Histidyl residues at the active site of the Na/succinate co-transporter in rabbit renal brush borders

Abstract

Mono-, dicarboxylic acid-, and D-glucose transport were measured in brush border vesicles from renal cortex after treatment with reagents known to modify terminal amino, lysyl, epsilon-amino, guanidino, serine/threonine, histidyl, tyrosyl, tryptophanyl and carboxylic residues. All three sodium-coupled co-transport systems proved to possess sulfhydryl (and maybe tryptophanyl sulfhydryl, disulfide, thioether and tyrosyl) residues but not at the substrate site or at the allosteric cavity for the Na co-ion. Histidyl groups seem to be located in the active site of the dicarboxylic transporter in that the simultaneous presence of Na and succinate protects the transporter against the histidyl specific reagent diethylpyrocarbonate. Lithium, which specifically competes for sodium sites in the dicarboxylic acid transporter, substantially blocked the protective effect of Na and succinate. Hydroxylamine specifically reversed the covalent binding of diethylpyrocarbonate to the succinate binding site. The pH dependence of the Na/succinate cotransport is consistent with an involvement of histidyl and sulfhydryl residues. We conclude that a histidyl residue is at, or is close to, the active site of the dicarboxylate transporter in renal brush border membranes.