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 (τ) produced during free radical-induced sterol oxidation in lipid vesicles reflects the potency of the antioxidant. The ascorbic acid-induced τ value varies with sterol mol% in a biphasic manner, showing a minimum at the critical sterol mole fraction for maximal superlattice formation (Cr), in ascorbic acid concentrations ⩽120 μM, indicating that the sterol superlattice affects antioxidant potency. In contrast, the biphasic change in τ at Cr was observed only at doses of ascorbyl palmitate <15 μM, 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 τ value, it can easily perturb sterol lateral organization by insertion into membrane bilayers, which could impose detrimental effects on cells. The threshold antioxidant concentration (Cth) to abolish biphasic change in τ at Cr may vary with antioxidant and could be used to assess potential adverse effects of other lipid-soluble antioxidants.