Abstract
The inhibitory activity of inositol phosphate (IP) against calcium phosphate precipitation was studied in vitro. Each inositol phosphate was fractionated by ion-exchange column chromatography after a mixture of inositol phosphates had been obtained by enzymatic dephosphorylation of sodium phytate. The activity of IP was determined by monitoring the amount of soluble calcium, including free ionizable calcium and soluble complexes of calcium, and used as an index of calcium bioavailability. The effect of IP concentration on its activity was studied between 1×10-3, and 10-5 M. The activities of all IP types were quite similar at 1×10-5M, which was equal to that of the control. The intensity of activity at 1×10-4M was IP2=IP4>IP3>IP5>IP6 (the numbers standing for the numbers of phosphate ions bound to a single inositol). The activities were variable between 1×10-4 and 1×10-3M.The activity of IP2 increased between 1×10-4 and 3×10-4M, but was unchanged at more than 3×10-4 M, whereas those of IP3 and IP4 were unchanged and decreased, respectively. Furthermore, tests for phosphate behavior in the reaction mixture suggested that calcium ions were bound to phosphate in IP, and not to inorganic phosphate. The molar ratio of calcium to phosphate was 1: 1 in any calcium IP precipitation.
