Interaction between an organic hydroperoxide and an unsaturated phospholipid and alpha-tocopherol in model membranes

The behavior of an organic hydroperoxide in the presence of lipid and/or alpha-tocopherol in model membranes has been studied using 14C-labeled cholesterol-5 alpha-hydroperoxide as the organic hydroperoxide. Cholesterol-F alpha-hydroperoxide in saturated phospholipid micelles is rapidly isomerized to cholesterol-7 alpha-hydroperoxide. Such an isomerization is inhibited by alpha-tocopherol, but not by an alpha-tocopheryl analogue with a substituted OH groups, present in the micelles, indicating the formation of hydrogen bonds between OH group in alpha-tocopherol and OOH group in the 5 alpha-hydroperoxide. The double bonds in unsaturated phospholipid can also serve to form a hydrogen bond with OOH in the 5 alpha-hydroperoxide moiety in the micelles. The resulting hydrogen-bonded complex could be decomposed by iron-induced lipid peroxidation, accompanied by isomerization of the 5 alpha-hydroperoxide and the further degradation to the 7-ketocholesterol and 7 alpha-hydroxycholesterol. When three components, such as unsaturated phospholipid, 5 alpha-hydroperoxide and alpha-tocopherol, are present in the same micelles, they form hydrogen bonded complexes. Such complexes could be decomposed by iron in the ferrous state, yielding mainly 5 alpha-hydroxycholesterol without significant change in the structure of alpha-tocopherol and peroxidative cleavage of unsaturated phospholipid.