Catechol-O-methyltransferase-catalyzed rapid O-methylation of mutagenic flavonoids: metabolic inactivation as a possible reason for their lack of carcinogenicity in vivo

Quercetin is highly mutagenic in vitro, yet is not carcinogenic when administered chronically at large doses to rodents for 12 months. We hypothesized that catechol-O-methyltransferase-catalyzed O-methylation of quercetin and other mutagenic catechol-containing flavonoids may provide an efficient inactivation in vivo and may therefore prevent tumor induction by these flavonoids. After one intraperitoneal administration of 50 mg/kg quercetin to hamsters, a urinary ether extract contained 2% quercetin and 97% 3'-O-methylquercetin. When the urine was treated first with beta-glucuronidase and sulfatase, 13% quercetin and 87% 3'-O-methylquercetin were recovered. Quercetin was rapidly O-methylated by either porcine liver or hamster kidney catechol-O-methyltransferase, with Km values of 6.1 and 6.9 micromolars and Vmax values of 14,870 and 200 pmol/mg of protein/min, respectively. S-Adenosyl-L-homocysteine exhibited a potent feedback inhibition of the catechol-O-methyltransferase-catalyzed O-methylation of quercetin by a competitive mechanism with respect to S-adenosyl-L-methionine and by a competitive plus noncompetitive mechanism with respect to the substrate. A comparison of the O-methylation rates and kinetic characteristics (Km, Vmax, and Vmax/Km) demonstrated that rates of O-methylation of quercetin and fisetin were up to three orders of magnitude higher than those of catechol estrogens and catecholamines. In conclusion, the rapid metabolic inactivation of mutagenic flavonoids catalyzed by catechol-O-methyltransferase may be a major reason for the lack of their carcinogenic activities in vivo.