High concentrations of antioxidants may not improve defense against oxidative stress.

It is often assumed that the oxygen radical defense could be further improved by higher concentration of antioxidants. But this has not been demonstrated over a wide range of concentrations. There are different types of oxygen radicals produced in the body and the antioxidant protection against them may not positively related to their concentrations. We report here that by using H(2)O(2) with Cu(2+) as an hydroxyl-radical generator in vitro, ascorbic acid shows no oxygen-radical absorbing capacity. We also found that the net hydroxyl-radical absorbing capacity of a water soluble alpha-tocopherol analogue (Trolox) and uric acid increases with concentration only when the concentration is lower than the normal value found for alpha-tocopherol and uric acid in human serum. At higher concentrations, the hydroxyl-radical absorbing capacity of the alpha-tocopherol analogue and uric acid decreases. The mechanism involved in the decrease of hydroxyl radical absorbance capacity of Trolox and uric acid at high concentration may be related to their reaction with hydroxyl radicals or other oxygen radicals produced in the presence of both H(2)O(2) and Cu(2+). This kind of reaction could lead to the formation of additional many Trolox or uric acid radicals at the same time. These results may be important not only in evaluating antioxidant activities of antioxidants in vitro but also in studying the potential efficiency of antioxidants in vivo in affecting oxidative stress status.