Urochrome Pigment in Uric Acid Crystals

Renal stones are heterogeneous composites of numerous microscopic crystals (e.g., calcium oxalate, calcium phosphate, uric acid, etc.) and 2–3 wt % amorphous organic “matrix”. Uric acid kidney stones are often red–orange–brown in color, though uric acid crystals are colorless. The stone color originates from a variety of components in the matrix, some of which are a broad range of urinary pigments or urochrome. Herein, we report the first definitive structure of one of these pigments, urorosein, and its ability to form intracrystalline inclusions in single crystals of both anhydrous uric acid and uric acid dihydrate. The preferred orientation of the included urorosein molecules in the uric acid crystals was determined through polarized light microspectroscopy. On the basis of these results, it seems likely that other urochrome pigments can locate in both intercrystalline and intracrystalline spaces in urinary precipitates. This expands the conventional picture of where “matrix” resides in these composite materials.