Protein location and elemental composition of urine spheres in different avian species.

We examined the internal morphology, location of protein, and identity and location of elements, in avian urate-containing spheres in 9 species of birds. The urine spheres were collected from voided samples. The spheres ranged in size from 0.5-5.0 microm, except in the domestic fowl, where they ranged up to 10 microm in diameter. The internal morphology of the spheres was examined using freeze-fracture microscopy. Protein location within the spheres was identified using fluorescein isothiocyanate (FITC). The urine spheres were analyzed for content and internal location of elements using Energy Dispersal System Analysis (EDS). Internally, the spheres consisted of a central nidus surrounded by 3-4 concentric narrow rings of protein. Elements found within the spheres included nitrogen, potassium, calcium, sodium, phosphorus, chloride and sulfur; however, only nitrogen, potassium and chloride were common in the spheres of all species. Nitrogen comprised the majority of the elemental content of the spheres (77-90%) followed by potassium (8-45%), with all other ions present in trace amounts. Unlike protein, the location of elements was random within the spheres. Protein and urate are both negatively charged and known to associate to form the spheres and as potassium is the only cation common to all spheres, it too may play a role in their formation.