A new approach to the study of urinary macromolecules as a participant in calcium oxalate crystallization

Despite intense investigation, the relationship of urinary proteins to urinary stone matrix formation remains poorly understood. In an attempt to gain more information regarding this interaction, the binding of urinary proteins to calcium oxalate crystals formed in urine in vitro was studied. 0.1 M calcium chloride and 0.1 M sodium oxalate were added to an aliquot of urine collected from five non-infected, non-stone forming males. The resulting calcium oxalate crystals were centrifuged and the pellet demineralized with 5% EDTA. Gel filtration chromatography was used to isolate the protein fraction from the urine samples before and after crystallization of calcium oxalate. The proteins recovered from the crystals and urine were separated by two-dimensional polyacrylamide gel electrophoresis. Many, but not all of the urinary proteins were bound by the crystals. Albumin, seen to the most abundant urinary protein, was absent or markedly diminished in all instances, and the second most abundant urinary protein, PC-30, became the predominant protein component of the crystals. An unidentified protein with approximate molecular weight of 22,000 daltons and isoelectric point of 6.4 was highly concentrated by the crystals even when undetected in the urine. The study suggests that the binding of urinary proteins to calcium oxalate crystals formed in urine in vitro is not a random event but rather a selective phenomenon.