Molecular Characterization of Quercetin and Quercetin Glycosides in Allium Vegetables

Flavonol levels in the edible portions of Allium vegetables (leeks, shallots, green onions, garlic and onions) range from <0.03 to > 1 g/kg of vegetable. Shallots contained uniformly high concentration of total flavonols, >800 mg/kg in each of five independent samples. However, onions varied widely in the amounts of flavonoids they contained. White onions contained no detectable flavonols, but 20 cultivars of yellow and red onions contained between 60 mg/kg and >1000 mg/kg. Individual samples of leeks, garlic, and scallions contained undetectable levels of flavonols. The primary flavonols present in shallots were quercetin 4-glucoside and quercetin aglycone; flavonols identified in onions were three quercetin diglucosides, quercetin 4'-glucoside, quercetin aglycone, and, in some cases, isorhamnetin monoglycoside or kaempferol monoglycoside. The relative concentrations of the various onion flavonols varied significantly in different onion cultivars. Treatment of vegetable flavonol extracts with human gut bacterial glucosidases (fecalase) efficiently deglycosylated the quercetin glucosides with concomitant release of free quercetin. Quercetin selectively inhibits the growth of transformedtumorigenic cells ( ras /3T3 and H35) and prevents the neoplastic transformation of NIH/3T3 cells with the oncogene H- ras .