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.