Antioxidant Activity Assays For Assessing Flavonoids And Structure-Activity Relationships For Cellular Antioxidant Protection

Flavonoids are the main antioxidant components in daily food.The excessive reactive species and oxidative stress play key role in pathology of certain diseases.Epidemiological surveys have identified the inverse relationships between intake of flavonoid-rich food and risk of chronic diseases such as cancer,cardiovascular disease and neurodegenerative diseases.In this dissertation,chemical reasons for the varying antioxidant activities of flavonoids under different assays were analyzed.The roles of flavonoids structures on cellular protection were also investigated.The primary findings are listed as follows:1.The results of ABTS·+ assay were poorly correlated with those of other three assays(DPPH,FRAP,and FCR assays).It was shown that the oxidizing strength in ABTS·+ assay,as indicated by the cyclic voltammograms of flavonoids,is deeper than those in DPPH,FRAP,and FCR assays.In addition,the oxidizing potentials in these assays are much lower than those of biological reactive species.Therefore,the four assays cannot accurately reflect the."total reducing capacity" of flavonoids.2.ORAC values of flavonoids were affected by the reaction reactivity of probes towards peroxyl radical.Different probes in the two ORAC assays influenced the flavonoids to exhibit specific aspect of their structural properties.The results in ORACFL were mainly determined by electron transfer capacity of deprotonated flavonoid,whereas the ease of proton dissociation affects the radical-scavenging activities of flavonoids in ORACPGR assay.3.In oleic acid-induced fatty liver cell model,it was observed that EGCG and quercetin had the highest efficacies in the inhibition of TG over-accumulation and ROS generation.In general,there were obviously significant correlations between inhibitory effects of the selected flavonoids on cellular TG over-accumulation and ROS generation.This indicates that the alleviating effects of the selected flavonoids on cellular TG over-accumulation are interdependent with their attenuating oxidative stress activities.In structural aspect,it was observed that the flavonoids with 3,4-o-dihydroxyl group in the B-ring had inhibitory effects on the TG over-accumulation.Moreover,the flavonoids with medium hydrophobicity behaved higher efficacies in the TG and ROS inhibition than those with high or low Clog P values.4.The induction of heme oxygeanse-1(Hmoxl)by flavonoids in human aortic endothelial cells is determined by the specific structure:3-(3,4-Dihydroxyphenyl)-2-propenoic acid(caffeic acid).It was found that this structural skeleton,with high redox activity,could probably induce Hmoxl expression through its oxidant form "ortho-quinone".In addition,Hmox1 induction by quercetin may be due to a sulfhydryl-related redox stress.Overall,the findings in this dissertation are helpful to provide theoretical guide to develop new antioxidant activity assay and to use flavonoids for health maintenance.