In vitro effects of a flavonoid-rich extract on LDL oxidation.

Flavonoids are phenolic compounds of vegetable origin with antioxidant effects. The present study aimed to determine their properties as LDL antioxidants. LDL were incubated with increasing concentrations of flavonoids (0-16 micrograms/ml) and LDL oxidation was started by adding CuCl2 (2 microM) to the media. When flavonoids were present in the media, vitamin E consumption, the lag phase of conjugated diene formation, LDL electrophoretic mobility in agarose gels and the appearance of thiobarbituric acid reacting substances (TBARS) were delayed in a concentration-dependent manner. To determine whether flavonoids could terminate LDL oxidation once initiated, two sets of experiments were performed. In the first, LDL oxidation was initiated as described above. At 2 or 4 h of incubation, flavonoids were added (4 micrograms/ml) and their effect compared to samples where butylated hydroxytoluene or EDTA were added. At 5 h, in the LDL samples where flavonoids were added, the electrophoretic mobility and TBARS production were the same as those present in LDL samples incubated for the whole period in the absence of flavonoids. However, when either butylate hydroxytoluene or EDTA was added, as would be expected, the LDL oxidation process was completely arrested as shown by a reduction in the appearance of TBARS and a lower LDL electrophoretic mobility. In the second experiment, LDL oxidation was initiated as described above and at 0, 10 and 20 min, flavonoids were added (4 micrograms/ml). When vitamin E was still present in the LDL solution, the flavonoids were able to both increase the lag phase in the formation of conjugated dienes and to delay the consumption of vitamin E. The present results show that in vitro, flavonoids prevent LDL oxidation in a concentration-dependent manner, delaying the consumption of vitamin E, but they cannot terminate or delay LDL oxidation once vitamin E in LDL is consumed.