Content of flavonol glucosides and some properties of enzymes metabolizing the glucosides in onion (Flavonoid in fruits and vegetables, part II)

Abstract

The contents of flavonol glucosides in yellow, red and white onions harvested in Hokkaido were measured. Seven varieties of yellow onion averaged 16.8 mg/100 g of quercetin-3, 4'-di-O-β-glucoside, 18.5 mg/100 g of quercetin-4'-O-β-glucoside, 0.8 mg/100 g of quercetin-3-O-β-glucoside and 2.9 mg/100 g of isorhamnetin-4'-O-β-glucoside. The red onion contained about 3-fold the amount of these glucosides while the white onion contained no detectable amounts of flavonols. The content of each flavonoid was higher in the outer scales than in the inner ones. Flavonol glucosidases and UDPG : flavonol glucosyltransferases were detected in the onions. The optimum pH of quercetin-3-O-β-glucosyltransferase, 4'-O-β-glucosyltransferase, 3-O-β-glucosidase and 4'-O-β-glucosidase was 6.0, 8.0, 4.5 and 7.0, respectively. In all the onion varieties, the activity of quercetin-4'-β-glucosyltransferase was higher than that of 4'-O-β-glucosidase, but the activity of 3-O-β-glucosyltransferase was lower than that of 3-O-β-glucosidase. Since quercetin-4'-O-β-glucosyltransferase was not able to transglucosylate quercetin-3-O-β-glucoside and rutin, the precursor of quercetin-3, 4'-di-O-β-glucoside is estimate to be the 4'-O-β-glucoside and not the 3-O-β-glucoside.