Nitrogen dioxide ((NO2)-N-.) participates in a variety of biological reactions. Of great interest are the reactions of (NO2)-N-. with oxymyoglobin and oxyhemoglobin, which are the predominant hemeproteins in biological systems. Although these reactions occur rapidly during the nitrite-catalyzed autoxidation of hemeproteins, their roles in systems producing (NO2)-N-. in the presence of these hemeproteins have been greatly underestimated. In the present study, we employed pulse radiolysis to study directly the kinetics and mechanism of the reaction of oxymyoglobin (MbFe(II)O(2)) with (NO2)-N-.. The rate constant of this reaction was determined to be (4.5 +/- 0.3) x 10(7) M(-1)s(-1), and is among the highest rate constants measured for (NO2)-N-. with any biomolecule at pH 7.4. The interconversion among the various oxidation states of myoglobin that is prompted by nitrogen oxide species is remarkable. The reaction of MbFeIIO(2) with (NO2)-N-. forms MbFeIIIOONO(2), which undergoes rapid heterolysis along the O-O bond to yield MbFe(V)=O and NO3-. The perferryl-myoglobin (MbFe(V=)O) transforms rapidly into the ferryl species that has a radical site on the globin ((.)MbFe(IV)=O). The latter oxidizes another oxymyoglobin (10(4) M(-1)s(-1) < k(17) < 10(7) M(-1)s(-1)) and generates equal amounts of ferrylmyoglobin and metmyoglobin. At much longer times, the ferrylmyoglobin disappears through a relatively slow comproportionation with oxymyoglobin (k(18) = 21.3 +/- 5.3 M(-1)s(-1)). Eventually, each (NO2)-N-. radical converts three oxymyoglobin molecules into metmyoglobin. The same intermediate, namely MbFe(III)OONO(2), is also formed via the reaction peroxynitrate (O2NOO-/O2NOOH) with metmyoglobin (k(19) = (4.6 +/- 0.3) x 10(4) M(-1)s(-1)). The reaction of (NO2)-N-. With ferrylmyoglobin (k(20) = (1.2 +/- 0.2) x 10(7) M(-1)s(-1)) yields MbFe(III)ONO(2), which in turn dissociates (k(21) = 190 +/- 20 s(-1)) into metmyoglobin and NO3-. This rate constant was found to be the same as that measured for the decay of the intermediate formed in the reaction of MbFe(II)O(2) with (NO2)-N-. which suggests that MbFe(III)ONO(2) is the intermediate observed in both processes. This conclusion is supported by thermokinetic arguments. The present results suggest that hemeproteins may detoxify (NO2)-N-. and thus preempt deleterious processes, such as nitration of proteins. Such a possibility is substantiated by the observation that the reactions of NO2 with the various oxidation states of myoglobin lead to the formation of metmyoglobin, which, though not functional in the gas transport, is nevertheless nontoxic at physiological pH.