A comparative study of the adsorption of amino acids on to calcium minerals found in renal calculi

To assess the binding of individual amino acids to the principal calcium mi nerals found in human kidney stones, the adsorption of 20 amino acids on to calcium oxalate monohydrate, CaHPO4.2H(2)O, Ca-3(PO4)(2) and Ca-5(PO4)(3)O H 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 t his decreased in all cases as the pH was increased. The amino acids which a dsorbed most strongly were aspartic acid, glutamic acid and gamma -carboxyg lutamic acid, with the last displaying the strongest affinity. All amino ac ids bound more avidly to calcium oxalate monohydrate than to any of the pho sphate minerals. Adsorption on to CaHPO4.2H(2)O was generally higher than f or Ca-3(PO4)(2) and Ca-5(PO4)(3)OH, for which all amino acids, with the exc eption of gamma -carboxyglutamic acid, had only a weak affinity. The bindin g affinity of these acids is thought to be due to their zwitterions being a ble to adopt conformations in which two carboxyl groups, and possibly the a mino group, can interact with the mineral surface without further rotation. The strong binding affinity of di-and tri-carboxylic acids for calcium sto ne minerals indicates that proteins rich in these amino acids are more like ly to play a functional role in stone pathogenesis than those possessing on ly a few such residues. These findings, as well as the preferential adsorpt ion of the amino acids for calcium oxalate monohydrate rather than calcium phosphate minerals, have ramifications for research aimed at discovering th e true role of proteins in stone formation and for potential application in the design of synthetic peptides for use in stone therapy.