Effects of Native and Oxidized Apolipoprotein A-I on Lipid Bilayer Microviscosity of Erythrocyte Plasma Membrane

Using a pyrene as a fluorescent probe, we investigated the influence of native and oxidative modified apolipoprotein A-I (apo A-I) and their complexes with tetrahydrocortisol (THC) on the microviscosity of the erythrocyte plasma membrane. The addition of THC to isolated membranes led to a 17% increase in the membrane microviscosity. In contrast, native apo A-I reduced the microviscosity (i.e., increased the fluidity) of the membranes by 15%. More marked increase (by 25%) in the membrane fluidity was found in the presence of the complex of apo A-I with THC. Unlike native apo A-I, modified apo A-I and its complex with THC did not change the membrane viscosity. In view of the fact that apo A-I plays an important role in the binding of membrane cholesterol we suggest that the observed increase in the membranes fluidity under the influence of the native apo A-I is associated with the cholesterol efflux from plasma membrane. Oxidative modification of apo A-I likely disturbs the mechanisms of the cholesterol efflux and prevents the decrease in the membrane microviscosity.