Biochemical and physiological properties of Tamm-Horsfall glycoprotein from cats and sheep

Tamm-Horsfall protein (THP) is an N-linked glycoprotein produced by the mammalian kidney and excreted into the urine. The normal function of THP remains uncertain. The overall objective of this study was to gain additional information on the biochemical, physiological, and pathological characteristics of THP. Primary emphasis was placed on investigating the potential importance of cat THP (cTHP) as an etiologic factor in feline urolithiasis. Additional experiments with sheep THP (sTHP) also were performed. A two-site, noncompetitive enzyme-linked immunosorbent assay (ELISA) that measured cTHP in cat urine was developed and evaluated. The method of storing and processing cat urine significantly affected the ability of the ELISA to detect accurately cTHP. Therefore, a standard handling protocol was adopted prior to using this ELISA to measure urine cTHP in normal cats. There was no statistically significant difference between mean cTHP concentrations in normal male (N = 27) and female (N = 25) cats. The overall mean cTHP concentration ($\pm$SE) in these cats was 48.9 $\pm$ 4.9 $\mu$g/ml. To begin to test the hypothesis that cats with high urine cTHP concentrations were predisposed to developing urolithiasis, cTHP was quantified in the urine of "normal" and "urolithiasis" male cats at least one year of age. The mean cTHP concentration in "urolithiasis" cats (95.4 $\pm$ 11.4 $\mu$g/ml, N = 9) was significantly higher (p $<$ 0.05) than the mean cTHP concentration in "normal" cats (49.2 $\pm$ 7.4 $\mu$g/ml, N = 23). Additional experiments determined some aggregation properties of cTHP. CTHP self-associated in solutions with approximately 6mM calcium, or 8mM magnesium, or 100mM sodium. CTHP appeared to aggregate at lower cation concentrations as the pH increased. These cTHP aggregation characteristics and the significantly higher cTHP concentrations detected in "urolithiasis" cats support, but do not prove, the theory that cTHP is important in feline urolithiasis. Experiments performed with sTHP identified a protein that contaminated these samples as sheep IgG (sIgG). An ELISA was used to demonstrate that sTHP actually bound sIgG with a very high affinity (K$\sb{\rm f}$ of $\approx$ 10$\sp{12}$ M$\sp{-1}$). This sTHP/sIgG interaction was inhibited competitively by 10mM sialic acid. Similar binding was detected between human THP and human IgG.