Role of anionic proteins in kidney stone formation: Interaction between model anionic polypeptides and calcium oxalate crystals

Purpose: We tested the effect of molecular weight and amino acid compositio n (aspartate versus glutamate) in model peptides on calcium oxalate dihydra te (COD) formation to understand how known urinary inhibitor proteins might control spontaneous crystallization. Materials and Methods: Supersaturated solutions of CaCl2 and Na2C2O4 in HEP ES buffered saline solution were prepared at various calcium (Ca) to oxalat e (Ox) ratios, but constant supersaturation, in the presence of protein inh ibitors (polyaspartic acid molecular weight series or polyglutamic acid). T he resulting crystals were collected and evaluated with optical microscopy. Results: With no added inhibitors, the crystal size increased with Ca to Ox ratio, while the number of crystals decreased. With protein inhibitors at equivalent mass concentrations, intermediate molecular weight proteins prod uced a greater proportion of COD in Ca rich conditions than did either extr eme. In Ox rich conditions, the proportion of COD was directly related to p rotein molecular weight. However, at equivalent molar concentrations, the p roportion of COD produced was directly related to molecular weight under al l conditions. Larger protein concentrations were required to produce COD at high Ox conditions, in proportion to the increased number of crystals prod uced. Polyglutamic acid had a much weaker effect on crystal structure, but it changed the COM morphology. Conclusions: The results suggest that a discrete number of protein molecule s per crystal were required to direct crystallization toward GOD, and that a characteristic size of polypeptide chain can be defined. The charge of th e side group was not the sole determinant of this effect, as polyglutamic a nd polyaspartic acids behaved differently. Calcium oxalate crystal nucleati on rates appeared to increase with Ox content.