Catecholamines, hydrogen peroxide

Electrochemical biosensors based on detection of hydrogen peroxide at platinized electrodes were found to be more versatile allowing a decrease in detection limit down to 1 i,mol L 1 [109]. However, all biological liquids contain a variety of electrochemically easily oxidizable reductants, e.g. ascorbate, urate, bilirubin, catecholamines, etc., which are oxidized at similar potentials and dramatically affect biosensor selectivity producing parasitic anodic current [110]. 

The most commonplace substrates in energy-transfer analytical CL methods are aryl oxalates such as to(2,4,6-trichlorophenyl) oxalate (TCPO) and z s(2,4-dinitrophenyl) oxalate (DNPO), which are oxidized with hydrogen peroxide [7, 8], In this process, which is known as the peroxyoxalate-CL (PO-CL) reaction, the fluorophore analyte is a native or derivatized fluorescent organic substance such as a polynuclear aromatic hydrocarbon, dansylamino acid, carboxylic acid, phenothiazine, or catecholamines, for example. The mechanism of the reaction between aryl oxalates and hydrogen peroxide is believed to generate dioxetane-l,2-dione, which may itself decompose to yield an excited-state species. Its interaction with a suitable fluorophore results in energy transfer to the fluorophore, and the subsequent emission can be exploited to develop analytical CL-based determinations. 

Monoamine oxidase, tyrosine hydroxylase, and L-amino acid oxidase generate hydrogen peroxide as their reaction product. Hydrogen peroxide is also produced by auto-oxidation of catecholamines in the presence of vitamin C. Moreover, phospholipase A2 (PLA2), cyclooxygenase (COX), and lipoxygenase (LOX), the enzymes associated with arachidonic acid release and the arachidonic acid cascade,... 

Monoamine oxidases catalyze oxidative deamination of many primary, secondary, and tertiary amines. They have a wide tissue distribution including brain, liver, and intestine. A variety of endogenous amines, such as catecholamines, and pharmacological substances are metabolized. The products of primary amines are the corresponding aldehydes, ammonia, and hydrogen peroxide. 

Figure 7.7 The phenolic hydroxyl group of catecholamine reacts with oxygen in aqueous solution. The reaction was catalyzed by imidazole to produce hydrogen peroxide at alkaline pH...

O. Nozaki, T. Iwaeda, H. Moriyama and Y. Kato, Chemiluminescent detection of catecholamines by generation of hydrogen peroxide with imidazole. Luminescence, 14, 123-127 (1999). 

Allowing that the above transfer system involving Mn does occur, the cause of manganese neurotoxicity is still little understood. Archibald and Tyree [75] propose that the ability of to attack catecholamines indicates that this metal ion is toxic in itself, while other workers [76-78] see the toxicity as being due to a variety of causes such as autooxidation of dopamine, decreased glutathione (GSH) levels [64,79], reduced GSH peroxidase and brain catalase [79], and Mn -induced production either of toxic catecholamines and quinones [77] or of reactive oxo species such as superoxide, hydroxide radicals, or hydrogen peroxide [77,80,81]. A further excellent summary of much of the above detail is given by Donaldson and Barbeau [82]. 

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