Similarity in effects of Na+ gradients and membrane potentials ond-glucose transport by,and phlorizin binding to,vesicles derived from brush borders of rabbit intestinal mucosal cells

Both the presence of sodium and of an electrical potential difference across the membrane have been found to be necessary in order to achieve optimal D-glucose-protectable phlorizin binding to brush border membranes from rabbit small intestine. The effect of delta approximately muNa on phlorizin binding shows a close similarity to that on D-glucose transport, confirming that phlorizin is indeed bound to the D-glucose transporting protein. Possible modulations of binding by a transmembrane potential are discussed on the basis of some models.     

Small-intestinal Na+/d-glucose cotransport. Inactivation of sugar transport and phlorizin binding by thiol-group and amino-group reagents

It has previously been shown that mercurials acting from the cytoplasmic side or from within the hydrophobic part of the membrane inactivate the small intestinal Na⁺/d-glucose cotransporter by blocking essential SH-groups (Klip, A., Grinstein, S. and Semenza, G. (1979) Biochim. Biophys. Acta 558, 233–245). Another (set of) sulfhydryl(s) which are critical for phlorizin binding and sugar transport function and which may lie on the luminal side of the brush border membrane, can be blocked by DTNB and 4,4′-dithiopyridine but not by $\text{N-}\text{ethylmaleimide}$. In addition, modification of amino groups by fluorescamine, reductive methylation and (under certain conditions) DIDS also lead to inactivation of the carrier's binding and transport functions. No evidence was obtained that any of the above groups is directly involved in the binding of either Na⁺/d-glucose or phlorizin, since none of these compounds prevented inactivation of the cotransporter.