Novel absorbance peak of gentisic acid following the oxidation reaction

Gentisic acid (GA), a metabolite of acetylsalicylic acid (ASA), and homogentisic acid (HGA), which is excreted at high levels in alkaptonuria, are divalent phenolic acids with very similar structures. Urine containing HGA is dark brown in color due to its oxidation. We recently reported a new oxidation method of HGA involving the addition of sodium hydroxide (NaOH) with sodium hypochlorite pentahydrate (NaOCl·5H2O), which is a strong oxidant. In the present study, we attempted to oxidize GA, which has a similar structure to HGA, using our method. We herein observed color changes in GA solution and analyzed the absorption spectra of GA after the addition of NaOH with NaOCl·5H2O. We also examined the oxidation reaction of GA using a liquid chromatography time-of-flight mass spectrometer (LC/TOF-MS). The results obtained indicated that GA solution had a unique absorption spectrum with a peak at approximately 500 nm through an oxidation reaction following the addition of NaOH with NaOCl·5H2O. This spectrophotometric method enables GA to be detected in sample solutions without expensive analytical instruments or a complex method.