CONTRIBUTION OF HUMAN UROPONTIN TO INHIBITION OF CALCIUM-OXALATE CRYSTALLIZATION

Uropontin (UP) is known to inhibit the growth and nucleation of calciu m oxalate monohydrate (COM) crystals, and it also impedes attachment o f calcium oxalate crystals to cultured renal epithelial cells. However , its role in normal defense against renal crystallization, and in pat hogenesis of nephrolithiasis is unclear. In this study we determined t he effect of UP on aggregation of COM crystals as well as the inhibito ry 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 importan t urinary inhibitor. The mean urinary excretion of UP measured by ELIS A was 185 +/- 12 nmol/24 hr (mean +/- SEM) with a mean urine UP concen tration of 131 +/- 13 nM. Uropontin isolated by immunoaffinity chromat ography 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 require d for 50% reduction in COM crystal growth rate was 16 +/- 2 nM. Inhibi tion of secondary nucleation was measured at a single concentration of 200 nM, which reduced the nucleation rate to 42 +/- 3% of control. Us ing a theoretical model of growth and aggregation inhibition at varyin g urine flow rates, we showed that inhibitory activity of UP would be significant for all subjects over a wide range of urine flow rates. Ov erall, UP is a potent inhibitor of COM aggregation as well as growth a nd nucleation. The urinary concentration of UP is in the range in whic h 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.