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Abstract
Quercetin is an important dietary flavonoid with in vitro antioxidant activity. However, it is found in human plasma as conjugates with glucuronic acid, sulfate or methyl groups, with no significant amounts of free quercetin present. The antioxidant properties of the conjugates found in vivo and their binding to serum albumin are unknown, but essential for understanding possible actions of quercetin in vivo. We, therefore, tested the most abundant human plasma quercetin conjugates, quercetin-3-glucuronide, quercetin-3′-sulfate and isorhamnetin-3-glucuronide, for their ability to inhibit Cu(II)-induced oxidation of human low density lipoprotein and to bind to human albumin, in comparison to free flavonoids and other quercetin conjugates. LDL oxidation lag time was increased by up to four times by low (<2 μM) concentrations of quercetin-3-glucuronide, but was unaffected by equivalent concentrations of quercetin-3′-sulfate and isorhamnetin-3-glucuronide. In general, the compounds under study prolonged the lag time of copper-induced LDL oxidation in the order: quercetin-7-glucuronide>quercetin>quercetin-3-glucuronide=quercetin-3-glucoside>catechin>quercetin-4′-glucuronide>isorhamnetin-3-glucuronide>quercetin-3′-sulfate. Thus the proposed products of small intestine metabolism (quercetin-7-glucuronide, quercetin-3-glucuronide) are more efficient antioxidants than subsequent liver metabolites (isorhamnetin-3-glucuronide, quercetin-3′-sulfate). Albumin-bound conjugates retained their property of protecting LDL from oxidation, although the order of efficacy was altered (quercetin-3′-sulfate>quercetin-7-glucuronide>quercetin-3-glucuronide>quercetin-4′-glucuronide=isorahmnetin-3-glucuronide). Kq values (concentration required to achieve 50% quenching) for albumin binding, as assessed by fluorescence quenching of Trp214, were as follows: quercetin-3′-sulfate (∼4 μM)=quercetin≥quercetin-7-glucuronide>quercetin-3-glucuronide=quercetin-3-glucoside>isorhamnetin-3-glucuronide>quercetin-4′-glucuronide (∼20 μM). The data show that flavonoid intestinal and hepatic metabolism have profound effects on ability to inhibit LDL oxidation and a lesser but significant effect on binding to serum albumin.