Horne Donald W
Department of Veterans Affairs Medical Center, Nashville, TN 37212, USA.
J Nutr. 2003 Feb;133(2):476-8. doi: 10.1093/jn/133.2.476.
5,10-Methylenetetrahydrofolate occupies a key position in folate-dependent one-carbon metabolism. It is involved directly in the biosynthesis of deoxythymidine, it can be converted to 10-formyltetrahydrofolate for purine synthesis and it may be reduced to 5-methyltetrahydrofolate for methylation of homocysteine to methionine. We have developed a HPLC method for measuring 5,10-methylenetetrahydrofolate in liver and we have used this method to investigate two conditions that perturb one-carbon metabolism: 1) administration of methionine and 2) administration of the anesthetic gas, nitrous oxide (N(2)O). Rats were given 1.3 mmol/kg of methionine, and folate coenzymes in liver were measured. As expected, giving methionine resulted in an apparent increase in the concentration of 10-formyl- and tetrahydrofolate and an apparent decrease in 5-methyltetrahydrofolate concentration at 30 and 60 min. After 120 min, the concentrations of these coenzymes appeared to revert to control values. There was no apparent change in the concentration of 5,10-methylenetetrahydrofolate. Exposing rats to an atmosphere containing N(2)O results in inactivation of methionine synthase and accumulation of 5-methyltetrahydrofolate at the expense of other folate coenzymes. In liver from rats breathing N(2)O, 5-methyltetrahydrofolate increased, whereas there was no change in 5- or 10-formyltetrahydrofolates (P > 0.7 and P > 0.8, respectively). Tetrahydrofolate was not detected in liver from the N(2)O group, whereas it constituted 24% of folates in the control group. The concentration of 5,10-methylenetetrahydrofolate was not significantly affected by N(2)O (P > 0.18). These results suggest that the concentration of 5,10-methylenetetrahydrofolate is tightly regulated in liver.
5,10-亚甲基四氢叶酸在依赖叶酸的一碳代谢中占据关键地位。它直接参与脱氧胸苷的生物合成，可转化为10-甲酰四氢叶酸用于嘌呤合成，还可能还原为5-甲基四氢叶酸用于将同型半胱氨酸甲基化生成甲硫氨酸。我们开发了一种用于测定肝脏中5,10-亚甲基四氢叶酸的高效液相色谱法，并利用该方法研究了两种扰乱一碳代谢的情况：1）给予甲硫氨酸；2）给予麻醉气体一氧化二氮（N₂O）。给大鼠注射1.3 mmol/kg的甲硫氨酸，并测定肝脏中的叶酸辅酶。正如预期的那样，给予甲硫氨酸导致在30分钟和60分钟时10-甲酰四氢叶酸和四氢叶酸的浓度明显升高，而5-甲基四氢叶酸的浓度明显降低。120分钟后，这些辅酶的浓度似乎恢复到对照值。5,10-亚甲基四氢叶酸的浓度没有明显变化。将大鼠暴露于含N₂O的环境中会导致甲硫氨酸合酶失活，并以其他叶酸辅酶为代价使5-甲基四氢叶酸积累。在吸入N₂O的大鼠肝脏中，5-甲基四氢叶酸增加，而5-或10-甲酰四氢叶酸没有变化（分别为P＞0.7和P＞0.8）。在N₂O组的肝脏中未检测到四氢叶酸，而在对照组中它占叶酸的24%。5,10-亚甲基四氢叶酸的浓度不受N₂O的显著影响（P＞0.18）。这些结果表明，肝脏中5,10-亚甲基四氢叶酸的浓度受到严格调控。
