Anti-Platelet and Membrane-Rigidifying Flavonoids in Brownish Scale of Onion

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Abstract
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 epineph- rine. The anti-platelet extract (1.0 µg/ml) rigidified liposomal membranes by acting on the hydrocarbon core more intensively than the surface of membrane lipid bilayers. Serial solvent extractions and chromatographic purifica- tions provided four isolates which were structurally identified as different quercetin dimers (1 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 1 and 3 (2 mM) also dissociated the aggregates produced by ADP. The anti-platelet flavonoids (0.25-10 µM) acted on liposomes of the lipid composition resem- bling human platelets to cause membrane rigidification which was greatest in the order of 1, 2, 3 and 4. The interac- tion 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.

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