Cooperative nature of the binding of cholesterol on to synaptosomal plasma membranes of dog brain.

The 'binding' of cholesterol on to dog brain synaptosomal plasma membranes from aqueous cholesterol 'solutions' was studied. 'Binding' of exogenous cholesterol is a slow process, strictly depending on the concentration of cholesterol and the quantity of the membranes present. It appears that binding probably occurs in three distinct successive stages. The first stage occurs very rapidly, and consists of a large deposition-like accumulation of cholesterol onto the membranes. This stage is characterized by the lack of functional changes of integral proteins. It is followed or accompanied by a slower type of binding, probably at 'specific binding sites', the nature of which is, in all probability, cooperative. Thus, when the glucoside of cholesterol is used at lower concentrations as compared to cholesterol it increases the binding of cholesterol, while at higher concentrations relative to cholesterol, it antagonizes its binding. This stage, which evokes strong functional changes of integral proteins, merges without interruption into an incorporation of cholesterol as a structural element into the membranous framework (nonspecific binding).