Green J M, Ballou D P, Matthews R G
Department of Biological Chemistry, University of Michigan, Ann Arbor 48109.
FASEB J. 1988 Jan;2(1):42-7. doi: 10.1096/fasebj.2.1.3335280.
Most mammalian cells receive exogenous folate from the bloodstream in the form of 5-methyltetrahydropteroylmonoglutamate (CH3-H4PteGlu1). Because this folate derivative is a very poor substrate for folylpolyglutamate synthetase, the enzyme that adds glutamyl residues to intracellular folates, CH3-H4PteGlu1 must first be converted to tetrahydropteroylmonoglutamate (H4PteGlu1), 10-formyltetrahydropteroylmonoglutamate (CHO-H4PteGlu1), or dihydrofolate (H2folate), which are excellent substrates for folylpolyglutamate synthetase. Polyglutamylation is required both for retention of intracellular folates and for efficacy of folates as substrates for most folate-dependent enzymes. Two enzymes are known that will react with CH3-H4PteGlu1 in vitro, methylenetetrahydrofolate reductase and methyltetrahydrofolate-homocysteine methyltransferase (cobalamin-dependent methionine synthase). These studies were performed to assess the possibility that methylenetetrahydrofolate reductase might catalyze the conversion of CH3-H4PteGlu1 to CH2-H4PteGlu1. CH2-H4PteGlu1 is readily converted to CHO-H4PteGlu1 by the action of methylenetetrahydrofolate dehydrogenase/methenyltetrahydrofolate cyclohydrolase, and these enzyme activities show very little preference for folypolyglutamate substrates as compared with folylmonoglutamates. We conclude from in vitro studies of the enzyme that methylenetetrahydrofolate reductase cannot convert CH3-H4PteGlu1 to CH2-H4PteGlu1 under physiological conditions and that uptake and retention of folate will be dependent on methionine synthase activity.
大多数哺乳动物细胞从血液中以5-甲基四氢蝶酰单谷氨酸（CH3-H4PteGlu1）的形式接收外源性叶酸。由于这种叶酸衍生物是叶酸多聚谷氨酸合成酶（一种将谷氨酰残基添加到细胞内叶酸上的酶）的非常差的底物，CH3-H4PteGlu1必须首先转化为四氢蝶酰单谷氨酸（H4PteGlu1）、10-甲酰四氢蝶酰单谷氨酸（CHO-H4PteGlu1）或二氢叶酸（H2folate），这些是叶酸多聚谷氨酸合成酶的优良底物。多聚谷氨酸化对于细胞内叶酸的保留以及叶酸作为大多数叶酸依赖性酶的底物的功效都是必需的。已知有两种酶在体外会与CH3-H4PteGlu1反应，即亚甲基四氢叶酸还原酶和甲基四氢叶酸-同型半胱氨酸甲基转移酶（钴胺素依赖性甲硫氨酸合酶）。进行这些研究是为了评估亚甲基四氢叶酸还原酶催化CH3-H4PteGlu1转化为CH2-H4PteGlu1的可能性。CH2-H四氢蝶酰谷氨酸通过亚甲基四氢叶酸脱氢酶/亚甲基四氢叶酸环水解酶的作用很容易转化为CHO-H4PteGlu1，与叶酸单谷氨酸相比，这些酶活性对叶酸多聚谷氨酸底物的偏好非常小。我们从该酶的体外研究得出结论，在生理条件下亚甲基四氢叶酸还原酶不能将CH3-H4PteGlu1转化为CH2-H4PteGlu1，并且叶酸的摄取和保留将依赖于甲硫氨酸合酶活性。
