A permeability transition in liposomes induced by the formation of Ca2+/palmitic acid complexes

Formation of palmitic acid/Ca2+ (PA/Ca2+) complexes was suggested to play a key role in the non-classical permeability transition in mitochondria (NCPT), which seems to be involved in the PA-induced apoptosis of cardiomyocytes. Our previous studies of complexation of free fatty acids (FFA) with Ca2+ showed that long-chain (C:16-C:22) saturated FFA had an affinity to Ca2+ which was much higher than that of other FFA and lipids. The formation of FFA/Ca2+ complexes in the black-lipid membrane (BLM) was demonstrated to induce a nonspecific ion permeability of the membrane. In the present work, we have found that binding of Ca2+ to PA incorporated into the membrane of sulforhodamine B (SRB)-loaded liposomes results in an instant release of a part of SRB, with the quantity of SRB released depending on the concentration of PA and Ca2+. The pH-optimum of this phenomenon, similar to that of PA/Ca2+ complexation, is in the alkaline range. The same picture of SRB release has been revealed for stearic, but not for linoleic acid. Along with Ca2+, some other bivalent cations (Ba2+, Sr2+, Mn2(+), Ni2+, CO2+) also induce SRB release upon binding to PA-containing liposomes, while Mg2+ turns out to be relatively ineffective. As revealed by fluorescence correlation spectroscopy, the apparent size of liposomes does not alter after the addition of PA, Ca2+ or their combination. So it has been supposed that the cause of SRB release from liposomes is the formation of lipid pores. The effect of FFA/Ca2+-induced permeabilization of liposomal membranes has several analogies with NCPT, suggesting that both these phenomena are of similar nature. (C) 2002 Elsevier Science B.V. All rights reserved.