O₂⁻ generating activity of flavocytochrome b₅₅₈ purified from membranes of X-CGD, ▵488-497 gp91^phox, and WT gp91^phox PLB-985 cells. Cell-free O₂⁻ assays were performed as described in the Materials and Methods using the equivalent of 0.5 pmol flavocytochromeb₅₅₈ per well and 2.5 × 10⁶ cell equivalents of neutrophil cytosol. (□) The maximum rate of O₂⁻ production with 4.9 mmol/L NADPH as substrate (mean ± SEM); (▧) the maximum rate of O₂⁻ production with 4.8 mmol/L NADH as substrate.