Cation-induced aggregation of cat Tamm-Horsfall glycoprotein and its possible role in feline urolithiasis

The in vitro cation-induced aggregation properties of cat Tamm-Horsfall protein (cTHP), a urinary glycoprotein, were examined and related to the potential role of cTHP in feline urolithiasis. The aggregation assay involved adding either CaCl2, MgCl2, or NaCl to solutions containing purified cTHP, and then separating the aggregated cTHP by centrifugation. The concentration of cTHP remaining in the supernatant was quantified using a previously developed enzyme-linked immunosorbent assay (ELISA). The effect that buffer pH, cTHP concentration, and urea concentration had on cTHP aggregation also were examined. Of the three salts, CaCl2 consistently was most efficient at precipitating cTHP, while MgCl2 was slightly less efficient. At least ten times more NaCl than CaCl2 or MgCl2 was required for comparable cTHP aggregation. As the pH decreased, increasing concentrations of the salts were required to aggregate cTHP. Increased amounts of CaCl2 and MgCl2 also were required to aggregate cTHP when the urea concentration was increased. As cTHP concentration increased within the physiological range, lower concentrations of CaCl2 and MgCl2 were required to precipitate 50% of the cTHP. Several aspects of the in vitro aggregation properties of cTHP correlate closely with previously identified risk factors for feline urolithiasis, strengthening the theory that cTHP aggregation may be important in this disease.