Kamen B A, Smith A K, Anderson R G
Department of Pediatrics, University of Texas Southwestern Medical Center, Dallas 75235-9039.
J Clin Invest. 1991 Apr;87(4):1442-9. doi: 10.1172/JCI115150.
Previous studies have defined a novel route of internalization for the essential vitamin 5-methyltetrahydrofolate in MA104 cells that begins with binding of the vitamin to the membrane receptor for folate. One of the critical steps in the pathway is the passage of 5-methyltetrahydrofolate through the membrane into the cytoplasm. Utilizing both probenecid and low temperature as selective inhibitors, we have successfully blocked transmembrane movement of the vitamin into the cytoplasm without affecting binding to the receptor or the internalization of the vitamin-receptor complex, which suggests that passage is through an anion carrier. This anion carrier, which mediates inward movement of 5-methyltetrahydrofolate after it dissociates from the receptor, also appears to mediate the efflux of folylmonoglutamate, but not folylpolyglutamate, when the concentration of the former in the cytoplasm is sufficiently high. Since we also found that the synthesis of folylpolyglutamates is regulated in these cells, most likely the intracellular concentration of the vitamin is controlled by regulating the flux of folylmonoglutamate through this carrier.
先前的研究已经确定了必需维生素5-甲基四氢叶酸在MA104细胞中的一种新的内化途径，该途径始于维生素与叶酸膜受体的结合。该途径的关键步骤之一是5-甲基四氢叶酸穿过膜进入细胞质。利用丙磺舒和低温作为选择性抑制剂，我们成功地阻断了维生素跨膜进入细胞质的过程，而不影响其与受体的结合或维生素-受体复合物的内化，这表明其通过阴离子载体进行转运。这种阴离子载体在5-甲基四氢叶酸从受体解离后介导其向内转运，当细胞质中叶酸单谷氨酸浓度足够高时，它似乎也介导叶酸单谷氨酸的外流，但不介导叶酸多谷氨酸的外流。由于我们还发现这些细胞中叶酸多谷氨酸的合成受到调节，很可能维生素的细胞内浓度是通过调节叶酸单谷氨酸通过该载体的通量来控制的。
