CHOLESTEROL - FREE-RADICAL PEROXIDATION AND TRANSFER INTO PHOSPHOLIPID-MEMBRANES

Cholesterol, when sequested in saturated liposomes of dimyristoylphosphatidylcholine (DMPC) or dipalmitoylphosphatidylcholine (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 2k(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-degrees-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 P-31 and H-2 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.