Ionophore 4-BrA23187 transports Zn²⁺ and Mn²⁺ with high selectivity over Ca²⁺

The cation transport selectivities of the Ca²⁺ ionophores A23187, Ionomycin, and 4-BrA23187 have been determined using a model system comprised of phospholipid vesicles loaded with the chelator/indicator Quin-2. At pH 7.00 and a 100 μM concentration of the cations, A23187 displays the transport selectivity sequence Zn²⁺ > Mn²⁺ > Ca²⁺ > Co²⁺ > Ni²⁺ > Sr²⁺, with the absolute rates of transport spanning ~3 orders of magnitude. Similar data are obtained with Ionomycin, although the relative transport rates of Zn²⁺ and Mn²⁺ are equivalent, and the range of absolute rates is decreased by a factor of ~3. When values are normalized to those of Ca²⁺, transport selectivity is seen to be only weakly related to complexation or extraction selectivity. It is also seen that, when used to manipulate Ca²⁺ (or Mg²⁺), both ionophores can be expected to alter the distribution of additional divalent cations which have known biological activities. 4-BrA23187 is a low-activity ionophore for Ca²⁺, compared to A23187 and Ionomycin, while retaining comparable activities as an ionophore for the other cations. As a consequence, 4-BrA23187 is highly selective for the transport of Zn²⁺ and Mn²⁺, compared to Ca²⁺, with selectivity ratios approaching that of valinomycin for K⁺ over Na⁺ when conditions are optimal. Plots of the log of the rate of cation transport vs the log of the ionophore concentration indicate that Ca²⁺ is transported primarily as a 2:1 complex by A23187 and 4-BrA23187, but Zn²⁺ and Mn²⁺ are transported, in part, as 1:1 complexes. These findings, together with a postulated low stability of 2:1, compared to 1:1 complexes between 4-BrA23187 and divalent cations, partially explain the novel transport selectivity of this compound. Unlike A23187 or Ionomycin, 4-BrA23187 may be useful for investigating cell regulation by Zn²⁺ and Mn²⁺, without interference by regulatory mechanisms which respond to Ca²⁺.