Kinetics of Calcium Oxalate Crystal Growth in the Presence of Osteopontin Isoforms: An Analysis by Scanning Confocal Interference Microcopy

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} ≈ {010}, whereas bone OPN interacts in order of preference {100}≈{121} > {010}. As {100} is the most Ca²⁺-rich face of COM, while {010} is the least Ca²⁺-rich, it appears that the OPN-mediated inhibition of COM growth occurs through a nonspecific electrostatic interaction between Ca²⁺ ions of the crystal and phosphate groups of the protein.