The oxidative modification of lipids in vivo has been proposed
to play a central role in atherogenesis and to contribute
to the diverse vascular sequelae of diabetes and aging (1).
Oxidation of the lipid component of low density lipoprotein
(LDL), for example, leads to the loss of LDL recognition by
cellular LDL receptors and in the preferential uptake of
oxidized LDL by macrophage scavenger receptors (2-4). The
enhanced endocytosis of oxidized LDL by vascular wall
macrophages transforms them into the lipid-laden foam cells
that characterize early atherosclerotic lesions. This is followed
progressively by the development of fatty streaks and
the complex, proliferative lesions that ultimately cause arterial
insufficiency and occlusion (1-5).
Despite increased investigation into the biological role of
lipid oxidation, there has been little insight into the biochemical
processes that initiate lipid oxidation in vivo. In vitro
studies have demonstrated that metal-catalyzed peroxidation
reactions occur readily at the unsaturated bonds within fatty
acid residues. Polyunsaturated fatty acids are particularly
sensitive to peroxidation because bisallylic hydrogens are
more easily abstracted by free radical processes. Diene
conjugation then occurs and hydroperoxides form. Once lipid
oxidation is initiated, fatty acids decompose readily to a
variety ofreactive aldehydes that rapidly propagate oxidative
reactions (6, 7).
It is important to note that in the absence of transition
metals or free radicals, oxygen itself is a poor oxidant. The
electronic structure of triplet (ground state) oxygen has two