NO* release from MbFe(II)NO and HbFe(II)NO after oxidation by peroxynitrite

Area of Science:

• Biochemistry
• Chemical Kinetics

Background:

• Myoglobin (Mb) and hemoglobin (Hb) are crucial proteins involved in oxygen transport.
• Nitrosyl complexes of these proteins (MbFe(II)NO and HbFe(II)NO) are relevant in biological systems.
• Peroxynitrite is a reactive nitrogen species with known oxidative potential.

Purpose of the Study:

• To investigate the reaction kinetics of peroxynitrite with myoglobin and hemoglobin nitrosyl complexes.
• To elucidate the role of pH, carbon dioxide, and nitrite in these oxidation reactions.
• To determine the reactive species responsible for the oxidation of the iron center in MbFe(II)NO and HbFe(II)NO.

Main Methods:

• Stopped-flow spectrophotometry was used to monitor the reactions.
• Kinetic experiments were conducted under varying pH conditions.
• The influence of carbon dioxide and nitrite concentrations on reaction rates was assessed.

Main Results:

• The reaction proceeds in two steps, forming iron(III)myoglobin (MbFe(III)NO) and releasing nitric oxide (NO*).
• Second-order rate constants for peroxynitrite oxidation of MbFe(II)NO and HbFe(II)NO were determined.
• Reaction rates increased with decreasing pH, suggesting HOONO as the oxidant.
• Carbon dioxide enhanced the reaction rates, implicating ONOOCO2- or its radical products.
• Nitrite showed a slight acceleration of HbFe(II)NO oxidation, with a determined catalytic rate constant.

Conclusions:

• Peroxynitrite oxidizes MbFe(II)NO and HbFe(II)NO to their respective iron(III) forms.
• The pH-dependence suggests HOONO is the primary oxidant, while CO2 influences the reaction pathway.
• Nitrite can also oxidize the iron center, albeit at a much slower rate.