Activation of the leukocyte NADPH oxidase. In the resting cell, the subunits of the oxidase are distributed between the cytosol (p40^PHOX, p47^PHOX, p67^PHOX and Rac2) and the membranes (Rap1A and cytochrome b₅₅₈, a p22^PHOX · gp91^PHOXcomplex). Rac2 and Rap1A are low-molecular-weight guanine nucleotide-binding proteins that function in other processes besides oxidase activation. The other 5 proteins are unique to the NADPH oxidase. When the cell is activated, p47^PHOXbecomes heavily phosphorylated and the cytosolic subunits migrate to the membrane, where they bind to cytochrome b₅₅₈ to assemble the active oxidase.