Thermodynamic and Kinetic Aspects of Calcium Oxalate Crystallization and Renal Lithiasis

Thermodynamic factors (supersaturation of substances that form crystals) and kinetic factors (heterogeneous nucleants and crystallization inhibitors) affect the formation of crystals and stones in the urinary tract. We studied the effect of five different polyhydroxycarboxylic acids and phytate on the formation of calcium oxalate crystals in artificial urine. All tested molecules are known to inhibit the crystallization of this calcium salt, and to also form complexes with calcium ions. Considering the typical concentration of polyhydroxycarboxylic acids in urine (similar to that of the calcium ion) and their ability to inhibit crystallization, their most important effect is the capacity to complex calcium-a thermodynamic effect. For phytate and its metabolites, which are present in concentrations much lower than that of the calcium ion, the most important effect is as a crystallization inhibitor-a kinetic effect. Among the five polyhydroxycarboxylic acids examined here, hydroxycitrate had the strongest complexing capacity, and the addition of phytate to hydroxycitrate led to greater inhibition of crystallization. Therefore, because oral consumption of hydroxycitrate does not increase the urinary pH, it is likely that the combined consumption of hydroxycitrate and phytate can provide certain benefits for patients with increased risk of developing calcium oxalate stones. We also discussed the effects of these different molecules on the different calcium oxalate stones, including papillary calcium oxalate monohydrate stones, cavity calcium oxalate monohydrate stones, calcium oxalate dihydrate stones, and mixed calcium oxalate dihydrate/hydroxyapatite stones.