Aminochrome Formation

Oxidations of Catecholamines in Dilute Solution 1. General Comments 

The oxidations in dilute solution discussed in this section mostly involve molecular oxygen as the oxidizing agent. The oxidative step in several catecholamine assay procedures, which usually involves the participation of an inorganic oxidizing agent, and which also occurs in dilute solution, is considered in Section V, E. 

There is still not complete agreement as to whether catecholamines undergo a true autoxidation in dilute aqueous solution at neutral 

As a result of kinetic studies carried out on the oxidation of adrenaline with molecular oxygen at pH 5 in acetate buffer, Sokoloski and Higuchi10 concluded that the reaction was relatively complex and involved free radical intermediates. At low concentrations of both adrenaline and oxygen, the reaction was effectively first order with respect to both components. With high concentrations of both reactants the orders with respect to oxygen and adrenaline were said to be 0.5 and 0.5, respectively, the appearance of fractional orders being indicative of free radical-mediated sequences. 

It has been known for many years that some metal ions and metal-containing proteins (e.g. ceruloplasmin and ferritin) readily catalyze 

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Adrenaline and noradrenaline were oxidized in solution by hydrogen peroxide in the absence of metals via a free radical mechanism on irradiation. However, no aminochrome formation was observed.61... 

B. Aminochrome Formation as a Possible Minor Pathway for Catecholamine Metabolism... 

Finally, evidence in support of the possibility of aminochrome formation in vivo is present in several publication, referring to the excretion of aminochromes (or closely related substances) in human urine.241-245... 

De Mol NJ, Beijersbergen van Henegouwen GMJ, Gerritsma KW. Photochemical decomposition of cathecolamines. II. The extent of aminochrome formation from adrenaline, isoprenaline and noradrenaline under ultraviolet light. Photochem Photobiol 1979 29 479-482. 

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