Instability of brain synaptosomal membrane preparations to repeated ultracentrifugation in isoosmotic density gradients.

Synaptosomal membranes from rat brain were found to be remarkably unstable, from the criterion of buoyant density, to repeated density gradient ultracentrifugation. These membranes underwent progressive changes in buoyant density through at least three cycles of fractionation on density gradients generating large proportions of material of both higher and lower buoyant density than the original fraction. The observed density changes were not due to osmotic effects of the various gradient density zones, clumping due to the influence of divalent cations, age of the membrane preparation, length of centrifugation time, intermediate processing steps or a variety of other factors. In addition, the method of homogenization including duration and rate of homogenization had only minimal effects on this property of buoyant density instability.