Interaction between flavonoids and α-tocopherol in human low density lipoprotein

Oxidative modification of low density lipoprotein (LDL) may play an important role in the development of atherosclerosis. α-Tocopherol functions as a major antioxidant in human LDL. The present study was to test whether four natural flavonoids (kempferol, morin, myricetin, and quercetin) would protect or regenerate α-tocopherol in human LDL. The oxidation of LDL incubated in sodium phosphate buffer (pH 7.4, 10 mM) was initiated by addition of either 5.0 mM CuSO4 at 37°C or 1.0 mM of 2,2′-azo-bis (2-amidinopropane) dihydrochloride (AAPH) at 40°C. It was found that α-tocopherol was completely depleted within 1 hour. Under the same experimental conditions, all four flavonoids demonstrated a dose-dependent protecting activity to α-tocopherol in LDL at the concentration ranging from 1 to 20μM. All flavonoids showed a varying protective activity against depletion of α-tocopherol in LDL, with kempherol and morin being less effective than myricetin and quercetin. The addition of flavonoids to the incubation mixture after 5 minutes demonstrated a significant regeneration of α-tocopherol in human LDL. The protective activity of four flavonoids to LDL is related to the number and location of hydroxyl groups in the B ring as well as the stability in sodium phosphate buffer.