Phosphate Inhibition of Protein-Polysilicic Acid Complex Formation In Vitro: A Factor in Preventing Silica Urolithiasis

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Abstract
Phosphate concentration and pH were studied as factors influencing the formation of insoluble protein-polysilicic acid complexes of bovine serum albumin (BSA) or urinary proteins (nondialyzable urinary solids, NDUS) with silicic acid under conditions of constant ionic strength (IS) equivalent to 0.1724 N. In the pH range 5.5–7.0, the amount of protein-silicic acid complex measured turbidimetrically increased with decreasing pH. Only a trace of precipitate occurred with use of either of the protein sources at pH 7 or with NDUS at pH 6.5. With BSA at pH 6.5, phosphate supplying the total IS of the solution completely prevented precipitation of the complex. The phosphate effect was linear when it supplied 20–50% of the IS. In this range, there was a 12.3% reduction in the amount of precipitate for each 10% of the IS supplied by phosphate. With NDUS at pH 6.0, the phosphate effect was linear over its full range of concentrations. The phosphate effect resulted in an 8.7% reduction in the amount of precipitate for each 10% of the IS supplied by phosphate. On removal of the precipitated protein-silicic acid complex from the silicic acid solution, dissolution was facilitated by increases in phosphate concentration and pH. It is concluded that phosphate inhibition of protein-polysilicic acid complex formation may play a role in the reduction of silica urolithiasis related to increases in dietary phosphate.