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.