Contribution of human uropontin to inhibition of calcium oxalate crystallization. Uropontin (UP) is known to inhibit the growth and nucleation of calcium oxalate monohydrate (COM) crystals, and it also impedes attachment of calcium oxalate crystals to cultured renal epithelial cells. However, its role in normal defense against renal crystallization, and in pathogenesis of nephrolithiasis is unclear. In this study we determined the effect of UP on aggregation of COM crystals as well as the inhibitory activity of UP on COM crystal growth and nucleation in a series of normal subjects, in order to assess the potential of UP as an important urinary inhibitor. The mean urinary excretion of UP measured by ELISA was 185 ± 12 nmol/24 hr (mean ± SEM) with a mean urine UP concentration of 131 ± 13 nM. Uropontin isolated by immunoaffinity chromatography was a very potent inhibitor of COM crystal aggregation, with a mean UP concentration of 28 ± 4 nM required for a 50% reduction in aggregation. The kDa for COM crystal growth inhibition determined from Langmuir type isotherms was 21 ± 3 nM and the concentration required for 50% reduction in COM crystal growth rate was 16 ± 2 nM. Inhibition of secondary nucleation was measured at a single concentration of 200 nM, which reduced the nucleation rate to 42 ± 3% of control. Using a theoretical model of growth and aggregation inhibition at varying urine flow rates, we showed that inhibitory activity of UP would be significant for all subjects over a wide range of urine flow rates. Overall, UP is a potent inhibitor of COM aggregation as well as growth and nucleation. The urinary concentration of UP is in the range in which its contribution to inhibition of growth and aggregation are likely to be substantial. Thus, UP appears to be an important natural defense against renal crystallizations and nephrolithiasis.