Inverse deuterium kinetic isotope effect for peroxidation in human low-density lipoprotein (LDL): a simple test for tocopherol-mediated peroxidation of LDL lipids.

alpha-Tocopherol (alpha-TOH) can act as a pro- or antioxidant for isolated ubiquinol-10-free human low density lipoprotein (LDL). We demonstrate that alpha-TOH is a more potent pro-oxidant than other forms of vitamin E for LDL peroxidation initiated by mild fluxes of aqueous peroxyl radicals and low concentrations of Cu2+. A simple deuterium exchange test shows that alpha-TOH switches from pro- to anti-oxidant at Cu2+:LDL ratios > 2.5. The results suggest that this test may be useful to distinguish 'inhibited' peroxidation of emulsion lipids propagated via the lipid peroxyl radical from that mediated via the antioxidant radical.