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Calprotectin is a protein complex composed of the S100A8 and S100A9 protein subunits. It is abundant in neutrophils and can restrict microbial access to manganese and zinc through metal binding. This is an example of nutritional immunity: changing nutrient availability can impair microbial functions without directly cutting apart the microorganism.^[ref]

A primary study compared ordinary calprotectin with a variant defective in its transition-metal-binding sites. Isothermal titration calorimetry tested binding, while growth and enzyme-activity assays tested functional consequences. Loss of metal binding strongly reduced antimicrobial activity; manganese add-back restored an affected bacterial superoxide dismutase activity. These comparisons linked metal availability to the phenotype rather than relying on calprotectin abundance alone.^[ref]

An original one-metal model separates total from free metal:If the stipulated system contains $1.0\mu \mathrm{M}$ metal and $0.8\mu \mathrm{M}$ is protein-bound, $0.2\mu \mathrm{M}$ remains free. The total metal concentration has not changed, although its availability has. Other ligands and competing metals would require additional terms.

Calprotectin therefore illustrates how a host protein can alter a microbe's chemical environment. Its metal-binding behavior depends on conditions and should not be reduced to a universal fixed number of ions bound in every sample; neither the simplified balance nor the presence of the protein alone determines an organism's response.

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.