Tamm-Horsfall protein in recurrent calcium kidney stone formers with positive family history: abnormalities in urinary excretion, molecular structure and function

Tamm-Horsfall protein (THP) powerfully inhibits calcium oxalate crystal aggregation, but structurally abnormal THPs from recurrent calcium stone formers may promote crystal aggregation. Therefore, increased urinary excretion of abnormal THP might be of relevance in nephrolithiasis. We studied 44 recurrent idiopathic calcium stone formers with a positive family history of stone disease (RCSF_fam) and 34 age- and sex-matched healthy controls (C). Twenty-four-hour urinary THP excretion was measured by enzyme linked immunosorbent assay. Structural properties of individually purified THPs were obtained from analysis of elution patterns from a Sepharose 4B column. Sialic acid (SA) contents of native whole 24-h urines, crude salt precipitates of native urines and individually purified THPs were measured. THP function was studied by measuring inhibition of CaOx crystal aggregation in vitro (pH 5.7, 200 mM sodium chloride). Twenty-four-hour urine excretion of THP was higher in RCSF_fam (44.0 ± 4.0 mg/day) than in C (30.9 ± 2.2 mg/day, P = 0.015). Upon salt precipitation and lyophilization, elution from a Sepharose 4B column revealed one major peak (peak A, cross-reacting with polyclonal anti-THP antibody) and a second minor peak (peak B, not cross-reacting). THPs from RCSF_fam eluted later than those from C (P = 0.021), and maximum width of THP peaks was higher in RCSF_fam than in C (P = 0.024). SA content was higher in specimens from RCSF_fam than from C, in native 24-h urines (207.5 ± 20.4 mg vs. 135.2 ± 16.1 mg, P = 0.013) as well as in crude salt precipitates of 24-h urines (10.4 ± 0.5 mg vs. 7.4 ± 0.9 mg, P = 0.002) and in purified THPs (75.3 ± 9.3 μg/mg vs. 48.8 ± 9.8 μg/mg THP, P = 0.043). Finally, inhibition of calcium oxalate monohydrate crystal aggregation by 40 mg/L of THP was lower in RCSF_fam (6.1 ± 5.5%, range −62.0 to +84.2%) than in C (24.9 ± 6.0%, range −39.8 to +82.7%), P = 0.022, and only 25 out of 44 (57%) THPs from RCSF_fam were inhibitory (positive inhibition value) vs. 25 out of 34 (74%) THPs from C, P < 0.05. In conclusion, severely recurrent calcium stone formers with a positive family history excrete more THP than healthy controls, and their THP molecules elute later from an analytical column and contain more SA. Such increasingly aggregated THP molecules predispose to exaggerated calcium oxalate crystal aggregation, an important prerequisite for urinary stone formation.