Panayotova-Heiermann M, Loo D D, Wright E M
Department of Physiology, UCLA School of Medicine 90095-1751, USA.
J Biol Chem. 1995 Nov 10;270(45):27099-105. doi: 10.1074/jbc.270.45.27099.
The rat Na+/glucose cotransporter (SGLT1) was expressed in Xenopus oocytes and steady-state and transient currents were measured using a two-electrode voltage clamp. The maximal glucose induced Na(+)-dependent inward current was approximately 300-500 nA. The apparent affinity constants for sugar (alpha-methyl-D-glucopyranoside; alpha MDG) (K alpha MDG 0.5) and sodium (KNa0.5) at a membrane potential of -150 mV were 0.2 mM and 4 mM. The KNa0.5 increased continuously with depolarizing potentials reaching 40 mM at -30 mV, K alpha MDG 0.5 was steeply voltage dependent, 0.46 mM at -30 mV and 1 mM at -10 mV. From all tested monovalent cations only Li+ could substitute for Na+, but with lower affinity. The relative substrate specificity was D-glucose > alpha MDG approximately D-galactose > 3-O-Me-Glc >> beta-naphthyl-D-glucoside >> uridine. Phlorizin (Pz), the specific blocker of sugar transport, showed an extremely high affinity for the rat cotransporter with an inhibitor constant (KPzi) of 12 nM. SGLT1 charge movements in the absence of sugar were fitted by the Boltzmann equation with an apparent valence of the movable charge of approximately 1, a potential for 50% maximal charge transfer (V0.5) of -43 mV, and a maximal charge (Qmax) of 9 nanocoulombs. The apparent turnover number for the rat SGLT1 was 30 s-1. Model simulations showed that the kinetics of the rat SGLT1 are described by a six-state ordered nonrapid equilibrium model, and comparison of the kinetics of the rat, rabbit and human cotransporters indicate that they differ mainly in their presteady-state kinetic parameters.
大鼠钠/葡萄糖协同转运蛋白（SGLT1）在非洲爪蟾卵母细胞中表达，采用双电极电压钳测量稳态电流和瞬态电流。葡萄糖诱导的最大钠依赖性内向电流约为300 - 500 nA。在膜电位为-150 mV时，糖（α-甲基-D-吡喃葡萄糖苷；αMDG）的表观亲和常数（KαMDG 0.5）和钠的表观亲和常数（KNa0.5）分别为0.2 mM和4 mM。KNa0.5随着去极化电位持续增加，在-30 mV时达到40 mM，KαMDG 0.5强烈依赖电压，在-30 mV时为0.46 mM，在-10 mV时为1 mM。在所有测试的单价阳离子中，只有Li⁺可以替代Na⁺，但亲和力较低。相对底物特异性为D-葡萄糖＞αMDG≈D-半乳糖＞3-O-甲基葡萄糖＞＞β-萘基-D-葡萄糖苷＞＞尿苷。糖转运的特异性阻滞剂根皮苷（Pz）对大鼠协同转运蛋白表现出极高的亲和力，抑制常数（KPzi）为12 nM。在无糖情况下，SGLT1的电荷移动符合玻尔兹曼方程，可移动电荷的表观价数约为1，最大电荷转移50%时的电位（V0.5）为-43 mV，最大电荷量（Qmax）为9纳库仑。大鼠SGLT1的表观周转数为30 s⁻¹。模型模拟表明，大鼠SGLT1的动力学由六态有序非快速平衡模型描述，大鼠、兔和人类协同转运蛋白动力学的比较表明，它们主要在稳态前动力学参数上有所不同。
