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

Monensin (Mon)-mediated decay of the pH difference (DeltapH) across soyabean phospholipid vesicular membrane has been studied as a function of K+ and Na+ ion concentrations. In these experiments, the DeltapH 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 DeltapH 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 DeltapH decay rate on adding carbonyl cyanide m-chlorophenylhydrazone (CCCP) also suggest that the dominant barrier to the DeltapH 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. Copyright 1998 Elsevier Science B.V.