May J M, Qu Z C, Mendiratta S
Department of Medicine, Vanderbilt University School of Medicine, Nashville, TN 37232-6303, USA.
Biochem Pharmacol. 1999 Jun 1;57(11):1275-82. doi: 10.1016/s0006-2952(99)00040-4.
The role of ascorbic acid in transferrin-independent ferric iron reduction and uptake was evaluated in cultured U-937 monocytic cells. Uptake of 55Fe by U-937 cells was doubled by 100 microM extracellular ascorbate, and by pre-incubation of cells with 100 microM dehydroascorbic acid, the two-electron-oxidized form of ascorbate. Reduction of extracellular ferric citrate also was enhanced by loading the cells with dehydroascorbic acid. Dehydroascorbic acid was taken up rapidly by the cells and reduced to ascorbate, such that the latter reached intracellular concentrations as high as 6 mM. However, some ascorbate did escape the cells and could be detected at concentrations of up to 1 microM in the incubation medium. Further, addition of ascorbate oxidase almost reversed the effects of dehydroascorbic acid on both 55Fe uptake and ferric citrate reduction. Thus, it is likely that extracellular ascorbate reduced ferric to ferrous iron, which was then taken up by the cells. This hypothesis also was supported by the finding that during loading with ferric citrate, only extracellular ascorbate increased the pool of intracellular ferrous iron that could be chelated with cell-penetrant ferrous iron chelators. In contrast to its inhibition of ascorbate-dependent ferric iron reduction, ascorbate oxidase was without effect on ascorbate-dependent reduction of extracellular ferricyanide. This indicates that the cells use different mechanisms for reduction of ferric iron and ferricyanide. Therefore, extracellular ascorbate derived from cells can enhance transferrin-independent iron uptake by reducing ferric to ferrous iron, but intracellular ascorbate neither contributes to this reduction nor modifies the redox status of intracellular free iron.
在培养的U-937单核细胞中评估了抗坏血酸在不依赖转铁蛋白的三价铁还原和摄取中的作用。100微摩尔的细胞外抗坏血酸盐使U-937细胞对55Fe的摄取增加了一倍，并且通过用100微摩尔脱氢抗坏血酸（抗坏血酸的双电子氧化形式）对细胞进行预孵育，也有同样效果。用脱氢抗坏血酸处理细胞后，细胞外柠檬酸铁的还原也得到增强。脱氢抗坏血酸被细胞迅速摄取并还原为抗坏血酸，使得后者在细胞内的浓度高达6毫摩尔。然而，一些抗坏血酸确实从细胞中逸出，并且在孵育培养基中可以检测到浓度高达1微摩尔。此外，添加抗坏血酸氧化酶几乎逆转了脱氢抗坏血酸对55Fe摄取和柠檬酸铁还原的影响。因此，细胞外抗坏血酸可能将三价铁还原为二价铁，然后被细胞摄取。这一假设也得到了以下发现的支持：在用柠檬酸铁加载期间，只有细胞外抗坏血酸增加了细胞内可与细胞穿透性二价铁螯合剂螯合的二价铁池。与它对抗坏血酸依赖性三价铁还原的抑制作用相反，抗坏血酸氧化酶对细胞外铁氰化铁的抗坏血酸依赖性还原没有影响。这表明细胞使用不同的机制来还原三价铁和铁氰化铁。因此，细胞来源的细胞外抗坏血酸可以通过将三价铁还原为二价铁来增强不依赖转铁蛋白的铁摄取，但细胞内抗坏血酸既不参与这种还原，也不改变细胞内游离铁的氧化还原状态。
