To assess the binding of individual
amino acids to the principal
calcium minerals found in human
kidney stones, the adsorption of 20
amino acids on to
calcium oxalate monohydrate, CaHPO4*2H2O,
Ca3(PO4)2 and Ca5(PO4)3OH crystals was determined over the physiological urinary pH range (pH 5-8) in aqueous solutions. All
amino acids adsorbed most strongly at pH 5, and this decreased in all cases as the pH was increased. The
amino acids which adsorbed most strongly were
aspartic acid,
glutamic acid and
gamma-carboxyglutamic acid, with the last displaying the strongest affinity. All
amino acids bound more avidly to
calcium oxalate monohydrate than to any of the
phosphate minerals. Adsorption on to CaHPO4*2H2O was generally higher than for
Ca3(PO4)2 and Ca5(PO4)3OH, for which all
amino acids, with the exception of
gamma-carboxyglutamic acid, had only a weak affinity. The binding affinity of these
acids is thought to be due to their zwitterions being able to adopt conformations in which two carboxyl groups, and possibly the amino group, can interact with the
mineral surface without further rotation. The strong binding affinity of di-and tri-
carboxylic acids for
calcium stone minerals indicates that
proteins rich in these
amino acids are more likely to play a functional role in stone pathogenesis than those possessing only a few such residues. These findings, as well as the preferential adsorption of the
amino acids for
calcium oxalate monohydrate rather than
calcium phosphate minerals, have ramifications for research aimed at discovering the true role of
proteins in stone formation and for potential application in the design of synthetic
peptides for use in stone
therapy.