Cholesterol : Free radical peroxidation and transfer into phospholipid membranes

Abstract

Cholesterol, when sequestered in saturated liposomes of dimyristoylphosphatidylcholine (DMPC) or dipalmitoylphos-phatidylcholine (DPPC), undergoes peroxidation thermally initiated either by a lipid-soluble or a water-soluble azo initiator and in both cases the reaction is inhibited effectively by the water-soluble antioxidant, 6-hydroxy-2,5,7,8-tetra-methylchroman-2-carboxylate (Trolox). Quantitative kinetic methods of autoxidation show that the oxidizability, k p (2k t) 1 2 (where k p and 2 k t are the rate constants of radical chain propagation and termination, respectively) of cholesterol in DMPC or DPPC multilamellar liposomes, where k p (2k t) 1 2 is 3.0 • 10 −3 to 4.3 · 10 −3 M − 1 2 s − 1 2 at 37–45°C, is similar to that measured in homogeneous solution in chlorobenzene, where K p (2k t) 1 2 is 3.32-10 −3. However, its oxidizability in smaller unilamellar vesicles of DMPC or DPPC increases by at least 3-times that measured in multilamellar systems. Autoxidation/antioxidant methods show that cholesterol partitions directly from the solid state into DMPC or DPPC liposomes by shaking and this is confirmed by 31P and 2H quadrupole NMR spectra of deuterated cholesterol when membrane bound. Analytical studies indicate that up to 21 mol% cholesterol will partition into the membranes by shaking.