Abstract
The growth-inhibitory effects of the brownish scale components of onion on tumor cells were studied with relating to their membrane activity. Quercetin, quercetin-4'-O-glucoside and two isomeric quercetin dimers (10 μM for each) isolated from the scale reduced the fluidity of tumor cell model membranes consisting of phospholipids and cholesterol more significantly than that of normal cell model membranes. In flavonoidal components, the membrane activity was greatest in the order of dimers, aglycone and glucoside. Quercetin and its dimers intensively acted on the membrane center rather than the membrane surface, while quercetin-4'-O-glucoside was relatively effective on the hydrophilic regions of membranes. Membrane-active flavonoids inhibited the growth of mouse myeloma cells at 10-100 μM with the same rank of order of potency as they rigidified liposomal membranes. Quercetin and its dimers rigidified cell membranes by acting on the hydrophobic inner regions simultaneously with inhibiting the cell growth, but not quercetin-4'-O-glucoside. Flavonoidal components in the brownish scale of onion have the potent anti-proliferative activity associated with the structure-specific rigidification of cell membranes, which is induced by the interaction with membrane lipid bilayers.
