Characterization of myo ‐inositol hexakisphosphate deposits from larval Echinococcus granulosus

The abundant metabolite myo ‐inositol hexakisphosphate (Ins P 6 ) can form vesicular deposits with cations, a widespread phenomenon in plants also found in the cestode parasite, Echinococcus granulosus . In this organism, the deposits are exocytosed, accumulating in a host‐exposed sheath of extracellular matrix termed the laminated layer. The formation and mobilization of Ins P 6 deposits, which involve precipitation and solubilization reactions, respectively, cannot yet be rationalized in quantitative chemical terms, as the solids involved have not been formally described. We report such a description for the Ins P 6 deposits from E. granulosus , purified as the solid residue left by mild alkaline digestion of the principal mucin component of the laminated layer. The deposits are largely composed of the compound Ca 5 H 2 L·16H 2 O (L representing fully deprotonated Ins P 6 ), and additionally contain Mg 2+ (6–9% molar ratio with respect to Ca 2+ ), but not K + . Calculations employing recently available chemical constants show that the precipitation of Ca 5 H 2 L·16H 2 O is predicted by thermodynamics in secretory vesicle‐like conditions. The deposits appear to be similar to microcrystalline solids when analysed under the electron microscope; we estimate that each crystal comprises around 200 Ins P 6 molecules. We calculate that the deposits increase, by three orders of magnitude, the surface area available for adsorption of host proteins, a salient ability of the laminated layer. The major inositol phosphate in the deposits, other than Ins P 6 , is myo ‐inositol (1,2,4,5,6) pentakisphosphate, or its enantiomer, inositol (2,3,4,5,6) pentakisphosphate. The compound appears to be a subproduct of the intracellular pathways leading to the synthesis and vesicular accumulation of Ins P 6 , rather than arising from extracellular hydrolysis of Ins P 6 .