Antioxidant ability of various flavonoids against DPPH radicals and LDL oxidation

Flavonoids, a group of polyphenolic compounds, exist naturally and serve as antioxidants in vegetables, fruits, and so on. The inhibition of low densi ty lipoprotein (LDL) oxidation may be an effective way to prevent or delay the progression of atherosclerosis. In the present study, we analyzed the r adical scavenging capacity of 10 flavonoids (catechin, epicatechin [EC], ep igallocatechin [EGC], epicatechin gallate [ECg], epigallocatechin gallate [ EGCg], myricetin, quercetin, apigenin, kaempferol, and luteolin) toward 1,1 -diphenyl-2-picryl-hydrazyl [DPPH]. After 20 min of incubation, EGCg was th e most effective DPPH radical scavenger, luteolin being the least active of this flavonoid group. The mutual antioxidant effect of flavonoids with alp ha -tocopherol (alpha -toc) on LDL oxidizability was investigated by using the lipophilic azo radical initiator 2,2'-azobis(4-methoxy-2,4-dimethylvale ronitrile) [AMVN-CH3O]. An inhibitory effect of flavonoids on LDL oxidation was observed in the order of luteolin>ECg>EC>quercetin>catechin>EGCg>EGC>m yricetin>kaempferol>apigenin. The shortened lag time induced by higher dose s of alpha -toc (6 mg/100 mL) was restored by flavonoids. These results sug gest that 1) radical trapping effects of flavonoids differ according to the ir structure, and 2) flavonoids act as hydrogen donors to alpha -toc radica l; furthermore, by interaction with alpha -toc, they have a greater potenti al to delay the oxidation of LDL.