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.