ANTIOXIDATIVE ACTIVITY OF QUERCETIN AND QUERCETIN MONOGLUCOSIDES IN SOLUTION AND PHOSPHOLIPID-BILAYERS

The antioxidative effect of quercetin, quercetin 3-O-beta-D-glucopyran oside (Q3G), quercetin 4'-O-beta-D-glucopyranoside (Q4'G) and querceti n 7-O-beta-D-glucopyranoside (Q7G) was examined in solution and liposo mal phospholipid suspension. First, their peroxyl radical-scavenging a ctivities were investigated by measuring the inhibition of hydroperoxi dation of methyl linoleate initiated by a radical initiator, 2,2'-azob is(2,4-dimethylvaleronitrile) (AMVN). Quercetin exhibited the highest peroxyl radical-scavenging activity judging from the rate of hydropero xidation during the induction period (R(inh)) and the length of induct ion period (t(inh)). Although Q7G showed an induction period, its R(in h) was higher and its t(inh) was lower than that of quercetin. Neither Q3G nor Q4'G gave a clear induction period in the curve of hydroperox ide formation. The rate of hydroperoxidation in the presence of Q3G wa s higher than R(inh) of quercetin and the oxidative loss of Q3G was mu ch slower than quercetin or Q7G when exposed to AMVN in solution. Q4'G exerted little inhibition compared to Q3G or Q7G. Next, the antioxida tive activity of quercetin and its monoglucosides in phospholipid bila yers was examined by measuring the inhibition of lipid peroxidation in large unilamellar vesicles composed of egg yolk phosphatidylcholine ( PC) and a water-soluble radical initiator. They retarded the accumulat ion of PC-hydroperoxides and the induction period increased in the ord er of Q4'G < Q3G similar to Q7G < quercetin. It is therefore concluded that quercetin acts as an antioxidant more efficiently than its monog lucosides when phospholipid bilayers are exposed to aqueous oxygen rad icals.