Comparison of the specific incorporation of intracrystalline proteins into urinary calcium oxalate monohydrate and dihydrate crystals

The aim of this study was to compare the comprehensive intracrystalline protein profiles of calcium oxalate monohydrate (COM) and dihydrate (COD) crystals precipitated from the same human urine samples. Three separate batches of COM and COD crystals were precipitated from pooled healthy human urine by the addition of sodium oxalate at calcium concentrations of 2 and 8 mM, respectively. Proteins in whole extracts of demineralised COM and COD crystals, as well as in spots excised from 2D-PAGE gels of the extracts, were identified using liquid chromatography and tandem mass spectrometry (LC-MS/MS). The number and type of individual proteins differed between COM and COD: 14 substantive proteins were found inside COM crystal extracts and 34 inside COD, with 9 proteins occurring in both crystal types. Numerous keratins were detected. However, in line with consensus in the proteomics literature, as well as a lack of published evidence linking them to urolithiasis, they were excluded as contaminants, leaving very few consistently detected proteins. On the basis of their known association with stone disease or identification in multiple runs, the principal proteins in COM crystal extracts were prothrombin fragment 1, protein S100A9, and IGkV1-5, while those in extracts of COD crystals included osteopontin, IGkV1-5, protein S100A9, annexin A1, HMW kininogen-1, and inter-α-inhibitor (IαI). In general, proteins incorporated into both hydromorphs were acidic (pI <6), smaller than 55 kDa, and calcium binders. We concluded that the incorporation of proteins into urinary COM and COD crystals is selective and that only a few of the urinary proteins associated with the two hydromorphs are likely to play any significant role in stone pathogenesis.