Methionine modification in cytochrome-c peroxidase.

Hydrogen peroxide oxidizes Met-119, Met-230 and Met-231 to the sulfoxide derivatives with equal initial rates in apocytochrome-c peroxidase at pH 4 in 0.1 M sodium acetate buffer. No detectable oxidation of Met-163 and Met-172 occurs under these conditions. Apoenzyme, in which up to two residues of methionine have been oxidized, binds heme stoichiometrically. Heme-reconstituted modified enzyme has an absorption spectrum with the Soret maximum red-shifted compared to that of the native enzyme, indicating a perturbation of the heme environment in the modified enzyme. Heme-reconstituted modified enzyme can bind cyanide with an affinity nearly identical to that of the native enzyme. The heme-reconstituted enzyme loses its ability to react with hydrogen peroxide to form Compound I. The loss of the ability to form Compound I is correlated with the modification of at least one of the residues in the Met-230/Met-231 pair.