NA K COMPETITIVE TRANSPORT SELECTIVITY OF (221)C-10-CRYPTAND - EFFECTS OF PH AND CARRIER CONCENTRATION/

The kinetics of the competitive transport of Na+ and K+ ions across th e membrane of large unilamellar vesicles (LUV) were determined when tr ansport was induced by (221)C-10-cryptand, an ionizable mobile carrier . The experiments were performed at various pH values (7.7 and 8.7) an d carrier concentrations (0.1, 0.5 and 1.0 mu M) in order to quantify the effects of these parameters on the Na/K competitive transport sele ctivity of this mobile carrier. At any given pH and carrier concentrat ion, the apparent affinity of (221)C-10 for Na+ was higher and less de pendent on the concentration of the other competing ion than that for K+. The Na/K competitive transport selectivity (S-C(Na/K)) of (221)C-1 0 increased linearly with the Na+ concentrations, decreased hyperbolic ally with increasing those of K+ and was independent of the pH and of the carrier concentration. In equimolecular ionic mixtures, this compe titive selectivity amounted to about 1.5 and when the pH rose, the car rier selectivity for Na+ over K+ ions was enhanced by cation competiti on compared to transport of cations as unique substrates. Equations we re established to describe the variations of the competitive transport selectivity (S-C) of cryptands, and for comparison of their noncompet itive selectivity (S-NC), With the ionic concentrations, the Michaelis parameters of the cations and the pH. The reaction order in Na+ (n(Na )) increased significantly with decreasing the pH and the K+ concentra tion. The results are discussed in terms of the structural, physico-ch emical and electrical characteristics of carriers and complexes.