Allosteric cotransport of sodium, chloride, and calcium by the intestine of freshwater prawns

Abstract

The apical membrane of the intestinal epithelium of the freshwater prawn,Macrobrachium rosenbergii, has been found to possess an apparently unique allosteric carrier mechanism for the simultaneous cotransport of sodium, chloride, and calcium from mucosal solution to cytosol. Influxes of the two monovalent ions individually were sigmoidal functions of their respective luminal concentrations, and their kinetics followed the Hill equation for homotropic cooperativity between identical binding ligands. Increased influx of chloride sigmoidally stimulated enhanced influx of sodium, suggesting the occurrence of heterotropic cooperativity between the dissimilar ligands. Calcium entry displayed hyperbolic (Michaelis-Menten) kinetics, and this cation was found to act both as an allosteric activator of sodium entry on the shared carrier system by associating with a discrete divalent cation binding site, as well as functioning as a possible competitive inhibitor of the monovalent cation binding process. Chloride was neither an allosteric activator nor inhibitor, but appeared to function mainly as an affinity modifier of the allosteric protein for sodium.