Wu X, Yuan G, Brett C M, Hui A C, Giacomini K M
School of Pharmacy, University of California, San Francisco 94143.
J Biol Chem. 1992 May 5;267(13):8813-8.
The overall goal of this study was to determine the mechanisms by which nucleosides are transported in choroid plexus. Choroid plexus tissue slices obtained from rabbit brain were depleted of ATP with 2,4-dinitrophenol. Uridine and thymidine accumulated in the slices against a concentration gradient in the presence of an inwardly directed Na+ gradient. The Na(+)-driven uptake of uridine and thymidine was saturable with Km values of 18.1 +/- 2.0 and 13.0 +/- 2.3 microM and Vmax values of 5.5 +/- 0.3 and 1.0 +/- 0.2 nmol/g/s, respectively. Na(+)-driven uridine uptake was inhibited by naturally occurring ribo- and deoxyribonucleosides (adenosine, cytidine, and thymidine) but not by synthetic nucleoside analogs (dideoxyadenosine, dideoxycytidine, cytidine arabinoside, and 3'-azidothymidine). Both purine (guanosine, inosine, formycin B) and pyrimidine nucleosides (uridine and cytidine) were potent inhibitors of Na(+)-thymidine transport with IC50 values ranging between 5 and 23 microM. Formycin B competitively inhibited Na(+)-thymidine uptake and thymidine trans-stimulated formycin B uptake. These data suggest that both purine and pyrimidine nucleosides are substrates of the same system. The stoichiometric coupling ratios between Na+ and the nucleosides, guanosine, uridine, and thymidine, were 1.87 +/- 0.10, 1.99 +/- 0.35, and 2.07 +/- 0.09, respectively. The system differs from Na(+)-nucleoside co-transport systems in other tissues which are generally selective for either purine or pyrimidine nucleosides and which have stoichiometric ratios of 1. This study represents the first direct demonstration of a unique Na(+)-nucleoside co-transport system in choroid plexus.
本研究的总体目标是确定核苷在脉络丛中运输的机制。从兔脑中获取的脉络丛组织切片用2,4 -二硝基苯酚消耗ATP。在存在内向Na⁺梯度的情况下，尿苷和胸苷在切片中逆浓度梯度积累。Na⁺驱动的尿苷和胸苷摄取是可饱和的，其Km值分别为18.1±2.0和13.0±2.3 μM，Vmax值分别为5.5±0.3和1.0±0.2 nmol/g/s。Na⁺驱动的尿苷摄取受到天然存在的核糖核苷和脱氧核糖核苷（腺苷、胞苷和胸苷）的抑制，但不受合成核苷类似物（双脱氧腺苷、双脱氧胞苷、阿糖胞苷和3'-叠氮胸苷）的抑制。嘌呤核苷（鸟苷、肌苷、间型霉素B）和嘧啶核苷（尿苷和胞苷）都是Na⁺-胸苷转运的有效抑制剂，IC50值在5至23 μM之间。间型霉素B竞争性抑制Na⁺-胸苷摄取，且胸苷可转刺激间型霉素B摄取。这些数据表明嘌呤核苷和嘧啶核苷都是同一系统的底物。Na⁺与核苷、鸟苷、尿苷和胸苷之间的化学计量偶联比分别为1.87±0.10、1.99±0.35和2.07±0.09。该系统与其他组织中的Na⁺-核苷共转运系统不同，其他组织中的该系统通常对嘌呤核苷或嘧啶核苷具有选择性，且化学计量比为1。本研究首次直接证明了脉络丛中存在一种独特的Na⁺-核苷共转运系统。
