Reduction of phenoxyl radicals mediated by monodehydroascorbate reductase

Monodehydroascorbate (MDA) reductase catalyzes the reduction of MDA, the only organic radical substrate for the enzyme reported so far. Here, we show that cucumber MDA reductase is also capable of reducing phenoxyl radicals which are generated by horseradish peroxidase (HRP) with H2O2. The addition of MDA reductase plus NADH suppressed the HRP/H2O2-dependent oxidation of quercetin, accompanied by the oxidation of NADH, The quenching of the quercetin radical by MDA reductase plus NADH was confirmed by ESR. MDA reductase with NADH also suppressed the HRP/H2O2-dependent oxidation of hydroxycinnamates, including ferulic acid, coniferyl alcohol, and chlorogenic acid. Thus, the phenoxyl radicals of plant phenols can be reduced to their respective parent phenols by MDA reductase via a mechanism similar to the reduction of MDA. (C) 2000 Academic Press.