Abstract

Imino sugars are used to treat type 2 diabetes mellitus [miglitol (Glyset)] and lysosomal storage disorders [miglustat (Zavesca)] based on the inhibition of α-glucosidases and glucosyltransferases. In this substrate specificity study, we examined the interactions of imino sugars with a novel human glucose sensor, sodium/glucose cotransporter type 3 (hSGLT3), using expression in Xenopus laevis oocytes and electrophysiology. The results for hSGLT3 are compared with those for α-glucosidases and human SGLT type 1 (hSGLT1), a well characterized sodium/glucose cotransporter of the SGLT family. In general, substrates have lower apparent affinities (K_0.5) for hSGLT3 than hSGLT1 (d-glucose, α-methyl-d-glucose, 1-deoxy-d-glucose, and 4-deoxy-4-fluoro-d-glucose exhibit K_0.5 values of 19, 21, 43, and 17 mM, respectively, for hSGLT3, and 0.5, 0.7, 10, and 0.07 mM, respectively, for hSGLT1). However, specificity of hSGLT3 binding is greater (d-galactose and 4-deoxy-4-fluoro-d-galactose are not hSGLT3 substrates, but have hSGLT1 K_0.5 values of 0.6 and 1.3 mM). An important deviation from this trend is potent hSGLT3 activation by the imino sugars 1-deoxynojirimycin (DNJ), N-hydroxylethyl-1-deoxynojirimycin (miglitol), N-butyl-1-deoxynojirimycin (miglustat), N-ethyl-1-deoxynojirimycin, and 1-deoxynojirimycin-1-sulfonic acid, with K_0.5 values of 0.5 to 9 μM. The diastereomer 1-deoxygalactonojirimycin activates hSGT3 with a K_0.5 value of 11 mM, a 3000-fold less potent interaction than is observed for DNJ (4 μM). These imino sugar binding characteristics are similar to those for α-glucosidases, but there are no interactions with hSGLT1. This work provides insights into hSGLT3 and -1 substrate binding interactions, establishes a pharmacological profile to study endogenous hSGLT3, and may have important ramifications for the clinical application of imino sugars.