Reduction of phenoxyl radicals mediated by monodehydroascorbate reductase

Area of Science:

• Biochemistry
• Enzymology
• Plant Science

Background:

• Monodehydroascorbate (MDA) reductase is known to reduce MDA, its sole reported organic radical substrate.
• Phenoxyl radicals are reactive intermediates implicated in various biological processes.

Purpose of the Study:

• To investigate if cucumber MDA reductase can reduce phenoxyl radicals.
• To explore the enzymatic mechanism and substrate scope of MDA reductase beyond MDA reduction.

Main Methods:

• Enzyme assays using horseradish peroxidase (HRP) and hydrogen peroxide (H2O2) to generate phenoxyl radicals.
• Spectrophotometric monitoring of substrate oxidation (quercetin, hydroxycinnamates) and NADH consumption.
• Electron Spin Resonance (ESR) spectroscopy to confirm radical quenching.

Main Results:

• Cucumber MDA reductase demonstrated the ability to reduce phenoxyl radicals generated by HRP/H2O2.
• The enzyme, with NADH, suppressed the oxidation of quercetin and various hydroxycinnamates (ferulic acid, coniferyl alcohol, chlorogenic acid).
• ESR confirmed the quenching of quercetin radicals by MDA reductase and NADH.

Conclusions:

• Cucumber MDA reductase exhibits broader radical-scavenging activity than previously known, reducing plant phenol-derived phenoxyl radicals.
• The reduction mechanism is analogous to that of MDA reduction, involving NADH as a reductant.
• This finding expands the understanding of MDA reductase's role in antioxidant defense and redox regulation in plants.