Mechanistic Insight into the Peroxidase Catalyzed Nitration of Tyrosine Derivatives by Nitrite and Hydrogen Peroxide

Peroxidases perform the nitration of tyrosine and tyrosyl
residues in proteins, in the presence of nitrite and hydrogen
peroxide. The nitrating species is still unknown but it is
usually assumed to be nitrogen dioxide. In the present
investigation, the nitration of phenolic compounds derived
from tyrosine by lactoperoxidase and horseradish peroxidase
was studied, with the aim of elucidating themechanism
of the reaction. The results indicate that nitrogen dioxide
cannot be the only nitrating species and suggest the presence
of two simultaneously operative pathways, one proceeding
through enzyme-generated nitrogen dioxide and another
through a more reactive species, assumed to be complexed
peroxynitrite, which is generated by reaction of hydrogen
peroxide with the enzyme–nitrite complex. The importance
of the two pathways depends on peroxide and nitrite concentrations.
With lactoperoxidase, nitration through the
highly reactive intermediate is preferred except at very low
nitrite concentration, while with horseradish peroxidase, the
nitrogen dioxide driven mechanism is preferred except at
very high nitrite concentration.The preferred mechanism for
the two enzymes is that operative in the physiological nitrite
concentration range.