title: Characterization of myo-inositol hexakisphosphate deposits from larval Echinococcus granulosus.
abstract: The abundant metabolite myo-inositol hexakisphosphate (InsP6) 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 InsP6 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 InsP6 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 Ca5H2L.16H2O (L representing fully deprotonated InsP6), and additionally contain Mg2+ (6-9% molar ratio with respect to Ca2+), but not K+. Calculations employing recently available chemical constants show that the precipitation of Ca5H2L.16H2O 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 InsP6 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 InsP6, 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 InsP6, rather than arising from extracellular hydrolysis of InsP6.
authors.affiliation: Cátedra de Inmunología, Facultad de Química/Ciencias, Universidad de la República, Montevideo, Uruguay.
authors.forename: Cecilia
authors.initials: C
authors.lastname: Casaravilla
authors.forename: Charles
authors.initials: C
authors.lastname: Brearley
authors.forename: Silvia
authors.initials: S
authors.lastname: Soulé
authors.forename: Carolina
authors.initials: C
authors.lastname: Fontana
authors.forename: Nicolás
authors.initials: N
authors.lastname: Veiga
authors.forename: María I
authors.initials: MI
authors.lastname: Bessio
authors.forename: Fernando
authors.initials: F
authors.lastname: Ferreira
authors.forename: Carlos
authors.initials: C
authors.lastname: Kremer
authors.forename: Alvaro
authors.initials: A
authors.lastname: Díaz
mesh_terms: D000818:Animals; D002118:Calcium / Q000032:analysis; D002417:Cattle; D002851:Chromatography, High Pressure Liquid; D048209:Echinococcus granulosus / Q000737:chemistry* / Q000254:growth & development; D005089:Exocytosis; D005109:Extracellular Matrix / Q000737:chemistry / Q000648:ultrastructure; D006868:Hydrolysis; D007814:Larva / Q000737:chemistry; D008274:Magnesium / Q000032:analysis; D051379:Mice; D019906:Nuclear Magnetic Resonance, Biomolecular; D010833:Phytic Acid / Q000032:analysis* / Q000096:biosynthesis / Q000302:isolation & purification; D011188:Potassium / Q000032:analysis; D012964:Sodium / Q000032:analysis; D013237:Stereoisomerism; D013816:Thermodynamics
chemical_list: D010833:Phytic Acid; D012964:Sodium; D008274:Magnesium; D011188:Potassium; D002118:Calcium