STOPPED-FLOW INVESTIGATION OF ANTIOXIDANT ACTIVITY OF ESTROGENS IN SOLUTION

A kinetic study of the reaction between estrogens (female hormone) and substituted phenoxyl radical has been performed, as a model for the reactions of estrogens with lipid peroxyl radical in biological systems. The rates of reaction of estrogens (estrone 1, estradiol 2, 2-methoxyesterone 3, 3-methoxyestrone 4, and 2-hydroxyestrone 5) with substituted phenoxyl radical in benzene have been determined spectrophotometrically, using stopped-flow technique. The second-order rate constants, k2, obtained are 84 M-1 · s-1 for 3, < 10-4 M-1 for 4, and 2.6 · 105 M-1 · s-1 for 5 at 25.0°C. 2-Hydroxyestrone 5 was found to be 2.9-times more active than α-tocopherol, which has the highest antioxidant activity among natural tocopherols. The order of magnitude of k2 value (1<2<3<α-Toc<5) is in agreeement with that of in vitro tests of their antioxidant activities, as measured by the inhibition of lipid peroxidation. Further, similar measurements have been performed for the reaction between the above estrogens 1-5 and tocopheroxyl 6 in benzene solution. It was found that the estrogens having an OH group at the aromatic ring have an ability to regerate the tocopheroxyl 6 to tocopherol. Especially, the 2-hydroxyestrone 5 showed about three orders of magnitude higher reactivity than ascorbic acid. © 1990.