Said H M, Strum W B
J Pharmacol Exp Ther. 1983 Jul;226(1):95-9.
The intestinal transport of the naturally occurring folate coenzyme, 5-methyltetrahydrofolate, was studied using everted sacs of rat jejunum. The study provides evidence that intestinal transport of 5-methyltetrahydrofolate is composed of two systems: 1) an active, carrier-mediated system which is demonstrable at low concentrations; and 2) a diffusion system which is demonstrable at high concentrations. The active system is characterized by: 1) saturation kinetics with Km congruent to 0.3 microM; 2) accumulation against a concentration gradient with a serosal-to-mucosal ratio of 1.8; 3) inhibition by metabolic poisons; 4) inhibition by oxidized and reduced folate analogs; 5) temperature dependence; 6) sodium dependence; 7) glucose dependence; and 8) specificity for the jejunum. These features are strongly pH-dependent, and demonstration of active transport of 5-methyltetrahydrofolate requires a buffer pH of 6, glucose in the incubation medium and a substrate concentration of less than 10(-6) M. The diffusion process is characterized by: 1) linear increase in the mucosal-to-serosal transport of 5-methyltetrahydrofolate with increasing mucosal concentration to 10(-6) M and above; 2) energy independence; 3) pH independence; and 4) temperature independence. These studies clarify the mechanism of intestinal transport of 5-methyltetrahydrofolate, show the similarities to transport of other folate compounds and provide a unified concept of intestinal folate transport.
利用大鼠空肠外翻囊研究了天然存在的叶酸辅酶5-甲基四氢叶酸的肠道转运。该研究提供的证据表明，5-甲基四氢叶酸的肠道转运由两个系统组成：1）一种主动的、载体介导的系统，在低浓度时可显示；2）一种扩散系统，在高浓度时可显示。主动系统的特征为：1）具有与0.3 microM相当的Km的饱和动力学；2）逆浓度梯度积累，浆膜与黏膜的比率为1.8；3）被代谢毒物抑制；4）被氧化型和还原型叶酸类似物抑制；5）温度依赖性；6）钠依赖性；7）葡萄糖依赖性；8）对空肠具有特异性。这些特征强烈依赖于pH值，5-甲基四氢叶酸主动转运的证明需要缓冲液pH值为6、孵育介质中有葡萄糖且底物浓度小于10^(-6) M。扩散过程的特征为：1）随着黏膜浓度增加至10^(-6) M及以上，5-甲基四氢叶酸从黏膜到浆膜的转运呈线性增加；2）不依赖能量；3）不依赖pH值；4）不依赖温度。这些研究阐明了5-甲基四氢叶酸的肠道转运机制，显示了与其他叶酸化合物转运的相似性，并提供了肠道叶酸转运的统一概念。
