The inhibitory effect of onion peel extract on platelet aggregation

Quercetin shows strong antioxidant activity, and it exerts prophylactic effects on arterial dysfunction through inhibitory actions on LDL cholesterol oxidation and platelet aggregation. Onion is the main dietary source of quercetin; however, the quercetin content in onion peel is 10 times higher than that in the bulb, and the peel is usually discarded before cooking. To use onion peel by-product, we prepared two onion peel extracts and examined their anti-platelet coagulation activity using rabbit and human platelets in vitro. Water was added to the onion peel, the mixture was heated at 110 degrees C for 90 min, and the whole peel extract was obtained. Next, the whole peel extract was subjected to centrifugation at 10 000 x g for 5 min, and the resultant precipitate was recovered. The ratios of quercetin and quercetin-4'-monoglucoside contents were 7:3 and 9.5:0.5 in the whole extract and precipitate, respectively. Using platelet activating factor, adenosine di-phosphate, and thrombin as stimulators of platelet aggregation, the whole peel extract and precipitate were assayed for anti-platelet coagulation. Both the peel extract and precipitate strongly inhibited the platelet coagulation stimulated by the three factors as well as quercetin. Further, we examined the activities of quercetin-4'-glucoside and quercetin-3-glucuronide; the inhibitory activity of quercetin-4'-glucoside was weaker than that of quercetin-3-glucuronide on human platelet aggregation. Following absorption of quercetin into human blood, it is metabolized to quercetin-3-glucuronide. The data obtained in this study indicated that onion peel may be a useful source of quercetin to prevent arterial dysfunction.