Rapid scavenging of peroxynitrous acid by monohydroascorbate

Area of Science:

• Biochemistry
• Chemical Kinetics
• Free Radical Chemistry

Background:

• Peroxynitrite is a reactive nitrogen species implicated in oxidative stress.
• Ascorbic acid (vitamin C) is a major biological antioxidant.
• The reaction kinetics between peroxynitrite and ascorbic acid are crucial for understanding oxidative damage.

Purpose of the Study:

• To reinvestigate the reaction kinetics between peroxynitrous acid and monohydroascorbate.
• To determine the rate constants and reaction mechanism under specific conditions.
• To clarify the role of monohydroascorbate as a scavenger of peroxynitrous acid.

Main Methods:

• Stopped-flow kinetics experiments were conducted.
• Reactions were studied at pH 5.8 and temperatures of 25°C and 37°C.
• Electron Paramagnetic Resonance (EPR) spectroscopy was used to detect radical species.

Main Results:

• New rate constants for the reaction were determined, significantly higher than previously reported.
• The reaction mechanism involves the formation of two intermediates, with the second having an absorption maximum at 345 nm.
• Monohydroascorbate concentrations influence the decay products of the second intermediate, leading to nitrate or nitrite and dehydroascorbate.
• The ascorbyl radical is a minor product, not a primary reaction product, with a yield of 0.24%.
• Homolysis of peroxynitrous acid was not observed under the experimental conditions.

Conclusions:

• Monohydroascorbate is a potent scavenger of peroxynitrous acid, forming adducts even at low concentrations.
• The reaction proceeds via intermediate species, with distinct pathways at different monohydroascorbate concentrations.
• Ascorbic acid's role in mitigating oxidative stress may involve direct scavenging of peroxynitrite.