アフリカツlt;KエルSGLT1LとウサギSGLT1から合成した輸送担体の基質特異性

抄録

To determine the sugar binding site of the Na⁺/glucose cotransporter type1 (SGLT1), substrate specificity of a chimera transporter which was made from rabbit SGLT1 and Xenopus SGLT1-like protein (SGLT1L) was examined. The order of the substrate specificity of SGLT1 is D-glucose:=D-galactose:=α-methyl-D-glucoside (αMDG)>>myo-inositol while that of Xenopus SGLT1L is myo-inositol>D-glucose>D-galactose:=αMDG. αMDG is a specific substrate for SGLT. A chimera transporter was made by substituting the extracellular loop between the transmembrane domain (TM)12 and TM13 of Xenopus SGLT1L with the same portion of rabbit SGLT1. Then the chimera was expressed in the Xenopus oocyte membrane and its transport activity was measured by the two-microelectrode voltage-clamp method. The order of the substrate specificity of the chimera was myo-inositol:=D-glucose>D-galactose:=αMG showing that by the substitution the affinity of the chimera with D-glucose increased but the affinities with myo-inositol, D-galactose and αMG changed little when compared to the native Xenopus SGLT1L. As a result, the substrate specificity of the chimera transporter was similar to neither　SGLT1 nor Xenopus SGLT1L. Therefore, the extracellular loop between TM12 and TM13 is probably a part of the sugar binding site of SGLT1. [Jpn J Physiol 54 Suppl:S87 (2004)]