Unusual Low Proton Permeability of Liposomes Prepared from the Endogenous Myelin Lipids

Abstract

In contrast with most other lipid substrates, in this article we show that liposomes prepared from the total myelin lipids exhibited a negligible proton permeability. Neither the generation of valinomycin‐induced potassium diffusion potentials as high as ‐177 mV nor the imposition of large pH gradients (up to three units) was able to produce a substantial flux of protons through liposomal membranes, as determined by the distribution of [14C]‐methylamine, or the changes in the fluorescence of the probes 9‐aminoacridine, acridine orange, and pyranine. The presence of cations (Na+, K+, Ca2+) did not alter this behavior. Voltage clamping did not increase the trans‐membrane ApH‐driven proton permeability. However, II‐posome diameter was found to be critical because small unilamellar vesicles displayed a much higher proton permeability than large unilamellar or multilamellar vesicles. This abnormally low proton permeability is interpreted by virtue of the characteristic biochemical composition of myelin lipid matrix, with a high content of cholesterol and sphingolipids and a very low level of free fatty acids. These results could be important for elucidating the role of myelin in the regulation of pH in the brain. In addition, the myelin lipid extract could be useful for reconstituting proteins that participate in the transport of H+ through the membrane.

Myelin water mapping by spatially regularized longitudinal relaxographic imaging at high magnetic fields

“…The value estimated by Koenig et al was based on successive permeation of myelin lamellae. Though serial permeation of stacked bilayers may be modeled , it is difficult to estimate the decreased water permeability coefficient of a myelin membrane to reflect its unusual lipid composition , rich in cholesterol, long‐chained galactoside cerebrosides (exofacial leaflet), and plasmalogen phosphatidylethanolamine (endofacial leaflet). Another approach to assess the exchange time of myelin water is to neglect permeation and to calculate the mean time required for myelin water molecules to circumferentially diffuse from the innermost myelin layers to reach the outermost layers.…”

Hypothesis of an Energetic Function for Myelin

“…It is tempting to presume that the high surface/volume ratio of myelin sheath be necessary for both its insulating and O 2 absorption function, similarly to the rod disk membrane, congenial to both light and O 2 capture [20]. Myelin is unusually impermeant to protons [32], a fundamental prerequisite for the formation of a H ? gradient.…”