Erythrocyte membrane lateral sterol domains: A dehydroergosterol fluorescence polarization study

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Abstract
Structural domains of cholesterol and their regulation in the erythrocyte membrane are poorly understood. Dehydroergosterol fluorescence polarization change was used to continuously monitor the kinetics of sterol exchange and sterol domain size in erythrocyte ghost membranes. Direct correlation between molecular sterol exchange and steady-state dehydroergosterol fluorescence polarization measurements was obtained without separation of donor and acceptor membranes. Three important observations were made. First, sterol exchange between small unilamellar vesicles (SUV) with the same cholesterol/phospholipid ratio as the erythrocyte membrane (1-palmitoyl-2-oleoylphosphatidylcholine/cholesterolc = 1:1) was resolved into three kinetic cholesterol domains: 23 +/- 9% of total sterol was rapidly exchangeable, with t(1/2) = 23 +/- 6 min; 59 +/- 9% of total sterol was slowly exchangeable, with t(1/2) 135 +/- 3 min; and 19 +/- 9% of total sterol was essentially nonexchangeable, with a t(1/2) of days. Second, the substitution of erythrocyte ghosts for SUV as an acceptor significantly altered the kinetic parameters of sterol exchange from donor SUV, graphically showing that both the properties of the acceptor and spontaneous desorption of cholesterol from the donor SUV influenced spontaneous cholesterol transfer. Third, studies of exchange between erythrocyte ghosts revealed multiple kinetic pools of sterol differing from those in the SUV: 4 +/- 2% of total sterol was rapidly exchangeable, with t(1/2) 32 +/- 9 min; 29 +/- 3% of total sterol was very slowly exchangeable, with a t(1/2) 23 +/- 7 h; and a surprisingly large 67 +/- 2% of total sterol was nonexchangeable, with a t(1/2) of days.