The mechanism of activation of NADPH oxidase in the cell-free system: the activation process is primarily catalytic and not through the formation of a stoichiometric complex

It is commonly assumed that activation of the superoxide-generating NADPH oxidase requires the formation of a stable complex between flavocytochrome b_-245 (the gp91^phox/p22^phox heterodimer) and the cytosolic cofactors p47^phox, p67^phox and Rac2. This association is thought to convert flavocytochrome b_-245, which contains the NADPH-binding site, flavin and haem centres, from an inactive into an active state. Here we provide evidence that, in the cell-free system, this activation process does not necessarily require the formation of a stable stoichiometric complex between the phox proteins. To explain this data we propose the hypothesis that p67^phox (and possibly Rac2), are capable of activating flavocytochrome b_-245 in a catalytic fashion, where a single molecule of p67^phox (or Rac2) is capable of activating multiple flavocytochrome b_-245 molecules.