Evidence for Dimer Participation and Evidence Against Channel Mechanism in A23187-Mediated Monovalent Metal Ion Transport Across Phospholipid Vesicular Membrane

AbstractThe decay of the pH difference (ΔpH) across soybean phospholipid vesicular membrane by ionophore A23187 (CAL)-mediated H+/M+ exchange (M+=Li+, Na+, K+, and Cs+) has been studied in the pH range 6–7.6. The ΔpH in these experiments were created by temperature jump. The observed dependence of ΔpH relaxation rate 1/τ on the concentration of CAL, pH, and the choice of M+ in vesicle solutions lead to the following conclusions. 1) The concentrations of dimers and other oligomers of A23187 in the membrane are small compared to the total concentration of A23187 in the membrane, similar to that in chloroform solutions reported in the literature. 2) In the H+ transport cycle leading to ΔpH decay, the A23187-mediated H+ translocation across the membrane is a fast step, and the rate-limiting step is the A23187-mediated M+ translocation. 3) Even though the monomeric Cal-H is the dominant species translocating H+, Cal-M is not the dominant species translocating M+ (even at concentrations higher than [Cal-H]), presumably because its dissociation rate is much higher than its translocation rate. 4) The pH dependence of 1/τ shows that the dimeric species Cal2LiLi, Cal2NaNa, Cal2KH, and Cal2CsH are the dominant species translocating M+. The rate constant associated with their translocation has been estimated to be ∼5×103 s−1. With this magnitude for the rate constants, the dimer dissociation constants of these species in the membrane have been estimated to be ∼4, 1, 0.05, and 0.04M, respectively. 5) Contrary to the claims made in the literature, the data obtained in the ΔpH decay studies do not favor the channel mechanism for the ion transport in this system. 6) However, they support the hypothesis that the dissociation of the divalent metal ion-A23187 complex is the rate limiting step of A23187-mediated divalent metal ion transport