arsenazo-iii and dioleoylphosphatidic-acid

The permeability behaviour of dioleoylphosphatidate/dioleoylphosphatidylcholine (20:80, mol%) large unilamellar vesicles at low millimolar calcium concentrations is different for various solutes. Between 0.5 mM and 2.5 mM of calcium a selective influx of calcium and efflux of enclosed calcium chelating anions is observed. At higher calcium concentrations the membrane loses its barrier function for a large variety of solutes. These permeability increases are a specific consequence of calcium phosphatidate interactions, because control experiments in which calcium was replaced by magnesium or in which dioleoylphosphatidate was replaced by dioleoylphosphatidylglycerol showed under the same conditions no permeability changes. These results are discussed on the basis of various putative mechanistic models for phosphatidate-mediated calcium translocation across membranes. Furthermore a kinetical model is presented by which the observed selective calcium and calcium-chelator translocation can be explained.

Topics: Arsenazo III; Calcium; Cell Membrane Permeability; Magnesium; Membranes, Artificial; Phosphatidic Acids; Phosphatidylcholines; Phosphatidylglycerols; Solubility; Sulfates

An adapted version of the Ca2+-influx assay of Weissmann et al. (Weissmann, G., Anderson, P., Serhan, C., Samuelson, E. and Goodman, E. (1980) Proc. Natl. Acad. Sci. USA 77, 1506-1510) is presented for studies on the possible ionophoretic properties of acidic phospholipids. This method is based on the use of the metallochromic dye arsenazo III enclosed in liposomal vesicles, to indicate the Ca2+ influx. An essential control is introduced to discriminate between Ca2+-arsenazo III complex formation inside the vesicles, as a consequence of Ca2+ influx, and outside the vesicles, as a consequence of arsenazo III leakage from the vesicles. Furthermore, some minor improvements are added, like the use of large unilamellar vesicles instead of multilamellar vesicles, and the use of dual wavelength spectrophotometry. Using this method, it was found that dioleoylphosphatidylcholine vesicles, containing 20 mol% dioleoylphosphatidylglycerol, were impermeable to Ca2+. In this system a selective Ca2+ permeability could be induced by the addition of the fungal Ca2+ ionophore A23187. In contrast, dioleoylphosphatidylcholine vesicles, containing 20 mol% dioleoylphosphatidic acid, incubated in the presence of Ca2+ were permeable to both Ca2+ and arsenazo III.

Topics: Arsenazo III; Azo Compounds; Biological Transport, Active; Calcimycin; Calcium; Lipid Bilayers; Liposomes; Methods; Methoxyhydroxyphenylglycol; Permeability; Phosphatidic Acids; Phosphatidylcholines; Phospholipids