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Uric acid

Determination

Uric acid is oxidized by the action of uricase, and transforms to allantoin whereby H2O2 is formed. There are a number of methods to directly measure the conversion of uric acid to allantoin, or to otherwise determine the peroxide thus formed.
The conversion of uric acid to allantoin is directly measured by UV-light, because the absorption maximum of uric acid is at 293 nm, while allantoin is reabsorbed. Uric acid is determined from the difference in absorbance decline before and after the action of uricase.
There are many methods to determine the concentration of uric acid via hydrogen peroxide formed in the reaction with uricase. In some of these methods, hydrogen peroxide oxidizes a chromogen by the catalytic action of peroxidase, and the intensity of chromogen staining is proportional to the uric acid concentration. In other methods, a hydrogen peroxide reacts with 2,4-dichlorophenol and 4-aminophenazone or 4-aminoantipyrine (PAP method) to produce a red-stained complex.

PAP method
Under the action of uricase, uric acid is oxidized and converted to allantoin, whereby H2O2 is formed. Thus formed H2O2 reacts with 2,4-dichlorophenol sulfate and 4-aminoantipyrine in the presence of peroxidase to produce a red-stained complex quinonimine with maximal absorbance at 510 nm.