An electrophoretic study of suspensions of intact mitochondria and fragments of mitochondrial membranes

Estimation of membrane surface potential and charge density from the phase equilibrium of a paramagnetic amphiphile

The distribution of a paramagnetic amphiphile, N,N-dimethyl-N-nonyl-N-tempoylammonium ion, between the membranes of charged phospholipid vesicles and the surrounding aqueous medium was studied by electron paramagnetic resonance spectroscopy. By systematically varying the surface charge of the vesicles and the aqueous electrolyte concentration, the distribution was shown to indicate vesicle surface potential. At each fixed phospholipid composition, the surface potential exhibited a dependence on aqueous NaCl concentration very similar to that predicted by the Gouy equation. The ability to sense and quantitate surface potentials makes this facile and sensitive technique of value in the study of cell and organelle surfaces.

Fixed charge in the cell membrane

1. Focal electric field was generated by passing a current of 5 x 10(-7) to 1 x 10(-5) A from a micropipette into the culture medium. Movement of cells at a distance of 5-50 mu from the electrode tip was observed. In case of cells embedded in the culture only local deformation of the membrane was observed.2. The cell species explored included neurones, glia, muscle fibres, connective cells, malignant cells and erythrocytes. All cells responded in a similar manner to the electric field, and the current required was in the same range.3. Cells were attracted to a positive micropipette and repelled from a negative one: the only exception was observed in certain malignant cells which moved in the opposite direction.4. Movement and membrane deformation could be obtained with electrodes filled with various concentrated and isotonic solutions. The composition of the culture medium also had no qualitative influence on these effects.5. Metabolic poisons or rupture of the cell membrane had no effect on the movement. Isolated membrane fragments showed movement similar to that of intact cells.6. The possibility of artifacts due to proximity of the focal electrode is considered. It is shown that electro-osmosis cannot account for the present observations. Some other artifacts are also excluded.7. It is proposed that the most satisfactory way to account for the present observations is by a membrane carrying negative fixed charge of the order of 2.5 x 10(3) e.s.u./cm(2). Some physiological consequences of presence of negative charge in the membrane are briefly discussed.

Physicochemical differences between fragments of plasma membrane and endoplasmic reticulum

Specific turbidities, densities, and refractive indices of fragments of plasma membrane (PM) and endoplasmic reticulum (ER) from Ehrlich ascites carcinoma have been measured. A spherical shell model of specified dimensions and refractive index was established for PM fragments. The ionic composition of the dispersion medium was varied systematically. Increases in Gamma/2 caused increases in the turbidity of both PM and ER suspensions, the greatest effects being observed with Ca(2+) and Mg(2+). In the case of PM this effect is attributable mainly to aggregation, whereas "structural" changes account for most of the turbidity increase with ER. The pH was also varied systematically to obtain pH- density and turbidity profiles and to establish the isoelectric pH of the two membrane types (PM-3.6; ER-4.35). Turbidity was maximum at "isoelectric" pH, which corresponds in each case to the region of minimum charge on the particle surfaces. Both PM and ER show large increases of density at the "isoelectric" pH, but only ER shows substantial structurally based turbidity increase under these conditions. Both PM and ER show operation of electrostatic attractions near "isoelectric" pH. PM has been shown to have ionically distinctive inner and outer surfaces while ER shows no such dissymmetry. The necessary theoretical background for interpretation of turbidity and density measurements is included, as well as a discussion of the limitations of our conclusions and the biological importance of our results.

The electrophoretic behaviour of mitochondria from kidney and liver of rats

1. The electrophoretic mobility values for mitochondria prepared from rat kidney and liver measured in 0.125m-potassium chloride-0.02m-tris, pH7.4, are: -0.78(s.e.m.+/-0.02)mu/sec./v/cm. (29 experiments) and -1.06(s.e.m.+/-0.01) mu/sec./v/cm. (21 experiments) respectively. 2. These mobility values are unaffected by washing and spontaneous swelling at 25 degrees , indicating a stable electrokinetic surface. 3. The mobility of rat-kidney mitochondria is unaffected by thyroxine-induced swelling, or by the state of hydration of the rat. 4. pH-mobility curves show similar surface ionogenic groups for kidney and liver mitochondria; their isoelectric points are pH3.9 and pH4.4 respectively.