Spectrofluorometric determination of kynurenic acid with horseradish peroxidase in the presence of hydrogen peroxide

The catalytic activity of horseradish peroxidase (HRP) in the presence of hydrogen peroxide has been investigated for the fluorescent derivatization of kynurenic acid under conditions with no exposure to light. Non-fluorescent kynurenic acid was converted into a fluorescent compound (Ex: 367 nm, Em: 470 nm) with HRP in the presence of hydrogen peroxide, and the optimum conditions of this fluorescent derivatization were investigated. Moreover, this fluorescent derivatization was developed for a spectrofluorometric determination of trace amounts of kynurenic acid by measuring the fluorescence intensity of the fluorescent compound. The calibration curve obtained was linear from 1.0 to 10.0 nmol of kynurenic acid in a 1.0 mL sample solution. The relative standard deviation at 5.0 nmol of kynurenic acid was 5.71% (n = 5). By adjusting the bandwidths for both the excitation and emission to 15 nm, the calibration curve was also linear in the range between 0.1 to 1.0 nmol of kynurenic acid in a 1.0 mL sample solution. This method was applied to the fluorometric determination of trace amounts of kynurenic acid in the control sera.