title: Cation ionophores A23187 and valinomycin enhance protein-mediated transfer of rat liver microsomal phosphatidylinositol to liposomes.
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. 174:413-420,1978) 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.
mesh_terms: D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D000001:Calcimycin / Q000494:pharmacology; D002352:Carrier Proteins / Q000378:metabolism*; D066298:In Vitro Techniques; D007476:Ionophores / Q000494:pharmacology*; D008081:Liposomes; D008862:Microsomes, Liver / Q000187:drug effects / Q000378:metabolism*; D010716:Phosphatidylinositols / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D014634:Valinomycin / Q000494:pharmacology
chemical_list: D002352:Carrier Proteins; D007476:Ionophores; D008081:Liposomes; D010716:Phosphatidylinositols; D014634:Valinomycin; D000001:Calcimycin