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Biochemical and Molecular Actions of Nutrients

Vitamin C Inhibits Lipid Oxidation in Human HDL^1,2

Department of Biochemistry, Chicago College of Osteopathic Medicine, Midwestern University, Downers Grove, IL 60515 and ^* Department of Biology, North Central College, Naperville, IL 60540

³To whom correspondence should be addressed. E-mail: slynch{at}midwestern.edu.

HDL are susceptible to oxidation, which affects their cardioprotective properties. Although several studies have reported inhibition of HDL oxidation by vitamin E, none has determined the potential protective effect of vitamin C, another important blood antioxidant. We investigated whether vitamin C protects HDL from oxidation by incubating HDL (0.2 g of protein/L) at 37°C with cupric (Cu²⁺) ions (10 µmol/L) in the absence (control) or presence of vitamin C (20–200 µmol/L). In the absence of vitamin C, lipid oxidation in HDL began immediately and proceeded rapidly. Cholesteryl linoleate declined to a minimum, whereas lipid oxidation products (lipid dienes and TBARS) increased to near-maximal levels within 1 h. Vitamin C (50–200 µmol/L) retarded initiation of lipid oxidation for at least 4 h under the same conditions. The ability of vitamin C to preserve the cardioprotective antioxidant function of HDL was also assessed. HDL (0.5 g of protein/L) preincubated with Cu²⁺ (10 µmol/L) for 2 h in the absence of vitamin C lost antioxidant activity (45.4 ± 6.2% inhibition of LDL oxidation compared with 93.2 ± 3.6% for native HDL, P < 0.05). The addition of vitamin C (50–200 µmol/L) during preincubation of HDL with Cu²⁺, however, resulted in no significant loss of HDL antioxidant activity (77.3 ± 0.3 to 89.8 ± 5.4% inhibition of LDL oxidation, P > 0.05 compared with native HDL). Our results demonstrate that vitamin C inhibits lipid oxidation in HDL and preserves the antioxidant activity associated with this lipoprotein fraction.

KEY WORDS: • ascorbic acid • antioxidants • lipoproteins • oxidant stress • cardiovascular diseases