Calcium diphosphatidate membrane traversal is inhibited by common phospholipids and cholesterol but not by plasmalogen

Osmotic behaviour and permeability properties of liposomes

In this overview liposomes are described as bilayer-bounded vesicles which under defined conditions act as ideal osmometers according to the Boyle van't Hoff law. Investigations on osmotic volume changes, directly or indirectly by taking advantage of changes in light scattering, are considered and applications in permeability measurements are discussed. Solute-solvent interactions occurring in isotonic swelling experiments are analysed in view of an irreversible thermodynamic description. In a second part liquid crystalline lipid bilayers are characterized as highly selective permeability bilayers and the physical principles underlying this selectivity are considered. Attention is given to special physical and chemical conditions that may cause structural defects in the bilayer organization and can affect the selective permeability properties of the bilayer or completely deteriorate its barrier function. Finally an evaluation is given of intrinsic ionophoretic activity in lipid bilayers containing negatively charged lipids.

Activation of protein kinase C by phosphatidylinositol 4,5-bisphosphate: possible involvement in Na+/H+ antiport down-regulation and cell proliferation

Phosphatidylinositol 4,5-bisphosphate (PIP2) as well as diacylglycerol (DG) activate protein kinase C (PKC) in the presence of calcium and phosphatidylserine. The pH at half-activation (pK) is 6.2 for DG.PKC and 7.7 for PIP2.PKC. Since the second monophosphate proton in position 5 of the PIP2 inositol (i.e., the last ionizable proton) has a pK of 7.7 (Van Paridon et al., (1986) Biochim. Biophys. Acta. 877, 216), the active effector is a fully deprotonated PIP2. Activation of PKC by PIP2 thus may follow intracellular alkalinization and be tied to the down-regulation of the Na+/H+ antiport mechanism. Since alkalinization is obligatory for cell proliferation, PIP2(5-).Ca.PKC may also be the gate that opens the pathways toward this and connected cellular reactions. A PIP2 analog in which inositol carbons 2-4 and the 4-phosphate have been removed, 1-phosphatidyl-rac-glycerol-3-phosphate (PGP), is completely inactive as PKC effector; this suggests that both 4-and 5-phosphate are engaged in the PIP2(5-).Ca.PKC complex. A model of the activated kinase takes this into account.

Mechanism of the ATP-dependent phosphatidylserine synthesis in liver subcellular fractions

It has been shown that the ATP-dependent incorporation of [14C]serine into phosphatidylserine in rat liver mitochondrial and microsomal fractions is prevented by EGTA. On the other hand, at low (microM) Ca2+ concentrations, serine incorporation is strongly stimulated by ATP and Mg2+. This stimulatory effect is reduced by calcium ionophore A23187. It is therefore suggested that the ATP-dependent process is that of serine base-exchange reaction, stimulated by endogenous Ca2+ accumulated inside the microsomal vesicles by Ca2+,Mg2+-ATPase. The mitochondrial activity can be accounted for by contamination by the endoplasmic reticulum.

Calcium translocation in liposome systems modeled on the mitochondrial inner membrane

Ca2+ uptake by liposomes consisting of phosphatidylcholine (PC) and cardiolipin (CL) has recently been reported (Smaal, E.B. et al. (1987) Biochim. Biophys. Acta 897, 191-196). In eukaryotic cells, CL is localized exclusively in the inner mitochondrial membrane, where it occurs in the presence of equimolar amounts of PC and phosphatidyl-ethanolamine (PE). We have therefore re-examined CL-mediated Ca2+ translocation in liposomes of more nearly physiological composition, i.e., PC/PE/CL (2:2:1 and 4:4:1, mol/mol). In addition, the effect on Ca2+ uptake of plasmalogens of PE, which may account for up to 50% of mitochondrial PE, was determined. Our findings can be summarized as follows. (1) Ca2+ uptake into CL-containing liposomes was increased dramatically by PE. (2) Ca2+ entry into PC/CL liposomes was biphasic; in the presence of PE, uptake was dominated by a slow process. (3) Ca2+ uptake by PC/CL liposomes saturated at less than or equal to 2 mM external Ca2+, whereas uptake into PC/PE/CL liposomes increased with increasing Ca2+ concentration up to 10 mM or until Ca2+ release ensued. (4) Ca2+ translocation by PE-containing liposomes and the slow phase of Ca2+ uptake into PC/CL liposomes were similarly and highly dependent on temperature. It can therefore be proposed that PE amplifies the slow component of CL-mediated Ca2+ translocation. This process is characterized by a requirement for high external Ca2+ concentrations and a large apparent activation energy. Ca2+ uptake was not significantly modified by plasmalogens of PE.

Cellular calcium and atherosclerosis: a brief review

Evidence for and against the theory that cell calcium is causally involved in the pathogenesis of atherosclerosis is presented and evaluated. In particular, it is argued that: (1) arterial calcium is increased in atherosclerosis; (2) this increase in tissue calcium content is largely intracellular; (3) this increased intracellular calcium content is caused by increased plasma membrane calcium permeability; (4) the increased calcium content is causally related to atherogenesis; (5) many of the cell physiological, cell biological, biochemical, and molecular biological processes, known to function abnormally in atherosclerosis, are also known to be calcium regulated; and (6) these processes are activated or inactivated in atherosclerosis in a manner consistent with increased cell calcium. It is concluded that the calcium-atherogenesis hypothesis has the potential to unify macroscopic clinical risk factors in terms of intracellular mechanisms that are controlled by cell calcium, and that this hypothesis deserves further experimental tests.