Spectrophotometric studies of the reaction of methaemoglobin with hydrogen peroxide. 1. The formation of methaemoglobin-hydrogen peroxide

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Abstract
The reaction between catalase-free human methemoglobin and hydrogen peroxide was studied spectrokinetically. With high concn. of peroxide at pH''s 6.0 and 8.5, methemoglobin is completely converted into methemoglobin-hydrogen peroxide, which then undergoes a relatively slow consecutive reaction. The absorption spectrum of the compound, which shows maxima at 418 m[mu], [epsilon]mEq. = 105 and 545 m[mu], [epsilon]mEq. = 10.5, is not affected by pH in the region 6.0-8.5. Reasonably reliable kinetic data for the formation of methemoglobin-hydrogen peroxide have been deduced from the spectrokinetic data of expts. in which the hydrogen peroxide was present in several excessive concns. The rate increases with the hydrogen-ion concn., the velocity constant at 17[degree] being 18.[image]-1sec.-1 at pH 8.5 and 220 [image]-1 sec.-1 at pH 6.0. Spectrokinetic data for the reaction at pH 6.0 with low peroxide concns. are consistent with a reversible reaction for which pK = 4.4 at 23[degree]. The results are discussed in relation to recent work on the reaction of metmyoglobin with hydrogen peroxide. Some observations on the reaction of methemoglobin-hydrogen peroxide with sodium dithionite at pH 8.5 are descr. The compound is mainly reduced to hemoglobin in a relatively slow reaction, but some protohematin is degraded to choleheme and other products in a rapid initial reaction with the excess of hydrogen peroxide. Comparative spectral and kinetic data for methemoglobin-hydrogen peroxide and the transient compound formed by the action of hydrogen peroxide on hemoglobin in the presence of dithionite are briefly discussed. The absorption maxima and extinction co-efficients of human methemoglobin in acid and alkaline soln. are recorded.