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

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Abstract
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, 1918) that under such conditions a maximum 70–80% of the homogenously 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 miture. Although both ionophores increase the relative amount of exchangeable phosphatidylinositol, neither ionophore results in all of the exchangeable phosphatidylinositol being transferred with singlestate 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.