Nitrite catalyzes ferriheme protein reductive nitrosylation.

Nitrite ion is found to catalyze the NO reduction of met-hemoglobin and met-myoglobin in pH 7.0 buffered aqueous solution. The catalysis rate constants for these ferriheme proteins and for two water-soluble ferriheme model systems follow the same order as do the FeIII/II reduction potentials of the ferric nitrosyl complexes. This is consistent with a proposed mechanism occurring via outer sphere reduction of the FeIII(NO) center by NO2- to give the FeII(NO) product plus NO2. Although the first step is thermodynamically uphill, the NO2 generated would be rapidly trapped by excess NO to form N2O3, which would hydrolyze. We speculate that, if formed in the proximity of the protein, the strong nitrosating agent N2O3 could also result in protein modifications.