Nitrite binding to metmyoglobin and methemoglobin in comparison to nitric oxide binding

Nitrite binds reversibly to the ferriheme pro- teins metmyoglobin and methemoglobin in aqueous bu er solution at a physiological pH of 7.4. The spectral changes recorded for the formation of metMb$NO_{2}^{-}$ di er signi®cantly from those observed for the nitrosy- lation of metMb, which can be accounted for in terms of the di erent reaction products. Nitric oxide binding to metMb produces a nitrosyl product with Fe(II)-$NO^{+}$ character, whereas the reaction with nitrite produces an Fe(III)-$NO_{2}^{-}$ complex. The kinetics of the binding and release of nitrite by metMb and metHb were investigated by stopped-flow techniques at ambient and high pres- sure. The kinetic traces recorded for the reaction of ni- trite with metMb exhibit excellent single-exponential fits, whereas nitrite binding to metHb is characterized by double-exponential kinetics which were assigned to the reactions of the $\alpha-$ and $\beta-$ chains of metHb with $NO_{2}^{-}$. The rate constants for the binding of nitrite to metMb and metHb were found to be much smaller than those reported for the binding of NO, such that nitrite impu- rities will not a ect the latter reaction. The activation parameters $(\Delta H^{\neq }, \Delta S^{ne}, \Delta V^{\neq })$ obtained from the temperature and pressure dependence of the reactions support the operation of a dissociative mechanism for the binding and release of nitrite, similar to that found for the binding and release of NO in metMb.