Preparation and Iron Redox Speciation Study of the Fe(II)-Binding Antimicrobial Protein Calprotectin

Calprotectin (CP, S100A8/S100A9 heterooligomer) is an abundant metal-sequestering host-defense protein expressed by neutrophils, other white blood cells, and epithelial cells. The apoprotein is a S100A8/S100A9 heterodimer that contains two sites for transition metal binding at the S100A8/S100A9 interface: a His₃Asp motif (site 1) and a His₆ motif (site 2). In this chapter, we provide a step-by-step protocol for the overexpression and purification of the human and murine orthologues of CP that affords each apo heterodimer in high yield and purity. In these procedures, the S100A8 and S100A9 subunits are overexpressed in Escherichia coli BL21(DE3), and each apo heterodimer is obtained following cell lysis, folding, column chromatography, and dialysis against Chelex resin to reduce metal contamination. Recent studies demonstrated that human CP coordinates Fe(II) and that the protein affects the redox speciation of Fe in solution. An Fe redox speciation assay employing ferrozine is described that demonstrates the ability of both the human and murine orthologues of CP to shift the redox speciation of Fe from the ferric to the ferrous oxidation state over time.