An oxalate-binding protein with crystal growth promoter activity from human kidney stone matrix.

OBJECTIVE

To fractionate renal-stone matrix proteins, identify the presence of oxalate-binding protein and assess its effect in a calcium oxalate (CaOx) crystal growth system.

METHODS

Proteins were isolated from the matrix of kidney stones containing CaOx as the major constituent, using EDTA as a demineralizing agent. The solubilized proteins were subjected to cellulose-column chromatography by eluting with increasing sodium chloride concentrations in Tris-HCl buffer. Three protein fraction peaks were eluted, i.e. fraction I in buffer, fraction II in 0.05 mol/L NaCl in buffer and fraction III in 0.3 mol/L NaCl in buffer. The protein fractions were tested for their effects on CaOx crystal growth.

RESULTS

All three fractions had maximum CaOx binding activity at pH 7.4 but fraction II also had activity at pH 4.5. Fraction I promoted in vitro CaOx crystal growth, while fractions II and III were inhibitory. When fraction I was further separated on a Sephadex G-200 column, two protein fractions (Ia and Ib) were obtained. Fraction Ia protein had high and fraction Ib low CaOx-binding activity. Fraction Ia had a molecular weight of 48 kDa on gel electrophoresis and Western blotting. The 48 kDa protein did not cross-react with crystal matrix protein antibody, band-3 protein antibody, or albumin. The protein promoted CaOx crystal growth, with an optimum temperature of 37 degrees C and pH 6.5. The inhibitory effect of citrate on crystal growth was significantly lower in the presence of the 48 kDa protein. The protein promoted nucleation and aggregation of CaOx crystals in the in vitro crystallization system at pH 6.5, whereas fraction Ib (29 kDa) inhibited both nucleation and aggregation. Using the 48 kDa antibody, the yield of the protein from the stone matrix was 32% by EDTA extraction and only 3% with other methods. The protein was also detected in the nucleus and mitochondria, and in other matrix fractions of calcium phosphate and uric acid stones.

CONCLUSIONS

The 48 kDa protein isolated from stone matrix is a potent promoter of CaOx crystal growth with high oxalate-binding activity; it is enriched in the nucleus and mitochondria.