Factors affecting resistance of low density lipoproteins to oxidation

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Abstract
Oxidation resistance (OR) of low density lipoproteins (LDL) is frequently determined by the conjugated diene (CD) assay, in which isolated LDL is exposed to Cu2+ as prooxidant in the range of 1–10 μM. A brief review on major findings obtained with this assay will be given. A consistent observation is that vitamin E supplements or oleic acid-rich diets increase OR. Oxidation indices measured by the CD assay and effects of antioxidants very significantly depend on the Cu2+ concentration used for LDL oxidation. For medium and high Cu2+ concentrations, the relationship between lag time and propagation rate can be described by a simple hyperbolic saturation function, which has the same mathematical form as the Michaelis-Menten equation. At medium and high Cu2+ concentrations (0.5 to 5 μM), vitamin E increases lag time in a dose-dependent manner. The increase is higher for 0.5 μM Cu2+ as compared to 5 μM. At low Cu2+ concentrations (0.5 μM or less), the mechanism of LDL oxidation changes. Significant oxidation occurs in a preoxidation phase, which commences shortly after addition of Cu2+. Preoxidation is not inhibited by vitamin E. It is concluded that much additional work is needed to validate the importance of oxidation indices derived from CD and similar assays.