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Several phenolic compounds and aromatic amines were tested with a system consisting of hydrogen peroxide, peroxidase and 4-aminophenazone for their chromogenic activity in the Emerson-Trinder reaction: sulphonated 2,4-diclorophenol had the greatest activity, about 4 times that of unsubstituted phenol. The mechanism of action of serum components (serum proteins, bilirubin and ascorbic acid) which can interfere in the Emerson-Trinder reaction was also investigated by the same analytical system. Binding of phenolic compounds to serum was found to be responsible for protein interference: this can be overcome by increasing the phenolic compounds concentration. The interference by bilirubin and ascorbic acid is due to hydrogen peroxide consumption, in competition with the chromogenic substrate.

We describe a direct colorimetric method for the enzymatic assay of uric acid. 50 microliters serum aliquots are incubated for 30 min with a diluted hexacyanoferrate solution, to remove interference by non-specific reducing agents. Uric acid is then oxidized to allantoin in the presence of uricase (Aspergillus flavus), with the simultaneous production of hydrogen peroxide; this is measured by a modified Emerson-Trinder reaction, in which phenol has been replaced by sulphonated 2,4-dichlorophenol, a more sensitive chromogen. The method gives linear results up to 20 mg of uric acid/100 ml.