Joe M. McCord

Joe M. McCord helped identify the enzymatic activity called superoxide dismutase. His 1981 first-person research recollection describes how an unexpected inhibition of cytochrome c reduction led to a different explanation of oxygen-dependent enzyme chemistry.^[ref]

The system contained xanthine oxidase, oxygen and a cytochrome c reporter. McCord initially sought direct binding between xanthine oxidase and an inhibitory protein. The eventual explanation involved freely diffusible superoxide: its competing removal could inhibit the reporter without the proposed binding. Preparations of other proteins contained a small amount of the catalytically active contaminant.^[ref]

The superoxide dismutase reaction is a disproportionation:In an original stoichiometry example, reaction of 40 superoxide equivalents produces 20 oxygen and 20 hydrogen peroxide equivalents. Decreasing the superoxide signal therefore need not mean producing only water. Likewise, a smaller cytochrome c reporter signal can reflect a competing reaction rather than less activity of the source enzyme. This is why a reporter's measured change must be connected to a reaction mechanism before interpreting an enzyme assay.