Effect of lipid mixing on the permeability and fusion of saturated lecithin membranes

The effect of phospholipid mixing on the permeability properties of multilamellar lipid vesicles (MLV) was studied. In the solid state, dimyristoylphosphatidylcholine/dipalmitoylphosphatidylcholine (DMPC/DPPC) vesicles exhibit ideal lipid miscibility; dimyristoylphosphatidylcholine/distearoylphosphatidylcholine (DMPC/DSPC) vesicles exhibit nonideal lipid miscibility at low DSPC molar fractions; dimyristoylphosphatidylcholine/dibehenoylphosphatidylcholine range of DBPC molar fractions. The rates of K+, ethylene glycol, and water diffusion from these vesicles in the solid state were measured by photometric and electrometric techniques. The following results were obtained: (1) The rate of solute diffusion, which is decreased monotonically, in DMPC/DPPC MLV, by increasing the molar fractions of DPPC, exhibits maxima at 0.2 molar fraction of DSPC in DMPC/DSPC MLV and at 0.4 molar fraction of DBPC in DMPC/DBPC MLV. (2) The activation energy of the solute diffusion process abruptly decreases in approximately the same range of lipid molar fractions where nonideal lipid miscibility is present. (3) The membrane pore radius is increased by increasing the lipid nonideal miscibility. The rate of vesicle size increase, measured by absorbance changes, is decreased monotonically in DMPC/DPPC monolamellar vesicles [small unilamellar lipid vesicles (SUV)] by increasing the molar fraction of DPPC. It, however, exhibits a maximum in DMPC/DSPC SUV at 0.15 molar fraction of DSPC. A model was suggested in which the solute diffusion and the membrane fusion processes are controlled by fractures. The average width of the fractures is increased by increasing the lipid immiscibility.

Phospholipid exchange and size enlargement in sonicated vesicles induced by a 'critical' fatty acid concentration

Size enlargement of dipalmitoyl phosphatidylcholine vesicles was greatly accelerated in the range of the phase-transition temperatures, when fatty acid concentration was above a threshold level ('critical' concentration). This 'critical' concentration varied with the length of the fatty acid chain. The size enlargement process had second-order kinetics dependent on the vesicle concentration. Alkaline pH and low ionic strength inhibited the rate of size enlargement. Phospholipid exchange between dimyristoyl and dipalmitoyl phosphatidyl-choline vesicles increased abruptly above a 'critical' fatty acid concentration. The donor vesicles were those vesicles in which fatty acids reached the 'critical' concentration. The phospholipid exchange occurred both in fluid- and in solidstate vesicles. The 'critical' fatty acid concentration accelerating the phospholipid exchange process was lower than that accelerating the size enlargement process. The phospholipid exchange process explained in terms of a diminished hydrophobic attraction among the phospholipid molecules of the bilayer occurs via a free phospholipid molecule transfer through the aqueous phase. The size enlargement process is interpreted in terms of high fatty acid concentration in the membrane fluid domains. The membrane structure is locally perturbed inducing vesicle sticking after collision.

Phospholipid release from sonicated vesicles induced by a "critical" fatty acid concentration

Dipalmitoyl phosphatidylcholine vesicles incubated in the presence of increasing amounts of myristic acid showed a progressive translocation of phospholipid molecules across a dialysis membrane. The rate of phospholipid translocation increased abruptly at a 'critical' value of myristic acid concentrations. The translocation rate of mixed dipalmitoyl phosphatidylcholine/myristic acid vesicles obtained by cosonicating the two components was also dependent on a 'critical' fatty acid concentration. A marked release of K+ and different responses of fluorescent probes to the fatty acid addition were observed at this concentration.

Interaction of cytochrome c with cytochrome c oxidase. Photoaffinity labeling of beef heart cytochrome c oxidase with arylazido-cytochrome c

Cytochrome c derivatives labeled with a 3-nitrophenylazido group at lysine 13, at lysine 22, or at both residues have been prepared. The interaction of the cytochrome c derivatives with beef heart cytochrome c oxidase (ferrocytochrome c:oxygen oxidoreductase, EC 1.9.3.1) in the presence of ultrviolet light results in formation of a covalent complex between cytochrome c and the oxidase. Using the lysine 22 derivative, the polypeptide composition of the oxidase is not modified, nor is its catalytic activity, whereas with the lysine 13 derivative, the gel electrophoretic pattern is altered and the catalytic activity of the complex diminished. The data are consisten with a specfic covalent interaction of the lysine 13 derivative of cytochrome c with the polypeptide of molecular weight 23,700 (Subunit II) of cytochrome c oxidase.

Cation transport in cytochrome oxidase reconstituted vesicles

Cation translocation across the membrane of cytochrome oxidase reconstituted vesicles may be followed with a simple spectrophotometric method. Cytochrome oxidase reconstituted vesicles, supplemented with ascorbate and cytochrome c. induce large spectral changes of the positive dye safranine, reversed by uncouplers and inhibitors of respiration. The dye is probably accumulated in the inner space of the vesicles, where it reaches high concentrations and aggregates. The spectral shifts and the absorbance changes, due to aggregation, are proportional to the amount of the dye taken up and depend on the respiratory control. In the presence of potassium, valinomycin causes an inhibition, whereas nigericin stimulates the dye uptake. The data are discussed in terms of electrical potential dependent fluxes.

Anion and amine uptake and uncoupling in submitochondrial particles

1. Unlike chloroplasts, submitochondrial particles are not uncoupled by nigericin + KCl or NH4Cl. Also the uncoupling effect of lipophilic anions is largely independent of the addition of weak bases. 2. Low concentrations of permeant anions cause a shift of the steady-state energy level rather than a cycle of energy utilization. The degree of inhibition of ATP synthesis by tetraphenylboron is larger than required for the uptake of the anion. 3. Lipophilic anions such as bromthymolblue, bromcresolpurple, and 8-anilino-1-napthalene sulphonate cause a pH-independent, 50% uncoupling in submitochondrial particles at concentrations of 3, 30 and 30 muM, respectively. The passive interaction of bromthymolblue and bromcresolpurple appears as a pH-dependent distribution between two pHases. ATP causes a pH-independent slight shift in the anion distribution, with negligible anion accumulation. 4. Addition of amines to energized submitochondrial particles results in two types of effects; uptake of amines and uncoupling. While in chloroplasts amine uptake and uncoupling are closely associated, this is not the case in submitochondrial particles. The uncoupling effect is observed only with lipophilic and not with hydrophilic amines, and the degree of uncoupling increases with the lipophilicity of the amines. The amine uptake, on the other hand, is accompanied by negligible uncoupling. 5. While the uptake of amines is dependent on the presence of non-permeant anions, such as Cl-, the uncoupling effect is independent of Cl-. Furthermore the amine uncoupling is markedly enhanced by lipophilic anions. 6. The view is discussed that the uncoupling effect of lipophilic anions and lipophilic amines in submitochondrial particles is due to a catalytic energy dissipation rather than to a stoichiometry energy utilization. The molecular mechanism of uncoupling presumably involves a cycling of charges after a perturbation of the membrane structure.