Covalent binding of the flavonoid quercetin to human serum albumin.

Quercetin is an abundant flavonoid in the human diet with numerous biological activities, which may contribute to the prevention of human disease but also may be potentially harmful. Quercetin is oxidized in cells to products capable of covalently binding to cellular proteins, a process that may be important for its biological activities. In the present study, using radiolabeled drug and quantifying the products after electrophoretic separation, proteins to which oxidized quercetin is binding irreversibly were identified. The binding of quercetin to human serum albumin (HSA) in human blood and the effect of stimulation of neutrophilic myeloperoxidase on this binding were also measured. The in vitro binding of quercetin to eight proteins in the presence of catalytic amounts of horseradish peroxidase and hydrogen peroxide was highly selective for HSA. For all proteins the binding was dramatically decreased by reduced L-glutathione. In the blood samples, the release of neutrophilic myeloperoxidase by phorbol ester caused a 3-fold increase in the binding of quercetin to HSA. This study shows that quercetin in the presence of peroxidase/hydrogen peroxide covalently links to proteins with a particularly high affinity for HSA and that this also may occur in vivo after exposure to quercetin. This provides further insights into the complex behavior of this major dietary flavonoid.

Resveratrol transport and metabolism by human intestinal Caco-2 cells.

Resveratrol is a dietary constituent suggested to have protective effects against cancer as well as cardiovascular disease. The purpose of the study was to learn whether this agent could be absorbed in man and enter the systemic circulation. This was examined by measuring transport and metabolism of resveratrol (5-40 microM) by the human intestinal epithelial cell line Caco-2 cultured in Transwells. Transport across the Caco-2 monolayer occurred in a direction-independent manner with P(app) values of approximately 7 x 10(-6) cm s(-1), much higher than for the paracellular transport marker mannitol (approximately 0.4 x 10(-6) cm s(-1)), suggesting efficient absorption in-vivo. At the highest resveratrol concentration, the absorption increased, possibly due to saturation of metabolism. In sharp contrast to previous findings in the rat, the metabolism of resveratrol in Caco-2 cells involved mainly sulfation and, to a minor extent, glucuronidation. At low resveratrol concentrations, most of the sulfate conjugate was exported to the apical side, presumably by MRP2, which is well expressed in these cells. At high concentrations, there was a shift towards the basolateral side, possibly involving MRP3, which was recently shown also to be expressed in Caco-2 cells. These results indicate that absorption of resveratrol in-vivo may be high but with limited bioavailability due to efficient sulfate conjugation. Extensive accumulation of resveratrol in the Caco-2 cells, demonstrated in additional experiments, suggests enterocytes as a major target site for this cancer preventive agent.