Glycoside binding and translocation in Na+-dependent glucose cotransporters:: Comparison of SGLT1 and SGLT3

Using cotransporters as drug delivery vehicles is a topic of continuing interest, We examined glucose derivatives containing conjugated aromatic rings using two isoforms of the Na+/glucose cotransporter: human SGLT1 (hSGLT1) and pig SGLT3 (pSGLT3, SAAT1). Our studies indicate that there is similarity between SGLT1 and SGLT3 in the overall architecture of the vestibule leading to the sugar-binding site but differences in translocation pathway interactions. Indican was transported by hSGLT1 with higher affinity (K-0.5, 0.06 mM) and 2-naphthylglucose with lower affinity (K-0.5 mM) than alpha-methyl-D-glucopyranoside (alpha MDG 0.2 mM). Both were poorly transported (maximal velocities, I-max, 14% and 8% of alpha MDG). Other compounds were inhibitors (K(i)s 1-13 mM). In pSGLT3, indican and 2-naphthylglucose were transported with higher affinity than alpha MDG (K(0.5)s 0.9, 0.2 and 2.5 mM and relative I(max)s of 80, 25 and 100%). Phenylglucose and arbutin were transported with higher I(max)s (130 and 120%) and comparable K(0.5)s (8 and 1 mM). Increased affinity of indican relative to alpha MDG suggests that nitrogen in the pyrrole ring is favorable in both transporters. Higher affinity of 2-naphthylglucose for pSGLT3 than hSGLT1 suggests more extensive hydrophobic/aromatic interaction in pSGLT3 than in hSGLT1, Our results indicate that bulky hydrophobic glucosides can be transported by hSGLT1 and pSGLT3, and discrimination between them is based on steric factors and requirements for H-bonding. This provides information for design of glycosides with potential therapeutic value.