Cation ionophores A23187 and valinomycin enhance protein-mediated transfer of rat liver microsomal phosphatidylinositol to liposomes.

A standard reaction mixture has been established in which partially purified rat liver phosphatidylinositol exchange proteins sustain a maximal rate of phosphatidylinositol transfer from rat liver microsomes to liposomes. Determination of the transfer kinetics confirms the findings of Brophy et al. (Biochem J. 174:413-420,1978) that under such conditions a maximum 70-80% of the homogeneously radiolabeled, microsomal phosphatidylinositol is exchanged with biphasic kinetics. The phosphatidylinositol exchange proteins thus indicate the presence of three microsomal phosphatidylinositol pools: One pool is not subject to protein-mediated exchange; the other two pools are both exchangeable but are exchanged with significantly different half-lives. Both the divalent cation ionophore, A23187, and the monovalent cation ionophore, valinomycin, significantly enhance phosphatidylinositol transfer in the standard reaction mixture at concentrations 1 to 2 orders of magnitude greater than those sufficient for the ionophores to facilitate cation transport across membranes. The stimulatory effect of each ionophore, however, is not a function of the ionophore/microsome mass ratio in the reaction mixture. Although both ionophores increase the relative amount of exchangeable phosphatidylinositol, either ionophore results in all of the exchangeable phosphatidylinositol being transferred with single-state kinetics. The evidence demonstrates that A23187 and valinomycin are the first substances found to markedly enhance the reactivity of a microsomal phospholipid class with phospholipid exchange proteins.