MECHANISM AND SPECIFICITY OF LANTHANIDE SERIES CATION-TRANSPORT BY IONOPHORES A23187, 4-BRA23187, AND IONOMYCIN

A23187, 4-BrA23187, and ionomycin transport several lanthanide series trivalent cations at efficiencies similar to Ca2+, when compared at ca tion concentrations of similar to 10(-5) M, ionophore concentrations o f similar to 10(-6) M, and a pH of 7.00, Selectivity sequences and the range of relative rates are as follows: A23187, Nd3+ > La3+ > Eu3+ > Gd3+ > Er3+ > Yb3+ > Lu3+ (similar to 34-fold); 4-BrA23187, Nd3+ > Eu3 + > Gd3+ > La3+ > Er3+ > Yb3+ > Lu3+ (similar to 34-fold); ionomycin, La3+ > Yb3+ > Nd3+ > Lu3+ > Er3+ > Eu3+ > Gd3+ (similar to 4-fold). At concentrations between 9 and 250 mu M, La3+ is transported by an elec troneutral mechanism, predominately through mixed complexes of the typ e (ionophore)(2)La . OH (A23187 and 4-BrA23187) or (ionophore)La . OH (ionomycin), when no membrane potential is present. For all three iono phores, an induced potential of similar to 160 mV accelerates transpor t by similar to 50-100%, However, measured values of H+/La3+ exchange indicate that only 4-BrA23187 displays a significant electrogenic acti vity under these conditions. At a La3+ concentration of 17 mM, transpo rt by all three ionophores is electroneutral and apparently occurs thr ough complexes of type (ionophore)(3)La (A23187 and 4-BrA23187) or (io nophore)La . OH (ionomycin). Analysis of these patterns in a context o f comproportionation equilibria involving the transporting species and free La3+ indicates that the species containing three ionophore molec ules are formed on the membrane when aqueous phase solution conditions would strongly favor a 1:1 complex, based upon previous studies in so lution. The implications of this and other findings are discussed.