Further aspects of the Ca2+-dependent polymorphism of bovine heart cardiolipin

Abstract

The influence of cations on the structure of aqueous dispersions of the sodium salt of bovine heart cardiolipin was investigated using binding experiments, 31P-NMR, freeze-fracture electron microscopy, small angle X-ray diffraction and batch calorimetry techniques. In the 1–3 mM concentration range, Ca2 induces a bilayer → hexagonal HII transition for the lipid. During this transition there is a marked increase in Ca2+ binding from a maximum of 0.35 Ca/cardiolipin in the bilayer to 1.0 Ca/cardiolipin in the hexagonal HII phase. Only when the cardiolipin liposomes are exposed to locally high Ca2+ concentrations is the bilayer → hexagonal HII transition accompanied by the appearance of an intermediate ‘isotropic’ structure characterized by an isotropic 31P-NMR signal and lipidic particles. In contrast, in mixed dioleoylphosphatidylcholine/cardiolipin (1:1) liposomes, Ca2+ concentrations as low as 100 μM will induce an ‘isotropic’ structure under conditions where no locally high Ca2+ concentrations can occur. In this system at higher Ca2+ concentrations (above 5 mM) the hexagonal HII phase formation occurs. At least 80% of the phosphatidylcholine can be incorporated into this phase. The Ca2+-induced bilayer to hexagonal transition is an endothermic reaction with a ΔH of approx. 1.8 kcal/mol. Removal of Ca2+ from the hexagonally organized calcium-cardiolipin (1:1) complex by dialysis is an extremely slow process with a half-time in excess of 80 h. After 23 h of dialysis at a Ca/cardiolipin ratio of 0.86 an ‘isotropic’ structure is observed, characterized by an isotropic 31P-NMR signal and the presence of lipidic particles. After 70 h of dialysis (Ca/cardiolipin = 0.7) a new phase is observed. This phase which is optically isotropic and highly viscous separates from a lipid-free aqueous phase and contains 111 mM cardiolipin (15.5% by weight). The phospholipid molecules undergo rapid isotropic motion and the freeze-fracture morphology indicates the presence of a highly curved interconnected bilayer network separating various aqueous compartments. No defined X-ray diffraction bands can be observed for this phase. These characteristics are typical for cubic phases. This phase is metastable as mechanical agitation immediately induces the formation of large bilayer vesicles.