Continuous Fluorometric Assay of Ca²⁺ Transport by Liposomes with Quin 2 Entrapped: Effect of Phospholipase A₂ and Unsaturated Long-Chain Fatty Acids.

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The amount of free calcium in the cytoplasm is important in stimulation coupled with a number of cellular functions. The putative iono-phoretic action of membrane lipid metabolites on Ca²⁺ offers convenient explanation of the stimulation-coupled mobilization of cytoplasmic Ca²⁺. To analyze the ionophoretic action of the lipid metabolites, we devised a sensitive method to study Ca²⁺ transport that uses liposome-entrapped Quin 2. A cal-cium ionophore, A23187, increased the fluorescence intensity of the Ca²⁺-Quin 2 complex as a function of Ca²⁺ transport into liposomes.
A similar Ca²⁺ flux into the liposomes was induced by phospholipase A₂ (PLA₂) and by various long-chain fatty acids in liposomes that consist of phospholipids containing unsaturated fatty acids. The potencies of the fatty acids for Ca²⁺ transport is inversely correlated with their melting points. The oxidized products of the unsaturated fatty acids increased the Ca²⁺ and nons-pecific permeability of the biological membranes. These results suggest that stimulation-coupled PLA₂ activation might mediates the mobilization of cytoplasmic Ca²⁺.