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 N-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.