Reduction of Ferrylmyoglobin by Hydrogen Sulfide. Kinetics in Relation to Meat Greening

Abstract

The hypervalent meat pigment ferrylmyoglobin, MbFe(IV)═O, characteristic for oxidatively stressed meat and known to initiate protein cross-linking, was found to be reduced by hydrogen sulfide to yield sulfmyoglobin. Horse heart myoglobin, void of cysteine, was used to avoid possible interference from protein thiols. For aqueous solution, the reactions were found to be second-order, and an apparent acid catalysis could be quantitatively accounted for in terms of a fast reaction between protonated ferrylmyoglobin, MbFe(IV)═O,H(+), and hydrogen sulfide, H2S (k2 = (2.5 ± 0.1) × 10(6) L mol(-1) s(-1) for 25.0 °C, ionic strengh 0.067, dominating for pH < 4), and a slow reaction between MbFe(IV)═O and HS(-) (k2 = (1.0 ± 0.7) × 10(4) L mol(-1) s(-1) for 25.0 °C, ionic strengh 0.067, dominating for pH > 7). For meat pH, a reaction via the transition state {MbFe(IV)═O···H···HS}([symbol: see text]) contributed significantly, and this reaction appeared almost independent of temperature with an apparent energy of activation of 2.1 ± 0.7 kJ mol(-1) at pH 7.4, as a result of compensation among activation energies and temperature influence on pKa values explaining low temperature greening of meat.

Improving meat quality through cattle feed enriched with mate extract: an integrated approach of the metabolic profile and redox chemistry of meat

“…21,94,194 Protonation of ferrylmyoglobin (eqn (19)) and its intermediates has been shown to occur under acidic conditions to form highly reactive species that promptly oxidize relevant biologic reductants. 19,21,194 ( ) = + ⇌ ( ) = , pK a = 4.9 (19) The increasing rate of reduction of MbFe(IV)=O at decreasing pH may be explained by the presence of the strongly reactive MbFe(IV)=O, H + species which is produced from the protonation of ferrylmyoglobin according to the equilibrium of eqn (19). Also, decreasing pH accounted for increasing the rate of the so-called autoreduction of MbFe(IV)=O assigned to specific acid catalysis initiated through a intramolecular electron transfer to form a protein radical cation as seen in eqn (20).…”

“…Kinetic analysis for the reaction between the two acid/base couples using time-dependent spectral measurements for the reduction of ferrylmyoglobin to metmyoglobin is currently reported in literature based on the depletion in the absorbance at 580 nm. 21,193,195,206 In order to get reliable measurements from this depletion it is necessary to use 30×   mol L -1 as a minimum concentration of ferrylmyoglobin. However, the low aqueous solubility of uric acid especially under acidic conditions, requires the use of very low concentrations of ferrylmyoglobin to ensure pseudo first order conditions with an excess of uric acid.…”

“…pH On the basis of the possible four reactions between the acid/base forms of the reactants, the second-order rate constant, k', as dependent on pH was used to estimate the specific rate constants for these four reactions according to the following rate expression for conditions of excess of uric acid/urate 21,202 :…”

“…The role of hypervalent myoglobin species in the initiation of oxidative damage in meat and biological system has been further investigated by many researchers. 2,3,[15][16][17][18][19][20][21][22] Hypervalent meat pigments, ferryl myoglobin (MbFe(IV)=O) and perferrylmyoglobin (  MbFe(IV)=O), are generated from the activation of metmyoglobin (MbFe(III)) with H 2 O 2 and/or organic hydroperoxides present in foods, initiating the pseudoperoxidase catalytic cycle of myoglobin. 23 Both perferryl myoglobin and ferryl myoglobin are strong oxidant species known to initiate lipid and protein oxidation in muscle tissues, and meat products.…”

“…The formation of disulfides through the cross-linking of myosin may occur via a radicalar mechanism as described by Lund et al 21 Thyil radicals (RS • ) produced by the oxidation of cysteine residues may be trapped by DMPO yielding a product that can be detected by immoblotting ( Figure 40). …”