Catalase-like activity of human methemoglobin: a kinetic and mechanistic study

Hydrogen peroxide triggers a redox cycle between methemoglobin and ferrylhemoglobin, leading to protein inactivation and oxygen evolution. In the present paper, the catalase-like oxygen production by human methemoglobin in the presence of H₂O₂ was kinetically characterized with a Clark-type electrode. Progress curves showed a pseudo-steady state in the first minutes of the reaction, while double-reciprocal plots were upwardly concave, indicating positive co-operativity dependent upon protein concentration, which is a very unusual kinetic behavior. Addition of superoxide radical scavengers slightly increased activity, suggesting that most oxygen was produced biocatalytically. By considering all the experimental data obtained, a possible mechanism was proposed, including: (a) competition between the one-electron and the two-electron reductions of the oxoferryl free radical species of hemoglobin, giving rise to ferrylhemoglobin and methemoglobin, respectively; (b) competition between the superoxide-dependent inactivation of the protein and its reduction back to the met state. Computer simulations of that model have been performed by numerically integrating the differential equations set describing the mechanism, which was seen to yield predictions of the kinetic parameters variation consistently with the kinetic behavior experimentally observed. We suggest that the catalase-like activity of methemoglobin must predominantly be a biocatalytic reaction that protects the protein against H₂O₂-induced suicide inactivation.