Effects of Inositol Hexaphosphate (Phytate) on Calcium Binding, Calcium Oxalate Crystallization and In Vitro Stone Growth

Abstract

We measured and compared 3 activities of inositol hexaphosphate, also known as phytate, to explore their importance in relation to antilithogenic potential. Calcium binding activity by inositol hexaphosphate was measured with a calcium electrode in artificial and whole urine. Calcium oxalate crystallization inhibition was measured by a 96-well plate turbidimetric method with artificial and whole urine. Effects on stone growth were measured in an in vitro system of 12 stones grown simultaneously (a stone farm) using artificial urine alone or supplemented with urinary macromolecules. Phytate decreased ionized calcium, increased the metastable limit, decreased the crystallization turbidity rate index and decreased the in vitro stone growth rate. The effective concentration for calcium binding reduction was about 2 orders of magnitude higher than that required for crystallization inhibition, which in turn was about 2 orders of magnitude higher than that required for stone growth inhibition. When human urine or artificial urine supplemented with urinary macromolecules was used, the effective concentration of phytate for inhibiting crystallization and stone growth was increased by about 1 order of magnitude. Inhibition of crystallization by phytate does not depend on decreasing the effective ionized calcium concentration and inhibition of in vitro stone growth does not depend on inhibiting crystallization of the suspended crystals. To our knowledge this is the first demonstration of a quantitative distinction between the inhibition of crystallization and stone growth. Inhibition of in vitro stone growth in the presence of macromolecules occurred at concentrations consistent with urinary phytate excretion.