A Study of the Incorporation of Urinary Macromolecules onto Crystals of Different Mineral Compositions

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Abstract
A new method used to study the binding of proteins onto calcium oxalate crystals demonstrated this interaction to be a selective process regarding the types of adsorbed proteins. To determine whether the incorporation of urinary macromolecules onto crystals formed in urine in vitro is dependent on the specific crystalline composition, this technique was applied to both uric acid and calcium oxalate crystals. Powdered uric acid was dissolved in urine and the pH lowered to 5.5. The resulting uric acid crystals were centrifuged and the pellet dialyzed against water at pH 8.0. Calcium oxalate crystals produced by the addition of 0.1 M calcium chloride and 0.1 M sodium oxalate to an aliquot of the same urine sample were demineralized with 5% EDTA. Gel filtration chromatography was used to isolate the protein fraction of an aliquot of the urine samples before and after crystallization of uric acid and calcium oxalate. The proteins recovered from the crystals and urine samples were separated by two-dimensional polyacrylamide gel electrophoresis. Analysis of the gels confirmed that the binding of urinary proteins onto crystals is a selective process and indicate that this interaction is dependent on the specific inorganic composition of the crystals.