Baird F E, Beattie K J, Hyde A R, Ganapathy V, Rennie M J, Taylor P M
Division of Molecular Physiology, School of Life Sciences, University of Dundee, Dundee DD1 5EH, UK.
J Physiol. 2004 Sep 1;559(Pt 2):367-81. doi: 10.1113/jphysiol.2003.060293. Epub 2004 Jun 24.
System N (SNAT3 and SNAT5) amino acid transporters are key mediators of glutamine transport across the plasma membrane of mammalian cell types, including hepatocytes and astrocytes. We demonstrate that SNAT5 shows simultaneous bidirectional glutamine fluxes when overexpressed in Xenopus oocytes. Influx and efflux are both apparently Na+ dependent but, since they are not directly coupled, the carrier is capable of mediating net amino acid movement across the cell membrane. The apparent Km values for glutamine influx and efflux are similar (approximately 1 mm) and the transporter behaviour is consistent with a kinetic model in which re-orientation of the carrier from outside- to inside-facing conformations (either empty or substrate loaded) is the limiting step in the transport cycle. In perfused rat liver, the observed relationship between influent (portal) glutamine concentration and net hepatic glutamine flux may be described by a simple kinetic model, assuming the balance between influx and efflux through System N determines net flux, where under physiological conditions efflux is generally saturated owing to high intracellular glutamine concentration. SNAT5 shows a more periportal mRNA distribution than SNAT3 in rat liver, indicating that SNAT5 may have particular importance for modulation of net hepatic glutamine flux.
系统N（SNAT3和SNAT5）氨基酸转运体是谷氨酰胺跨哺乳动物细胞类型（包括肝细胞和星形胶质细胞）质膜转运的关键介质。我们证明，当在非洲爪蟾卵母细胞中过表达时，SNAT5表现出同时双向的谷氨酰胺通量。流入和流出显然都依赖于Na+，但由于它们不是直接偶联的，载体能够介导氨基酸在细胞膜上的净移动。谷氨酰胺流入和流出的表观Km值相似（约1 mM），转运体行为与动力学模型一致，在该模型中，载体从外向内构象（空的或装载底物的）重新定向是转运循环中的限速步骤。在灌注的大鼠肝脏中，假设通过系统N的流入和流出之间的平衡决定净通量，其中在生理条件下由于细胞内谷氨酰胺浓度高，流出通常饱和，那么观察到的流入（门静脉）谷氨酰胺浓度与肝脏谷氨酰胺净通量之间的关系可以用一个简单的动力学模型来描述。在大鼠肝脏中，SNAT5的mRNA分布比SNAT3更偏向门静脉周围，这表明SNAT5可能对调节肝脏谷氨酰胺净通量具有特别重要的意义。
