Critical carboxyl group(s) in Na+-dependent cotransporters of the intestinal brush-border membrane.

We have previously provided functional evidence for a role of carboxyl group(s) in the mechanism of coupling of Na+ and D-glucose fluxes by the small-intestinal cotransporter(s) (Kessler, M. and Semenza, G. (1983) J. Membrane Biol. 76, 27-56). We present here a study on the inactivation of the Na+-dependent transport systems, but not of the Na+-independent ones, in the small-intestinal brush-border membrane, by hydrophobic carbodiimides. Although marginal or insignificant protection by the substrates or by Na+ was observed, the parallelism between Na+-dependence and inactivation by these carbodiimides strongly indicates the role of carboxyl group(s) previously indicated. Contrary to the carboxyl group identified by Turner [1986) J. Biol. Chem. 261, 1041-1047) in the sugar binding site of the renal Na+/D-glucose cotransporter, the carboxyl group(s) studied here probably occur elsewhere in the cotransporter molecule.