We assessed the effects of intracrystalline urinary proteins on the ability of Type II Madin Darby canine kidney cells to bind and degrade calcium oxalate monohydrate (COM) crystals. Binding of [C-14]-labelled inorganic crystals (iCOM), and COM crystals precipitated from centrifuged and filtered (CF) or ultrafiltered (UF) human urine was quantified by radioactive analysis. SDS PAGE confirmed the presence of intracrystalline proteins > 10 kDa in CF crystals and their absence front UFcrystals. Morphological effects were assessed qualitatively by field emission scanning electron microscopy. iCOM crystals bound rapidly and extensively and were resistant to degradation. Binding of CF crystals was weaker than UF crystals. and both had markedly less affinity than iCOM. CF and UF crystals were extensively degraded within 90 min, the effect being more pronounced with CF. These results support our hypothesis that intracrystalline proteins protect against urolithiasis by facilitating intracellular proteolytic digestion and destruction of crystals phagocytosed by urothelial cells. (C) 2005 Elsevier Inc. All rights reserved.