Helbig H, Korbmacher C, Wiederholt M
Institut für Klinische Physiologie, Klinikums Steglitz der Freien Universität, Berlin.
Fortschr Ophthalmol. 1990;87(4):421-4.
The concentration of ascorbic acid in the aqueous humor in many species (including humans and oxen) is several times higher than the plasma concentration. The role of ascorbic acid as a protective mechanism in the intraocular tissues against light-induced damage is discussed. We have performed uptake studies using 14C-labeled ascorbic acid in cultured bovine pigmented ciliary epithelial cells in order to study the mechanisms for ascorbic acid transport from the ciliary body to the aqueous humor. We found that intracellular accumulation of ascorbic acid was Na(+)-dependent. Data from kinetic experiments suggest the presence of a co-transport system for 2 Na+ and 1 ascorbate. The oxidized form of the vitamin (dehydro-ascorbic acid, DHA) was taken up in the cell by a Na(+)-independent mechanism. Using thin-layer chromatographic assay, we found intracellularly only ascorbic acid after incubation with extracellular DHA, indicating the presence of DHA reductase activity in ciliary body epithelial cells. Thus, these cells possess two mechanisms for intracellular accumulation of ascorbic acid: (1) co-transport for 2 Na+ and 1 ascorbate and (2) a Na(+)-independent DHA transport in cooperation with intracellular DHA reductase. These mechanisms could be involved in the transport of ascorbic acid from the stroma to the aqueous humor.
在许多物种（包括人类和牛）中，房水中抗坏血酸的浓度比血浆浓度高出数倍。本文讨论了抗坏血酸在眼内组织中作为一种保护机制抵抗光诱导损伤的作用。为了研究抗坏血酸从睫状体转运至房水的机制，我们使用14C标记的抗坏血酸对培养的牛色素性睫状体上皮细胞进行了摄取研究。我们发现抗坏血酸在细胞内的积累依赖于Na(+)。动力学实验数据表明存在一个2个Na+和1个抗坏血酸的协同转运系统。维生素的氧化形式（脱氢抗坏血酸，DHA）通过一种不依赖Na(+)的机制被细胞摄取。使用薄层色谱分析法，我们发现在细胞外DHA孵育后细胞内仅存在抗坏血酸，这表明睫状体上皮细胞中存在DHA还原酶活性。因此，这些细胞具有两种抗坏血酸在细胞内积累的机制：（1）2个Na+和1个抗坏血酸的协同转运，以及（2）与细胞内DHA还原酶协同作用的不依赖Na(+)的DHA转运。这些机制可能参与了抗坏血酸从基质向房水的转运。
