Surface interaction between glycosaminoglycans and calcium oxalate.

Molecules and macromolecules are known to alter the process of crystallization, either through inhibition or promotion of nucleation, growth, and/or aggregation. One particular group of macromolecules, glycosaminoglycans (GAGs), has been of interest in our laboratory. The GAGs chondroitin A, chondroitin C, heparan sulfate, dermatan sulfate, hyaluronic acid, and keratan sulfate have all been shown to be inhibitors of calcium oxalate crystallization. Heparin, the only GAG which is not naturally present in urine, is the most potent inhibitor of all GAGs. Using the method of Langmuir isotherm adsorption, we studied the adsorption of certain GAGs onto calcium oxalate crystals. Under standardized conditions, heparin, chondroitin C, hyaluronic acid, and pentosan polysulfate (a synthetic polyanionic molecule similar to, but a weaker inhibitor than, heparin) were adsorbed onto calcium oxalate. The total amount of GAG required to maximally cover the crystal surface, as well as the equilibrium concentration at which surface was half-covered with GAG (inversely related to the desorption energy) were measured. Chondroitin C was adsorbed in the greatest amount, followed by heparin, pentosan polysulfate, and finally hyaluronic acid. Using the method of fiducial limits, the only insignificant difference was between heparin and chondroitin C, and between hyaluronic acid and pentosan polysulfate. Pentosan polysulfate required significantly higher equilibrium concentration than heparin and hyaluronic acid to cover half of the surface of the calcium oxalate crystals. The principle of Langmuir isotherm adsorption can be useful in predicting the effects of macromolecules on crystallization. Weaker inhibitors bind with less affinity than do stronger inhibitors. Further work is underway to characterize other inhibitors and promoters.