Deactivation of hypervalent meat pigments. Kinetics of reduction of ferrylmyoglobin by nitrite and iodide

The hypervalent heme pigment ferrylmyoglobin, MbFe(IV)=O, a potential pro-o xidant in meat products, is deactivated efficiently by the antioxidant nitr ite to a nitrite complex of metmyoglobin, MbFe(III). The second-order rate constant for direct reduction in aqueous 0.16 M NaCl with pH = 7.4 was foun d to have the value k(2) = 13.9 +/- 0.4 M-1 s(-1) at 25 degrees C with the activation parameters Delta H-# = 30 +/- 2 kJ mol(-1) and Delta S-# = -123 +/- 7 J mol(-1) x K-1. Iodide, an example of a simple spheric electron dono r, is less efficient with k(2) = 0.34 +/- 0.01 M-1 s(-1) as the result of a higher enthalpy of activation (Delta H-# = 43 +/- 3 kJ mol(-1) and Delta S -# = -109 +/- 8 J mol(-1) K-1). Although nitrite, like iodide, thus belongs to a class of deactivators, For which intra-molecular electron transfer to form a cation radical (+)MbFe(III)-O- is rate-determining and characterize d by a negative entropy of activation, it differs from the spheric iodide, by inducing conformation changes in the protein to lower the barrier of act ivation. (C) 2000 Elsevier Science Ltd. All rights reserved.