Abstract
Planar asymmetric bilayer membranes, formed by apposing a monolayer of the neutral lipid glyceroldioleate (GDO) with one of the negatively charged lipid oleyl acid phosphate (OAP), were used to measure the rate of transmembrane OAP migration. The assay for this lipid flip-flop was the interaction of Ca(2+) ions with negatively charged lipids which causes membranes to break: when Ca(2+) is added to the compartment limited initially by the neutral lipid, flip-flop of the charged lipid eventually results in membrane breakdown. At 22+/-2 degrees C, in the absence of an externally applied electric field, an upper limit to the half time of OAP flip-flop was measured as 18.7 h, with a tentative lower limit of 14.4 h.