Modifiers of calcium oxalate crystallization found in urine. II. Studies on their mode of action in an artificial urine.

The relative potencies of various modifiers of the crystallization of calcium oxalate (CaOx) were determined under "whole urine equivalent" conditions using a batch crystallizer. The system was used to measure changes in the degree of agglomeration of CaOx crystals produced spontaneously at a level of supersaturation within the range found in the urines of recurrent CaOx stone-formers. The modifiers tested included RNA, heparin, chondroitin-4-sulphate, pyrophosphate (at pH 5, 6 and 7), Tamm-Horsfall mucoprotein and a greater than 10,000 dalton fraction of macromolecules obtained from pooled normal urine by dialysis. The effects of these urinary constituents were also measured on the zeta potential produced at the surface of CaOx crystals. In the case of pyrophosphate, there was a clear correlation between the degree of inhibition of agglomeration and the zeta potential observed on the crystal surface indicating that, for this ion, repulsive electrostatic forces dominate the tendency for CaOx crystals to agglomerate. For the other modifiers tested, the tendency to agglomerate appeared to be dependent on the balance between the positive viscous binding ("sticky") forces and the negative electrostatic forces produced by these macromolecules on the CaOx crystal surface.