ON THE INTERACTION OF MYOGLOBIN AND HEMOGLOBIN WITH MOLECULAR OXYGEN AND ITS LOWER OXIDATION STATES AND WITH CYTOCHROMEc

Ferrimyoglobin and ferrihemoglobin are oxidized to the ferry1 (+4) state by the xanthine-xanthine oxidase system under the same conditions under which the system causes rapid reduction of ferricytochrome c. Catalytic amounts of ferrimyoglobin exert an apparent inhibition of ferricytochrome c reduction by the system. Oxidation of ferrimyoglobin and ferrihemoglobin is most likely due to their reacting with products of xanthine oxidase reduction of oxygen, O⁻₂ (HO₂) and/or H₂O₂. The apparent inhibition of ferricytochrome c reduction is direct (interception of O⁻₂ by ferrimyoglobin) and/or indirect (reoxidation of ferrocytochrome c by ferrylmyoglobin). The latter mechanism is supported by spectral evidence of one-equivalent redox reactions (intermolecular electron transfers) between ferrylmyoglobin and ferrocytochrome c and between ferromyoglobin and ferricytochrome c. Both electron transfers have favorable redox potentials and therefore would be expected to occur from that standpoint. During autoxidation of oxymyoglobin O⁻₂ dissociation was not detected. This is consistent with theoretical arguments which indicate that O⁻₂ cannot dissociate during autoxidation although the conjugate acid (HO₂) can. If so, this would at least partially explain the known decrease in stability of the oxyheme complex as the pH of fresh red meat, meat extracts or oxyhemoprotein solutions is lowered. Mechanisms of autoxidation that are consistent with known pH and oxygen partial pressure effects and chemical reactions, as well as with available quantitative data and pertinent theoretical arguments are discussed.