Reduction of ferrylmyoglobin to metmyoglobin by quinonoid compounds

Abstract

Several quinonoid compounds mediated the reduction of ferrylmyoglobin (Mb IV) to metmyoglobin (Mb III). The efficiency of the Mb IV reduction to Mb III was accomplished by the quinones in the following order: p-benzoquinone > 1,4-naphthoquinone > 2-OH-1,4-naphthoquinone > 2,3-epoxy-1,4-naphthoquinone. The quinone-mediated reduction of Mb IV to Mb III had the following characteristics: (a) it was stoichiometrically — rather than catalytically — related to the number of cycles of the Mb IV a3 Mb III transition involving the reduction of H 2O 2. (b) It proceeded with similar efficiencies under aerobic and anaerobic conditions. (c) It did not require the free radical form of Mb IV( •Mb IV), thus excluding a two-electron oxidation of the quinone. (d) the nucleophilic addition of − NH 2 groups of the apoprotein on the quinone seemed not to be involved through an alternative pathway in the reduction of Mb IV, especially since 2-OH-1,4-naphthoquinone, a compound which cannot undergo nucleophilic addition, also facilitated the reduction of the ferryl compound. (e) No two-electron oxidation products of the unsubstituted quinones, such as quinone epoxides, were detected in the spent reaction mixture analyzed by HPLC with electrochemical detection. On the basis of these observations, it is suggested that the reduction of Mb IV to Mb III by the above quinonoid compounds is a one-electron transfer process, with electron abstraction being probably accomplished at some site in the benzo ring of the quinone.