Superoxidized states of Escherichia coli sulfite reductase heme protein subunit

The oxidized forms of resting and sulfite-complexed Escherichia coli sulfite reductase heme protein subunit react with near-stoichiometric amounts of porphyrexide to produce what is best characterized as a ferrisiroheme pi cation radical. Addition of either sodium ascorbate or NADPH completely regenerates the parent form. Implications of these findings with respect to mechanisms of metal-radical J coupling and catalysis are discussed.