来抑制嘌呤生物合成；2）通过螯合THF来维持细胞叶酸浓度；3）耗尽叶酸激活的一碳单位供应；4）通过将叶酸辅因子导向其他叶酸依赖性酶来刺激THF激活的一碳单位合成。在不含可检测水平FDH的神经母细胞瘤中研究了FDH的代谢功能。低水平和高水平的FDH表达均使细胞总叶酸浓度降低60%，提高了叶酸分解代谢速率，并耗尽了细胞内5-甲基-THF和S-腺苷甲硫氨酸水平。低水平的FDH表达使甲酰-THF/THF比值增加近10倍，而在高水平FDH表达的神经母细胞瘤中，THF占总叶酸的近50%。FDH表达不影响外源甲酸掺入甲硫氨酸、丝氨酸或嘌呤，也不抑制嘌呤核苷酸的从头生物合成。我们得出结论，低水平的FDH表达有利于一碳单位掺入一碳库，而高水平的FDH表达通过催化10-甲酰-THF转化为THF来耗尽叶酸激活的一碳库。此外，在神经母细胞瘤中，FDH不会通过螯合THF来增加细胞叶酸浓度，也不会抑制或调节嘌呤的从头生物合成。FDH表达确实会耗尽细胞内5-甲基-THF和S-腺苷甲硫氨酸水平，表明FDH损害了叶酸依赖性同型半胱氨酸再甲基化循环。
