O'Kane Robyn L, Viña Juan R, Simpson Ian, Zaragozá Rosa, Mokashi Ashwini, Hawkins Richard A
Natural and Applied Science Department, LaGuardia Community College/City University of New York, New York, USA.
Am J Physiol Endocrinol Metab. 2006 Aug;291(2):E412-9. doi: 10.1152/ajpendo.00007.2006. Epub 2006 Mar 28.
Cationic amino acid (CAA) transport is brought about by two families of proteins that are found in various tissues: Cat (CAA transporter), referred to as system y+, and Bat [broad-scope amino acid (AA) transporter], which comprises systems b0,+, B0,+, and y+L. CAA traverse the blood-brain barrier (BBB), but experiments done in vivo have only been able to examine the BBB from the luminal (blood-facing) side. In the present study, plasma membranes isolated from bovine brain microvessels were used to identify and characterize the CAA transporter(s) on both sides of the BBB. From these studies, it was concluded that system y+ was the only transporter present, with a prevalence of activity on the abluminal membrane. System y+ was voltage dependent and had a Km of 470 +/- 106 microM (SE) for lysine, a Ki of 34 microM for arginine, and a Ki of 290 microM for ornithine. In the presence of Na+, system y+ was inhibited by several essential neutral AAs. The Ki values were 3-10 times the plasma concentrations, suggesting that system y+ was not as important a point of access for these AAs as system L1. Several small nonessential AAs (serine, glutamine, alanine,and glycine) inhibited system y+ with Ki values similar to their plasma concentrations, suggesting that system y+ may account for the permeability of the BBB to these AAs. System y+ may be important in the provision of arginine for NO synthesis. Real-time PCR and Western blotting techniques established the presence of the three known nitric oxide synthases in cerebral endothelial cells: NOS-1 (neuronal), NOS-2 (inducible), and NOS-3 (endothelial). These results confirm that system y+ is the only CAA transporter in the BBB and suggest that NO can be produced in brain endothelial cells.
阳离子氨基酸（CAA）转运由存在于各种组织中的两类蛋白质介导：Cat（CAA转运体），即系统y+，以及Bat [广谱氨基酸（AA）转运体]，后者包括系统b0,+、B0,+和y+L。CAA可穿越血脑屏障（BBB），但体内实验仅能从管腔（面向血液）侧检测血脑屏障。在本研究中，使用从牛脑微血管分离的质膜来鉴定和表征血脑屏障两侧的CAA转运体。从这些研究得出的结论是，系统y+是唯一存在的转运体，在无腔膜上活性占优势。系统y+依赖电压，对赖氨酸的Km为470±106μM（标准误），对精氨酸的Ki为34μM，对鸟氨酸的Ki为290μM。在有Na+存在的情况下，系统y+受到几种必需中性氨基酸的抑制。Ki值是血浆浓度的3至10倍，这表明对于这些氨基酸而言，系统y+作为进入途径的重要性不如系统L1。几种小的非必需氨基酸（丝氨酸、谷氨酰胺、丙氨酸和甘氨酸）以与它们血浆浓度相似的Ki值抑制系统y+，这表明系统y+可能解释了血脑屏障对这些氨基酸的通透性。系统y+在为一氧化氮合成提供精氨酸方面可能很重要。实时PCR和蛋白质印迹技术证实脑内皮细胞中存在三种已知的一氧化氮合酶：NOS-1（神经元型）、NOS-2（诱导型）和NOS-3（内皮型）。这些结果证实系统y+是血脑屏障中唯一的CAA转运体，并表明脑内皮细胞中可产生一氧化氮。
