Identification and in vitro biological activities of flavonols in garlic leaf and shoot: inhibition of soybean lipoxygenase and hyaluronidase activities and scavenging of free radicals

We isolated four flavonols from garlic (Allium sativum L) leaf and shoot and measured the in vitro antioxidant activity of the isolated flavonols and their aglycones. The chemical structures of the compounds were shown to be quercetin 3‐O‐β‐D‐glucopyranoside (isoquercitrin), quercetin 3‐O‐β‐D‐xylopyranoside (reynoutrin), kaempferol 3‐O‐β‐D‐glucopyranoside (astragalin) and isorhamnetin 3‐O‐β‐D‐glucopyranoside based on FAB‐MS and NMR analyses. Assays of DPPH (1,1‐diphenyl‐β‐picryl hydrazyl) and hydroxyl radical scavenging, inhibition of linoleic acid peroxidation, and soybean lipoxygenase (LO) and hyaluronidase (HYA) inhibition were used to evaluate the antioxidant activity. Quercetin and its glycosides showed the highest antioxidant activity among the compounds. In the LO assay, the IC50 values of quercetin, isoquercitrin and reynoutrin were 16.9, 40.1 and 32.9 µM respectively. Quercetin was the most effective among the flavonols. In the HYA assay the IC50 values of quercetin, isoquercitrin and reynoutrin were 23.0, 20.9 and 22.1 mM respectively. Isoquercitrin had the most potent inhibitory activity on HYA. The inhibition patterns of the flavonols on LO and HYA were elucidated as mixed types of competitive and non‐competitive inhibition according to Lineweaver–Burk plot results. Although most garlic shoots and leaves are discarded and not used at present, our results suggest that ancillary garlic parts could be utilised as functional foods or ingredients.