Changes in the surface conditioning of calcium-salt crystals treated with physiological and alkaline urine

To study, using an in vitro model, the early phases of deposition of urinary components onto the surface of calcium-salt crystals treated with physiological and alkaline urine.

Calcium carbonate, oxalate and phosphate crystals were incubated in either freshly collected 'physiological' urine (pH 5.5) or with urine at pH 8.0. The surface conditioning was characterized using sodium-dodecyl sulphate-polyacrylamide gel electrophoresis immunoblot profiles of the adsorbed proteins and by Fourier transform infrared spectroscopy. Crystal morphology and aggregation were assessed using scanning electron microscopy.

The patterns of protein adsorption from physiological urine showed the ubiquitous adsorption of bands within 51-86 kDa, while Tamm-Horsfall protein (THP) and alpha 1-microglobulin were found only in calcium oxalate crystals. Less aggregation was detected in calcium oxalate and phosphate crystals treated in urine at pH 5.5, while a new crystalline phase was deposited on calcium carbonate surfaces. Incubation in alkaline urine led to changes in the protein electrophoretic profiles, with a significant variation in the morphology of the inorganic phase only in calcium phosphate crystals.

The binding of urinary proteins onto crystals depends on the chemistry of the surface and on the physicochemical composition of the urine. THP, albumin, alpha 1-microglobulin and a 20 kDa protein were able to bind calcium crystals under different circumstances. Except for THP, there was no clear relationship between the adsorption of proteins on crystals and the re-arrangement of the inorganic phase.