Kinetic and specificity differences between rat, human, and rabbit Na+-glucose cotransporters (SGLT-1)
Kinetic and specificity differences between rat, human, and rabbit Na+-glucose cotransporters (SGLT-1)
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DOI：
10.1152/ajpgi.1996.270.6.g919
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发表时间：
1996-06-01
影响因子：
4.5
通讯作者：
Wright, EM
中科院分区：
文献类型：
--
作者：
Hirayama, BA;Lostao, MP;Wright, EM
采用爪蟾卵母细胞表达系统和双电极电压钳方法，对人、兔和大鼠Na+-葡萄糖协同转运蛋白（SGLT-1）亚型的Na+激活和底物特异性进行了表征。我们发现，有差异，主要和次要的，在这两个动力学和底物特异性之间的异构体，在半最大电流的底物浓度（K-0.5）对于己糖从0.2到>40 mM变化，取决于种类和糖;根皮苷，SGLT-1的经典竞争性抑制剂，变化超过两个数量级（大鼠Ki = 0.03 μ M对兔Ki = 1.4 μ M）;兔同种型的一些葡萄糖苷抑制剂，对硝基苯基葡萄糖和β-萘基葡萄糖，由人和大鼠转运蛋白转运。与兔相比，人和大鼠亚型中的Na+激活对膜电位更敏感。兔同种型对α-甲基葡萄糖和3-O-甲基葡萄糖的表观亲和力比人或大鼠高两倍。这些结果可以通过我们的SGLT-1六态动力学模型进行定量拟合，从而深入了解这些变化所涉及的过程。例如，该模型预测人和大鼠SGLT-1中的Na+结合（速率常数，k（12））相似，但比兔中的大4倍，而兔和大鼠中的糖结合（k（23））相似，但比人SGLT-1中的值大2倍。这三种同源蛋白之间的一级氨基酸序列差异必须解释动力学和底物特异性差异，并且SGLT-1亚型的功能特性和氨基酸序列的比较提供了有关结构/功能关系的有用信息。
The Na+ activation and substrate specificity of human, rabbit, and rat Na+-glucose cotransporter (SGLT-1) isoforms were characterized using the Xenopus oocyte expression system and the two-electrode voltage-clamp method. We find that there are differences, major and minor, in both the kinetics and substrate specificities between these isoforms; the substrate concentration at half-maximal current (K-0.5) for hexoses varies from 0.2 to >40 mM, depending on the species and sugar; the affinity constant (K-i) for phlorizin, the classic competitive inhibitor of SGLT-1, varies over two orders of magnitude (rat K-i = 0.03 mu M vs. rabbit K-i = 1.4 mu M); and some glucoside inhibitors of the rabbit isoform, p-nitrophenyl glucose and beta-naphthyl glucose, are transported by the human and rat transporters. Na+ activation is more sensitive to membrane potential in the human and rat isoforms compared with rabbit. The rabbit isoform has a higher apparent affinity for alpha-methylglucose and 3-O-methylglucose by a factor of two than either human or rat. These results can be quantitatively fitted by our six-state kinetic model of SGLT-1, providing insight into the processes involved in these changes. For example, the model predicts that Na+ binding (rate constant, k(12)) in human and rat SGLT-1 is similar but is fourfold larger than in rabbit, whereas sugar binding (k(23)) in rabbit and rat is similar but double the value in human SGLT-1. The differences in the primary amino acid sequences between these three homologous proteins must account for the kinetic and substrate specificity differences, and comparisons of the functional properties and amino acid sequences of SGLT-1 isoforms provide useful information about structure/function relationships.