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Int J Clin Pharmacol Ther. 2002 May;40(5):207-12.

Methylation of quercetin and fisetin, flavonoids widely distributed in edible vegetables, fruits and wine, by human liver.

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Department of Neuroscience, Section of Pharmacology, Medical School, Cisanello Hospital at University of Pisa, Italy.


The aim of this investigation was to study the methylation of quercetin and fisetin, 2 chemically related flavonoids, in human liver and to this purpose, an assay was set-up to measure the rates of quercetin and fisetin methylation in human liver. The methylation rates (pmol/min/mg) of quercetin and fisetin were measured in 10 liver samples and the mean +/- SD and the median were 170+/-30 and 177 (quercetin) and 183+/-15 and 178 (fisetin). The rates of quercetin and fisetin methylation were not different (p = 0.283). The fold of variation among samples was 2 (quercetin) and 1.3 (fisetin). Methyltransferase towards quercetin and fisetin followed Michaelis-Menten kinetics, and the Km values were 2.6+/-0.3 (quercetin) and 8.6+/-0.7 microM (fisetin, p = 0.009) and the Vmax values were 187+/-20 (quercetin) and 276+/-33 pmol/min/mg (fisetin, p = 0.009). Two, 4 and 8 microl of red Chianti wine added to the incubation mixture reduced the rate of quercetin methylation to 75+/-4%, 65+/-9% and 59+/-9%, respectively, and that of fisetin methylation to 62+/-3%, 51+/-3% and 44+/-4%, respectively. In conclusion, quercetin and fisetin are methylated in human liver and their rates of methylation have a limited variation among subjects.

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