Intracrystalline Proteins Promote Dissolution of Urinary Calcium Oxalate Crystals in Cultured Renal Epithelial Cells Available to Purchase

We have proposed that internalized calcium oxalate (CaOx) crystals containing intracrystalline proteins would be vulnerable to intracellular dissolution. The aims of this study were (1) to measure non‐uniform strain and crystallite size in CaOx monohydrate (COM) crystals containing increasing amounts of intracrystalline crystal matrix extract (CME) and (2) to compare the rates of crystal dissolution in Madin‐Darby canine kidney (MDCKII) cells. CME was isolated by demineralization of COM crystals generated from human urine. Cold and ¹⁴C‐oxalate‐labelled COM crystals were precipitated from ultrafiltered urine containing CME at final concentrations of 0–5mg/L. Non‐uniform strain and crystallite size were determined using synchrotron X‐ray diffraction with Rietveld whole‐pattern peak fitting and profile analysis, and the protein content of the crystals was analyzed using SDS‐PAGE and Western blotting for prothrombin fragment 1. Radiolabeled crystals were added to MDCKII cells and dissolution was expressed as radioactive label released into the medium relative to that in the crystals at zero time. Non‐uniform strain increased and crystallite size decreased proportionally with rising CME concentration, reaching saturation between approximately 1 and 5 mg/L, and demonstrating unequivocally the inclusion of increasing quantities of proteins in the crystals. This was confirmed by SDS‐PAGE and Western blotting. Crystal dissolution also followed saturation kinetics. These findings were confirmed by field emission scanning electron microscopy (FESEM), which showed that the degree of crystal degradation increased relative to CME concentration. We conclude that intracrystalline proteins enhance intracellular dissolution of CaOx crystals and thus may provide a natural defense against stone pathogenesis.