Anti-platelet and membrane-rigidifying flavonoids in brownish scale of onion

The bio-activity of the brownish scale of onion (Allium cepa) was studied together with identifying the active components and addressing the mode of action. A crude MeOH extract (0.5-1.0 mg/ml) showed the inhibitory effects on human platelet aggregation induced by collagen, adenosine 5'-diphosphate (ADP), thrombin and epinephrine. The anti-platelet extract (1.0 mug/ml) rigidified liposomal membranes by acting on the hydrocarbon core more intensively than the surface of membrane lipid bilayers. Serial solvent extractions and chromatographic purifications provided four isolates which were structurally identified as different quercetin dimers (I and 2), quercetin (3) and quercetin-4'-glucoside (4). The flavonoidal components 1, 3, 2 and 4 (0.5-2 mM) inhibited collagen-induced platelet aggregation in increasing order of intensity. More active I and 3 (2 mM) also dissociated the aggregates produced by ADP. The anti-platelet flavonoids (0.25-10 muM) acted on liposomes of the lipid composition resembling human platelets to cause membrane rigidification which was greatest in the order of 1, 2, 3 and 4. The interaction with membrane lipids to modify membrane fluidity appears to be partly responsible for the anti-aggregatory and disaggregatory effects on human platelets. Although the inedible scale of onion is usually regarded as waste, it has the possibility to be a medicinal resource.