Proteins that inhibit the growth and aggregation of calcium oxalate crystals play important roles in the prevention of kidney stone disease. One such protein is osteopontin (OPN), which inhibits the formation of calcium oxalate monohydrate (COM) in a phosphorylation-dependent manner. To determine the role of phosphate groups in the inhibition of COM growth by OPN, we used scanning confocal interference microscopy to compare the effects of highly phosphorylated OPN from cow milk, less phosphorylated OPN from rat bone, and nonphosphorylated recombinant OPN. COM growth was measured in the principal crystallographic directions <001>, <010>, and <100>, representing lattice-ion addition to {121}, {010}, and {100} faces, respectively. While the shapes of growth curves were very consistent from crystal to crystal, absolute growth rates varied widely. To control for this, results were expressed as changes in the aspect ratios <010>/<001> and <100>/<001>. Compared to control, bone OPN increased <010>/<001> and had no effect on <100>/<001>; milk OPN had no effect on <010>/<001>and decreased <100>/<001>; recombinant OPN had no significant effect on either aspect ratio. These findings indicate that milk OPN interacts with COM crystal faces in order of preference {100} > {121} approximately {010}, whereas bone OPN interacts in order of preference {100} approximately {121} > {010}. As {100} is the most Ca(2+)-rich face of COM, while {010} is the least Ca(2+)-rich, it appears that the OPN-mediated inhibition of COM growth occurs through a nonspecific electrostatic interaction between Ca(2+) ions of the crystal and phosphate groups of the protein.

Scanning confocal interference microscopy (SCIM) and molecular dynamics (MD) simulations were used to investigate the adsorption of the synthetic polypeptide poly(l-glutamic acid) (poly-glu) to calcium oxalate monohydrate (COM) crystals and its effect on COM formation. At low concentrations (1 μg/mL), poly-glu inhibits growth most effectively in ⟨001⟩ directions, indicating strong interactions of the polypeptide with {121} crystal faces. Growth in <010> directions was inhibited only marginally by 1 μg/mL poly-glu, while growth in <100> directions did not appear to be affected. This suggests that, at low concentrations, poly-glu inhibits lattice-ion addition to the faces of COM in the order {121} > {010} ≥ {100}. At high concentrations (6 μg/mL), poly-glu resulted in the formation of dumbbell-shaped crystals featuring concave troughs on the {100} faces. The effects on crystal growth indicate that, at high concentrations, poly-glu interacts with the faces of COM in the order {100} > {121} > {010}. This mirrors MD simulations, which predicted that poly-glu will adsorb to a {100} terrace plane (most calcium-rich) in preference to a {121} (oblique) riser plane but will adsorb to {121} riser plane in preference to an {010} terrace plane (least calcium-rich). The effects of different poly-glu concentration on COM growth (1-6 μg/mL) may be due to variations between the faces in terms of growth mechanism and/or (nano)roughness, which can affect surface energy. In addition, 1 μg/mL might not be adequate to reach the critical concentration for poly-glu to significantly pin step movement on {100} and {010} faces. Understanding the mechanisms involved in these processes is essential for the development of agents to reduce recurrence of kidney stone disease.

Because of its ability to inhibit the growth of calcium oxalate monohydrate (COM) crystals, citrate plays an important role in preventing the formation of kidney stones. To determine the mechanism of inhibition, we studied the citrate-COM interaction using a combination of microscopic and simulation techniques. Using scanning confocal interference microscopy, we found that addition of citrate preferentially inhibits crystal growth in <100> and, to a lesser extent, <001> directions, suggesting that citrate adsorbs to the faces of COM in the order {100} > {121} > {010}. Scanning electron microscopy showed that the resulting crystals are plate shaped, with large {100} faces and rounded ends. Molecular-dynamics simulations predicted, however, that citrate interacts with the faces of COM in a different order, i.e. {100} > {010} > {121}. Our simulations showed that citrate molecules align with the rows of Ca²⁺ ions on the {010} face but do not form close contacts, presumably because of electrostatic repulsion by the carboxylate groups that project from the Ca²⁺-rich plane. We propose that this weak interaction is responsible for citrate’s limited inhibition of COM growth in <010> directions. Overall, these findings indicate that electrostatic interactions with the Ca²⁺-rich faces of COM crystals are responsible for the growth-modulating properties of citrate.

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