Identification of a novel fluorophore, xanthurenic acid 8- O -beta-D-glucoside in human brunescent cataract

We have identified the chemical structure of a novel protein-unbound fluorescent glucoside (Fl-Glc), found to be far more abundant in the human brunescent cataractous lens nuclei than in non-brunescent ones. Our earlier experiments showed that long-term incubation of the protein-free filtrate of non-brunescent cataractous nuclei generated increasing amounts of a particular yet to be characterized fluorophore (Fl-X). High performance liquid chromatography (HPLC) analyses revealed Fl-X and Fl-Glc to be identical. HPLC-electrospray ionization-mass spectrometry (HPLC-ESI-MS) disclosed the molecular weights (MW) of Fl-X and its beta-glucosidase-digest (Fl-X-aglycon) to be 367 and 205, respectively. Fl-X-aglycon and authentic xanthurenic acid (MW = 205) not only eluted at exactly the same retention time on HPLC but also revealed their protonated ions at the same m/z of 206.1 by positive ion analysis on HPLC-ESI-MS. These results suggest that Fl-X ( = Fl-Glc) is a beta-glucoside of xanthurenic acid. Fl-Glc was finally identified as xanthurenic acid 8- O -beta- D -glucoside because the retention times of both completely agreed with three kinds of HPLC conditions.