Department Clinical Chemistry, VU University Medical Center, Amsterdam, The Netherlands.
Clin Chem Lab Med. 2013 Aug;51(8):1643-50. doi: 10.1515/cclm-2012-0694.
In randomized trails, folic acid (FA) lowered plasma homocysteine, but failed to reduce cardiovascular risk. We hypothesize this is due to a discrepancy between plasma and intracellular effects of FA.
In a double-blind trial, 50 volunteers were randomized to received 500 µg FA daily for 8 weeks, or placebo. Plasma and peripheral blood mononuclear cell (PBMC) concentrations of homocysteine, S-adenosylmethionine (SAM), S-adenosylhomocysteine, methionine, cystathionine and 5-methyltetrahydrofolate (bioactive folate) were measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS). PBMCs were used as a cellular model since they display the full spectrum of one-carbon (1C) enzymes and reactions.
At baseline, plasma concentrations were a poor reflection of intracellular concentrations for most 1C metabolites, except 5-methyltetrahydrofolate (R=0.33, p=0.02), homocysteine (Hcy) (R=0.35, p=0.01), and cystathionine (R=0.45, p=0.001). FA significantly lowered plasma homocysteine (p=0.00), but failed to lower intracellular homocysteine or change the concentrations of any of the other PBMC 1C metabolites. At baseline, PBMC homocysteine concentrations correlated to PBMC SAM. After FA supplementation, PBMC homocysteine no longer correlated with PBMC SAM, suggesting a loss of SAM's regulatory function. In vitro experiments in lymphoblasts confirmed that at higher folate substrate concentrations, physiological concentrations of SAM no longer effectively inhibit the key regulatory enzyme methylenetetrahydrofolate reductase (MTHFR).
FA supplementation does not reduce intracellular concentrations of Hcy or any of its closely related substances. Rather, FA may disturb physiological regulation of intracellular 1C metabolism by interfering with SAM's inhibitory effect on MTHFR activity.
在随机试验中，叶酸（FA）降低了血浆同型半胱氨酸水平，但未能降低心血管风险。我们假设这是由于 FA 的血浆和细胞内作用之间存在差异。
在一项双盲试验中，50 名志愿者随机分为每日接受 500μg FA 或安慰剂 8 周。通过液相色谱-串联质谱（LC-MS/MS）测量血浆和外周血单核细胞（PBMC）中同型半胱氨酸、S-腺苷甲硫氨酸（SAM）、S-腺苷同型半胱氨酸、蛋氨酸、胱硫醚和 5-甲基四氢叶酸（生物活性叶酸）的浓度。由于 PBMC 显示出一碳（1C）酶和反应的全谱，因此它们被用作细胞模型。
在基线时，大多数 1C 代谢物的血浆浓度与细胞内浓度相关性较差，除了 5-甲基四氢叶酸（R=0.33，p=0.02）、同型半胱氨酸（Hcy）（R=0.35，p=0.01）和胱硫醚（R=0.45，p=0.001）。FA 显著降低了血浆同型半胱氨酸（p=0.00），但未能降低细胞内同型半胱氨酸或改变 PBMC 中任何其他 1C 代谢物的浓度。在基线时，PBMC 同型半胱氨酸浓度与 PBMC SAM 相关。FA 补充后，PBMC 同型半胱氨酸不再与 PBMC SAM 相关，表明 SAM 的调节功能丧失。在淋巴细胞的体外实验中证实，在较高的叶酸底物浓度下，生理浓度的 SAM 不再能有效抑制关键的调节酶亚甲基四氢叶酸还原酶（MTHFR）。
FA 补充并不能降低细胞内 Hcy 或其任何密切相关物质的浓度。相反，FA 可能通过干扰 SAM 对 MTHFR 活性的抑制作用，干扰细胞内 1C 代谢的生理调节。
