Analysis of the metabolites of the sodium salt of 6-hydroxy-5-(phenylazo)-2-naphthalenesulfonic acid in Sprague–Dawley rat urine

The sodium salt of 6-hydroxy-5-(phenylazo)-2-naphthalenesulfonic acid (SS-AN), which is a subsidiary color present in Food Yellow No. 5 [Sunset Yellow FCF, disodium salt of 6-hydroxy-5-(4-sulfophenylazo)-2-naphthalenesulfonic acid], was orally administered to Sprague–Dawley rats. Metabolite A, metabolite B, and unaltered SS-AN were detected as colored metabolites in the rat urine. Analysis of the chemical structures showed that metabolite A (major peak) was 6-hydroxy-5-(4-sulfooxyphenylazo)-2-naphthalenesulfonic acid, the sulfuric acid conjugate of SS-AN, and metabolite B (minor peak) was 6-hydroxy-5-(4-hydroxyphenylazo)-2-naphthalenesulfonic acid (SS-PAP), which is a derivative of metabolite A without the sulfuric acid. The colorless metabolites p-aminophenol, o-aminophenol, and aniline present in the urine were analyzed by liquid chromatography–mass spectrometry. The orally administered SS-AN had been metabolized to the colorless metabolites (p-aminophenol 45.3%, o-aminophenol 9.4%, aniline 0.4%) in the 24-h urine samples. Analysis of the colored metabolites by high-performance liquid chromatography with detection at 482 nm indicated the presence of metabolite A (0.29%), SS-PAP (0.01%), and SS-AN (0.02%) were detected in the 24-h urine samples. Approximately 56% of SS-AN was excreted into the urine and the rest is probably excreted into feces.