Intracrystalline proteins and the hidden ultrastructure of calcium oxalateurinary crystals: Implications for kidney stone formation

The external appearance of urinary calcium oxalate (CaOx) crystals suggests that they are solid, homogeneous structures, despite their known associati on with proteins. Our aim was to determine whether proteins comprising the organic matrix of CaOx crystals are superficial or intracrystalline in orde r to clarify the role of urinary proteins in the formation of kidney stones . CaOx crystals were precipitated from centrifuged and filtered, or ultrafi ltered, healthy human urine. They were then treated with dilute NaOH to rem ove bound proteins, partially demineralized with EDTA, or fractured and sub jected to limited proteolysis before examination by low-resolution scanning electron microscopy or field emission scanning electron microscopy. Crysta ls precipitated from centrifuged and filtered urine had a complex interior network of protein distributed throughout the mineral phase, which appeared to comprise closely packed subcrystalline particles stacked in an orderly array among an amorphous organic matrix. This ultrastructure was not eviden t in crystals deposited in the absence of macromolecules, which were comple tely solid. This is the first direct evidence that crystals generated from cell-free systems contain significant amounts of protein distributed throug hout a complex internal cribriform ultrastructure. Combined with mineral er osion in the acidic lysosomal environment, proteins inside CaOx crystals wo uld render them susceptible to attack by urinary and intracellular renal pr oteases and facilitate their further dissolution or disruption into small p articles and ions for removal by exocytosis. The findings also have broader ramifications for industry and the materials sciences, as well as the deve lopment and resorption of crystals in biomineralization systems throughout nature. (C) 2001 Academic Press.