Sterol superlattice affects antioxidant potency and can be used to assess adverse effects of antioxidants.

We have developed a fluorescence method to examine how membrane sterol lateral organization affects the potency of antioxidants, and used this information to evaluate possible adverse effects of lipid-soluble antioxidants seen in recent clinical studies. In the presence of an antioxidant, the lag time (tau) produced during free radical-induced sterol oxidation in lipid vesicles reflects the potency of the antioxidant. The ascorbic acid-induced tau value varies with sterol mol% in a biphasic manner, showing a minimum at the critical sterol mole fraction for maximal superlattice formation (C r), in ascorbic acid concentrations <or=120 microM, indicating that the sterol superlattice affects antioxidant potency. In contrast, the biphasic change in tau at C r was observed only at doses of ascorbyl palmitate <15 microM, above which the biphasic change at Cr is abolished. Our data suggest that while ascorbyl palmitate is a more efficient antioxidant than its water-soluble counterpart as judged by the tau value, it can easily perturb sterol lateral organization by insertion into membrane bilayers, which could impose detrimental effects on cells. The threshold antioxidant concentration (C th) to abolish biphasic change in tau at C r may vary with antioxidant and could be used to assess potential adverse effects of other lipid-soluble antioxidants.