Role of anionic proteins in kidney stone formation: interaction between model anionic polypeptides and calcium oxalate crystals.
OBJECTIVE
We tested the effect of molecular weight and amino acid composition (aspartate versus glutamate) in model peptides on calcium oxalate dihydrate (COD) formation to understand how known urinary inhibitor proteins might control spontaneous crystallization.


METHODS
Supersaturated solutions of CaCl2 and Na2C2O4 in HEPES buffered saline solution were prepared at various calcium (Ca) to oxalate (Ox) ratios, but constant supersaturation, in the presence of protein inhibitors (polyaspartic acid molecular weight series or polyglutamic acid). The 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 produced a greater proportion of COD in Ca rich conditions than did either extreme. In Ox rich conditions, the proportion of COD was directly related to protein molecular weight. However, at equivalent molar concentrations, the proportion of COD produced was directly related to molecular weight under all conditions. Larger protein concentrations were required to produce COD at high Ox conditions, in proportion to the increased number of crystals produced. 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 molecules per crystal were required to direct crystallization toward COD, and that a characteristic size of polypeptide chain can be defined. The charge of the side group was not the sole determinant of this effect, as polyglutamic and polyaspartic acids behaved differently. Calcium oxalate crystal nucleation rates appeared to increase with Ox content.
Wesson
J A
JA
Medical College of Wisconsin and the Department of Veterans Affairs Medical Center, Milwaukee 53295, USA.
Worcester
E M
EM
Kleinman
J G
JG
D002129:Calcium Oxalate / Q000737:chemistry*; D003460:Crystallization; D007669:Kidney Calculi / Q000209:etiology*; D008970:Molecular Weight; D010455:Peptides / Q000494:pharmacology*; D011099:Polyglutamic Acid / Q000494:pharmacology*
D010455:Peptides; D011099:Polyglutamic Acid; C017645:polyaspartate; D002129:Calcium Oxalate