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 (Delta pH) across s oyabean phospholipid vesicular membrane has been studied as a function of K+ and Na+ ion concentrations. In these experiments, the Delta pH was created using temperature jump, and ionic strength was regulated a t 0.3 using CsCl. Rate constants associated with the translocation of Mon-H, Mon-K and Mon-Na have been estimated (without making any assump tions) from an analysis of the Delta pH decay data. These estimates co ntradict the claim made in the literature (E. Nachliel, Y. Finkelstein , M. Gutman, Biochim. Biophys. Acta, 1285 (1996) 131-145) that the tra nslocation rate constants of the three above-mentioned species are sig nificantly different. Our observations on the changes in Delta pH deca y rate on adding carbonyl cyanide m-chlorophenylhydrazone (CCCP) also suggest that the dominant barrier to the Delta pH 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 ca use large differences in their translocation rate constants. (C) 1998 Elsevier Science B.V.