ASCT2 is a neutral amino acid transporter, which catalyzes a sodium-dependent obligatory antiport among glutamine and other neutral amino acids. The human ASCT2 over-expressed in Pichia pastoris and reconstituted in proteoliposomes has been employed for identifying alternative substrates of the transporter. The experimental data highlighted that hASCT2 also catalyzes a sodium-dependent antiport of glutamate with glutamine. This unconventional antiport shows a preferred sidedness: glutamate is inwardly transported in exchange for glutamine transported in the counter direction. The orientation of the transport protein in proteoliposomes is the same as in the cell membrane; then, the observed sidedness corresponds to the transport of glutamate from the extracellular to the intracellular compartment. The competitive inhibition exerted by glutamate on the glutamine transport together with the docking analysis indicates that the glutamate binding site is the same as that of glutamine. The affinity for glutamate is lower than that for neutral amino acids, while the transport rate is comparable to that measured for the asparagine/glutamine antiport. Differently from the neutral amino acid antiport that is insensitive to pH, the glutamate/glutamine antiport is pH-dependent with optimal activity at acidic pH on the external (extracellular) side. The stimulation of glutamate transport by a pH gradient suggests the occurrence of a proton flux coupled to the glutamate transport. The proton transport has been detected by a spectrofluorometric method. The rate of proton transport correlates well with the rate of glutamate transport indicating a 1:1 stoichiometry H⁺: glutamate. The glutamate/glutamine antiport is also active in intact HeLa cells. On a physiological point of view, the described antiport could have relevance in some districts in which a glutamate/glutamine cycling is necessary, such as in placenta.

ASCT2是一种中性氨基酸转运蛋白，可催化谷氨酰胺和其他中性氨基酸之间的钠依赖性强制性反转运。人类ASCT2在巴斯德毕赤酵母重组脂质体中的脂质体已被用于鉴定转运蛋白的替代底物。实验数据强调，hASCT2还催化谷氨酸与谷氨酰胺的钠依赖性逆转运。这种非常规的反端口显示出优选的侧面：向内运输谷氨酸盐，以换取沿相反方向运输的谷氨酰胺。蛋白脂质体中转运蛋白的取向与细胞膜中的相同；然后，观察到的侧面对应于谷氨酸从细胞外到细胞内区室的转运。谷氨酸对谷氨酰胺转运的竞争性抑制以及对接分析表明，谷氨酸结合位点与谷氨酰胺相同。对谷氨酸的亲和力低于对中性氨基酸的亲和力，而转运速率与天冬酰胺/谷氨酰胺反转运所测得的相当。与对pH不敏感的中性氨基酸反转运不同，谷氨酸/谷氨酰胺反转运是pH依赖性的，在外部（细胞外）一侧在酸性pH下具有最佳活性。pH梯度对谷氨酸转运的刺激表明质子通量与谷氨酸转运耦合。质子传输已通过荧光光谱法检测到。质子传输速率与谷氨酸传输速率相关性很好，表明化学计量比为1：1 与对pH不敏感的中性氨基酸反转运不同，谷氨酸/谷氨酰胺反转运是pH依赖性的，在外部（细胞外）一侧在酸性pH下具有最佳活性。pH梯度对谷氨酸转运的刺激表明质子通量与谷氨酸转运耦合。质子传输已通过荧光光谱法检测到。质子传输速率与谷氨酸传输速率相关性很好，表明化学计量比为1：1 与对pH不敏感的中性氨基酸反转运不同，谷氨酸/谷氨酰胺反转运是pH依赖性的，在外部（细胞外）一侧在酸性pH下具有最佳活性。pH梯度对谷氨酸转运的刺激表明质子通量与谷氨酸转运耦合。质子传输已通过荧光光谱法检测到。质子传输速率与谷氨酸传输速率相关性很好，表明化学计量比为1：1⁺：谷氨酸。谷氨酸/谷氨酰胺逆向转运在完整的HeLa细胞中也有活性。从生理学的角度来看，所述的反转运蛋白可能在某些需要谷氨酸/谷氨酰胺循环的地区（例如胎盘）具有相关性。