Exploring calcium oxalate crystallization: a constant composition approach

Crystal growth rates have been extensively studied in calcium oxalate monohydrate (COM) crystallization, because COM crystals are the principal component in most kidney stones. Constant composition methods are useful for studying growth rates, but fail to differentiate concurrent nucleation and aggregation events. A constant composition method coupled with particle size determinations that addresses this deficiency was previously published for a calcium phosphate system, and this method was extended to COM crystallization in this report. A seeded constant composition experiment was combined with particle size determination and a separate near-equilibrium aggregation experiment to separate effects of growth rate, nucleation, and aggregation in COM crystal formation and to test the effects of various inhibitors relevant to stone formation. With no inhibitors present, apparent COM growth rates were heavily influenced by secondary nucleation at low seed crystal additions, but growth-related aggregation increased at higher seed crystal densities. Among small molecule inhibitors, citrate demonstrated growth rate inhibition but enhanced growth-related aggregation, while magnesium did not affect COM crystallization. Polyanions (polyaspartate, polyglutamate, or osteopontin) showed strong growth rate inhibition, but large differences in nucleation and aggregation were observed. Polycations (polyarginine) did not affect COM crystal growth or aggregation. Mixtures of polyanions and polycations produced a complicated set of growth rate, nucleation, and aggregation behaviors. These experiments demonstrated the power of combining particle size determinations with constant composition experiments to fully characterize COM crystallization and to obtain detailed knowledge of inhibitor properties that will be critical to understanding kidney stone formation.