arbutin and methylglucoside

Phenylglucosides are transported by the intestinal Na+/glucose cotransporter (SGLT1) and phlorizin, the classical competitive inhibitor of SGLT1, is also a phenylglucoside. To investigate the structural requirements for binding of substrates to SGLT1, we have studied the interactions between phenylglucosides and the cotransporter expressed in Xenopus oocytes using tracer uptake and electrophysiological methods. Some phenylglucosides inhibited the Na(+)-dependent uptake of 14C-alpha-methyl-D-glucopyranoside (alpha MDG) with apparent Kis in the range 0.1 to 20 mM, while others had no effect. Electrophysiological experiments indicated that phenylglucosides can act either as: (1) transported substrates, e.g., arbutin; (2) nontransported inhibitors, e.g., glucosylphenyl-isothiocyanate; or (3) noninteracting sugars, e.g., salicin. The transported substrates (glucose, arbutin, phenylglucoside and helicin) induced different maximal currents, and computer simulations showed that this may be explained by a difference in the translocation rates of the sugar and Na(+)-loaded transporter. Computational chemistry indicated that all these beta-phenylglucosides have similar 3-D structures. Analysis showed that among the side chains in the para position of the phenyl ring the -OH group (arbutin) facilitates transport, but the -NCS (glucosylphenyl-isothiocyanate) inhibits transport. In the ortho position, -CH2OH (salicin) prevents interaction, but the aldehyde (helicin) permits the molecule to be transported. Studies such as these may help to understand the geometry and nature of glucoside binding to SGLT1.

Topics: Animals; Arbutin; Binding, Competitive; Biological Transport; Carrier Proteins; Cell Membrane; Electrophysiology; Female; Glucose; Glucosides; Intestinal Mucosa; Intestines; Membrane Glycoproteins; Membrane Potentials; Membrane Proteins; Methylglucosides; Microvilli; Monosaccharide Transport Proteins; Oocytes; Phlorhizin; Rabbits; Sodium-Glucose Transporter 1; Xenopus laevis

Myo-Inositol depletion as a result of hyperglycemia is considered one of the leading contributors to chronic diabetic complications. We investigated the possible mechanisms through which elevated extracellular glucose levels affect the loss of intracellular myo-inositol in rat lens. Short-term incubation (up to 4 h) in solutions with elevated glucose concentrations revealed a concentration-dependent inhibition of myo-inositol influx. This inhibition was caused by both an increase of the transport coefficient and a decrease of maximal flux and thus was a mixed competitive and noncompetitive inhibition. If polyol accumulation was prevented with sorbinil, an aldose reductase inhibitor, the inhibition of myo-inositol influx was partially reduced. The remaining inhibition was the result of an increased transport coefficient without a change in maximal flux and therefore represents a strictly competitive inhibition. A similar competitive inhibition was observed with the nonmetabolizable glucose analogue L-glucose, which cannot be converted to polyol. Longer exposure (16 h) to solutions with high glucose concentrations resulted in an inhibition that correlated with high lens polyol levels. This inhibition persisted after the lenses were returned to solutions with normal glucose concentrations and was the result of a decrease of maximal flux without a significant change in transport coefficient, a strictly noncompetitive inhibition. The noncompetitive inhibition associated with polyol accumulation and the competitive inhibition due to extracellular glucose were additive. Lens myo-inositol depletion after exposure to elevated glucose concentrations thus resulted from a competitive inhibition caused by the interaction of extracellular glucose with the myo-inositol carrier and a noncompetitive inhibition associated with polyol accumulation.

Topics: 3-O-Methylglucose; Animals; Arbutin; Deoxyglucose; Glucose; In Vitro Techniques; Inositol; Kinetics; Lens, Crystalline; Male; Methylglucosides; Rats; Rats, Sprague-Dawley; Stereoisomerism; Time Factors