Inhibitory Action of Citrate on Calcium Oxalate Monohydrate Crystal Nucleation and Growth

Abstract

Because calcium oxalate monohydrate is an important urinary stone salt, considerable attention has been devoted to the analysis of COM crystallization in an attempt to understand the associated processes of nucleation, crystal growth, aggregation, and breakup. Considering the paracrystalline dendritic nature of oxalate stones, many urinary agents may bind to and thereby alter crystal growth and/or adhesion to other components in the stone lamina. In addition to calcium oxalate hydrates, other carboxylic ligands that may interact with COM crystals include citrate1, phosphocitrate2, the aspartate-rich uropontin3, γ-carboxyglutamate-containing nephrocalcin4, and the cytoplasmic membrane component phosphatidylserine (F Novin, CM Brown, and DL Purich, unpublished findings). These agents each contain one or more carboxyl groups which potentially could substitute for oxalate carboxyls within the crystal lattice. Recently, our laboratory conducted crystal growth and nucleation rate measurements to investigate how agents might promote or inhibit COM crystallization5. Our work has centered on the inhibitory action of citrate, a prominent urinary metabolite that can sequester calcium ions and thereby reduce the relative supersaturation of urine with respect to calcium oxalate monohydrate1. In the present study we describe nucleation and growth kinetics in experiments for which we used the EQUIL speciation software to maintain the free, uncomplexed calcium ion concentration and hence avoid effects of changes in relative supersaturation. We also explored the relationship of these kinetic data to crystal morphology changes arising in the presence of citrate, and we have rationalized structural changes in terms of crystal face-specific interactions with citrate and/or calcium citrate complex. This has required the application of molecular recognition theory6, 7 to calculate the electrostatic surface potential of stable calcium oxalate monohydrate crystal surfaces and to characterize stereochemical interactions of carboxylic ligands with COM crystal faces. Our work supports the conclusion that citrate binds selectively to the (010) face, thereby blocking further crystal growth on that face.