Relative Reactivity of Vitamin A vs. a Mixture of ß-Carotene Geometric Isomers with Electrochemically Generated Superoxide and Hydroperoxyl Radicals

The present study comprises a kinetic comparison of the reactivity of electrochemically generated superoxide, O _2• ^- , and hydroperoxyl, HO _2• , radicals toward a mixture of β-carotene geometric isomers versus all-trans -retinol (vitamin A). Rate constants were determined by computer simulation of experimental cyclic voltammetric data. It is reported that all-trans -retinol reacts more readily with both O _2• ^- and HO _2• than a mixture of geometric β-carotene isomers. Specifically, the latter is unreactive at ≤ 25 mM with O _2• ^- within the timescale of cyclic voltammetry, while it reacts with HO _2• at least an order of magnitude slower than all-trans -retinol. Our results indicate that all-trans -retinol is a better radical scavenger than a mixture of geometric β-carotene isomers and parallel the results of a recent epidemiological study which has shown that vitamin a may be a better anticarcinogenic agent than β-carotene.