THE REACTION OF ASCORBIC-ACID WITH DIFFERENT HEME IRON REDOX STATES OF MYOGLOBIN - ANTIOXIDANT AND PROOXIDANT ASPECTS

The interaction of ascorbate with different heme iron redox states of myoglobin (ferrylmyoglobin, Fe(IV)=O; metmyoglobin, Fe(III); and oxymy oglobin, Fe(II)O2) was examined by e.s.r. and absorption spectroscopy. The reaction of ascorbate with ferryl- or met-myoglobin resulted in a scorbyl radical production. The interaction of ascorbate with oxymyogl obin proceeded with formation of ascorbyl radical, hydrogen peroxide, and an overall oxidation of oxymyoglobin to metmyoglobin. The latter r eaction proceeded via an oxoferryl complex intermediate - correspondin g to ferrylmyoglobin and identified by treatment of the reaction mixtu re with Na2S. These observations are consistent with a concerted elect ron transfer mechanism, whereby the two electrons required for the red uction of oxygen to hydrogen peroxide are donated by ascorbic acid and the heme iron. The antioxidant and prooxidant aspects of these redox transitions are discussed in terms of their kinetic properties.