Haloperoxidase-Catalyzed Luminol Luminescence

Common peroxidase action and haloperoxidase action are quantifiable as light emission from dioxygenation of luminol (5-amino-2,3-dihydrophthalazine-1,4-dione). The velocity of enzyme action is dependent on the concentration of reactants. Thus, the reaction order of each participant reactant in luminol luminescence was determined. Horseradish peroxidase (HRP)-catalyzed luminol luminescence is first order for hydrogen peroxide (H₂O₂), but myeloperoxidase (MPO) and eosinophil peroxidase (EPO) are second order for H₂O₂. For MPO, reaction is first order for chloride (Cl⁻) or bromide (Br⁻). For EPO, reaction is first order for Br⁻. HRP action has no halide requirement. For MPO and EPO, reaction is first order for luminol, but for HRP, reaction is greater than first order for luminol. Haloperoxidase-catalyzed luminol luminescence requires acidity, but HRP action requires alkalinity. Unlike the radical mechanism of common peroxidase, haloperoxidases (XPO) catalyze non-radical oxidation of halide to hypohalite. That reaction is second order for H₂O₂ is consistent with the non-enzymatic reaction of hypohalite with a second H₂O₂ to produce singlet molecular oxygen (¹O₂*) for luminol dioxygenation. Alternatively, luminol dehydrogenation by hypohalite followed by reaction with H₂O₂ yields dioxygenation consistent with the same reaction order. Haloperoxidase action, Cl⁻, and Br⁻ are specifically quantifiable as luminol luminescence in an acidic milieu.