Effect of rat serum albumin-cholesterol on the physical properties of biomembranes.

Affinity chromatography, using Cibacron blue F3GA bound to Sepharose 4B, and Sephadex G-150 column chromatography, permit the isolation of large quantities of non-esterified cholesterol loaded albumin from rat serum. Unlabeled cholesterol and cholesterol oxides displaced 14C-cholesterol bound to albumin in a dose-dependent manner. Binding of 14C-cholesterol to rat serum albumin in vitro follows a sigmoid curve. Ca2+ ions (up to 10 mM) inhibited the cholesterol binding to albumin in vitro. Fluorescence anisotropy of diphenyl-hexatriene (DPH) embedded in the lipid core of rat brain synaptosomal plasma membranes (SPM) increased with albumin-cholesterol incorporation and decreased in parallel to cholesterol removal. The allosteric properties of the SPM-bound (Na+ + K+)ATPase by fluoride (F-) (as reflected by changes in the Hill coefficient) were modulated by albumin-cholesterol, suggesting that the physical state of the (Na+ + K+)ATPase lipid microenvironment changed from a liquid-crystalline to gel phase. The present studies concerning albumin-cholesterol complex, its behavior and its role in the structure of biomembranes, provide important new clues to the role of this fascinating molecule in normal and pathological states.