Relative magnitudes of the rate constants associated with monensin-mediated H +, Na + and K + translocations across phospholipid vesicular membranes

Abstract

Monensin (Mon)-mediated decay of the pH difference (ΔpH) across soyabean phospholipid vesicular membrane has been studied as a function of K + and Na + ion concentrations. In these experiments, the ΔpH was created using temperature jump, and ionic strength was regulated at 0.3 using CsCl. Rate constants associated with the translocation of Mon–H, Mon–K and Mon–Na have been estimated (without making any assumptions) from an analysis of the ΔpH decay data. These estimates contradict the claim made in the literature (E. Nachliel, Y. Finkelstein, M. Gutman, Biochim. Biophys. Acta, 1285 (1996) 131–145) that the translocation rate constants of the three above-mentioned species are significantly different. Our observations on the changes in ΔpH decay rate on adding carbonyl cyanide m-chlorophenylhydrazone (CCCP) also suggest that the dominant barrier to the ΔpH decay process is not the `polar region' of the membrane. Therefore, the differences in the electric dipole moments of Mon–H, Mon–K and Mon–Na are unlikely to cause large differences in their translocation rate constants.