Spectrophotometric studies of the reaction of methaemoglobin with hydrogen peroxide. 2. The degradation of methaemoglobin by hydrogen peroxide

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Abstract
The reactions of human methemoglobin and hydrogen peroxide at pH''s 6.0 and 8.5, which yield initially methemoglobin-hydrogen peroxide, were studied throughout their course by spectrokinetic observations and by conversion into hemochromogen derivatives. With low peroxide concns., the compound reverted, in part, to methemoglobin, but with high peroxide concns. the heme structure was ultimately destroyed. At pH 8.5, no evidence was obtained for the occurrence, in significant amt., of heme compounds other than methemoglobin and methemoglobin-hydrogen peroxide. At pH 6.0, a green pigment was formed, which, in the presence of sodium dithionite, yielded stable CO and hemochromogen derivatives showing bands in the red. The pigment itself was not stable towards sodium dithionite. The green pigment was formed by partial oxidative degradation of the prosthetic group of methemoglobin-hydrogen peroxide, which yields protoheme derivation in the presence of sodium dithionite. The green pigment has not been obtained free from protoheme compounds; the yield increased with the initial peroxide concn. up to a certain limit, but beyond this point, even on successive reaction of the products with small quantities of peroxide, simultaneous destruction of the residual protoheme and the green heme occurred. The residual protoheme was not entirely in the form of methemoglobin, but was apparently accounted for as carboxyhemoglobin after the addition of CO and dithionite. The green pigment is distinct from choleglobin and its formation is not prevented by CO, showing that oxyhemoglobin is not an intermediate in the reaction.