Synthesis and Ca(2+)-release activity of D- and L-myo-inositol 2,4,5-trisphosphate and D- and L-chiro-inositol 1,3,4-trisphosphate

Both d- and l-Glucose Polyphosphates Mimic d-myo-Inositol 1,4,5-Trisphosphate: New Synthetic Agonists and Partial Agonists at the Ins(1,4,5)P3 Receptor

Chiral sugar derivatives are potential cyclitol surrogates of the Ca²⁺-mobilizing intracellular messenger d-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P₃]. Six novel polyphosphorylated analogues derived from both d- and l-glucose were synthesized. Binding to Ins(1,4,5)P₃ receptors [Ins(1,4,5)P₃R] and the ability to release Ca²⁺ from intracellular stores via type 1 Ins(1,4,5)P₃Rs were investigated. β-d-Glucopyranosyl 1,3,4-tris-phosphate, with similar phosphate regiochemistry and stereochemistry to Ins(1,4,5)P₃, and α-d-glucopyranosyl 1,3,4-tris-phosphate are full agonists, being equipotent and 23-fold less potent than Ins(1,4,5)P₃, respectively, in Ca²⁺-release assays and similar to Ins(1,4,5)P₃ and 15-fold weaker in binding assays. They can be viewed as truncated analogues of adenophostin A and refine understanding of structure-activity relationships for this Ins(1,4,5)P₃R agonist. l-Glucose-derived ligands, methyl α-l-glucopyranoside 2,3,6-trisphosphate and methyl α-l-glucopyranoside 2,4,6-trisphosphate, are also active, while their corresponding d-enantiomers, methyl α-d-glucopyranoside 2,3,6-trisphosphate and methyl α-d-glucopyranoside 2,4,6-trisphosphate, are inactive. Interestingly, both l-glucose-derived ligands are partial agonists: they are among the least efficacious agonists of Ins(1,4,5)P₃R yet identified, providing new leads for antagonist development.

The "Other" Inositols and Their Phosphates: Synthesis, Biology, and Medicine (with Recent Advances in myo-Inositol Chemistry)

Cell signaling via inositol phosphates, in particular via the second messenger myo-inositol 1,4,5-trisphosphate, and phosphoinositides comprises a huge field of biology. Of the nine 1,2,3,4,5,6-cyclohexanehexol isomers, myo-inositol is pre-eminent, with "other" inositols (cis-, epi-, allo-, muco-, neo-, L-chiro-, D-chiro-, and scyllo-) and derivatives rarer or thought not to exist in nature. However, neo- and d-chiro-inositol hexakisphosphates were recently revealed in both terrestrial and aquatic ecosystems, thus highlighting the paucity of knowledge of the origins and potential biological functions of such stereoisomers, a prevalent group of environmental organic phosphates, and their parent inositols. Some "other" inositols are medically relevant, for example, scyllo-inositol (neurodegenerative diseases) and d-chiro-inositol (diabetes). It is timely to consider exploration of the roles and applications of the "other" isomers and their derivatives, likely by exploiting techniques now well developed for the myo series.

Identification of the inositol isomers present in Tetrahymena

The inositol isomer composition of phosphoinositides, polyphosphoinositols, phosphatidylinositol-linked glycans, and glycosyl phosphatidylinositol-anchored proteins of logarithmic phase Tetrahymena vorax was determined by GC-MS analysis of trimethylsilylimadazole derivatives. The most abundant inositol found was the myo-isomer; however, appreciable percentages of scylloinositol were present in the free inositol pool, phosphatidylinositol-linked glycan fraction, and glycosyl phosphatidylinositol-anchored protein fraction. Trace quantities of chiro- and neo-inositols also were present.

Synthesis of D- and L-myo-inositol 2,4,5-trisphosphate and trisphosphorothioate: structural analogues of D-myo-inositol 1,4,5-trisphosphate

The preparation of D- and L-myo-inositol 2,4,5-trisphosphate is described, together with the phosphorothioate counterparts. The known chiral diols D- and L-1,4-di-O-benzyl-5,6-bis-O-p-methoxybenzyl-myo-inositol were regioselectively protected at the 3-position using a benzyl group via a 2,3-O-stannylene acetal. Removal of the p-methoxybenzyl groups of each enantiomer gave D- and L-1,3,6-tri-O-benzyl-myo-inositol. Phosphitylation with bis(benzyloxy)diisoproplyaminophosphine and 1H-tetrazole gave the trisphosphite intermediate for each enantiomer. Oxidation with 3-chloroperoxybenzoic acid gave the fully protected D- and L-myo-inositol 2,4,5-trisphosphates. Sulphoxidation of the D- and L-2,4,5-trisphosphite intermediates gave the fully protected D- and L-myo-inositol 2,4,5-trisphosphorothioate compounds. The fully protected trisphosphates were deblocked using hydrogenolysis and the phosphorothioates were deprotected using sodium in liquid ammonia. The individual compounds were then purified using ion exchange chromatography to afford pure D- and L-myo-inositol 2,4,5-trisphosphates together with the corresponding phosphorothioates.

Divergent syntheses of all possible optically active regioisomers of myo-inositol tris- and tetrakisphosphates

Since the discovery of D-myo-inositol 1,4,5-trisphosphate, which plays a pivotal role as a second messenger in transmembrane signaling, the scope of the phosphoinositide-based signaling processes has been continually expanding. However, the clear understanding of the molecular signal transduction mechanisms including the functions of newly found IP(n) is still lacking. As a continuing effort to our previously reported syntheses of all possible 39 optically inactive regioisomers of myo-inositol phosphates (IP(n); n = 1-6), we synthesized all possible optically active regioisomers of myo-IP(3) and myo-IP(4) using chiral IBz(3)s and IBz(2)s, respectively. A series of procedures involving CRL-catalyzed enzymatic resolution of racemic 1,2:5,6-di-O-isopropylidene-myo-inositol and base-catalyzed benzoyl migration in tri- and dibenzoyl-isopropylidene-myo-inositol afforded eight enantiomeric pairs of IBz(3) and six enantiomeric pairs of IBz(2), respectively. Phosphorylation of these intermediates by the phosphitylation and oxidation procedure gave the target products.

Synthesis, calcium mobilizing, and physicochemical properties of D-chiro-inositol 1,3,4,6-tetrakisphosphate, a novel and potent ligand at the D-myo-inositol 1,4,5-trisphosphate receptor

The synthesis of a novel and potent ligand at the D-myo-inositol 1,4, 5-trisphosphate receptor (InsP3R) is described. D-chiro-Inositol 1,3, 4,6-tetrakisphosphate (7) and L-chiro-inositol 1,3,4, 6-tetrakisphosphate (ent-7) have been synthesized from D-2, 5-di-O-benzyl-chiro-inositol and L-2,5-di-O-benzyl-chiro-inositol, respectively. The potency of binding and Ca2+ release of 7 and ent-7 were examined in L15 and Lvec cells. 7 was a full agonist at the InsP3R in both cells, and ent-7 was inactive. The results are compared to those from D-myo-inositol 1,4,5-trisphosphate (1), DL-scyllo-inositol 1,2,4-trisphosphate (2), DL-myo-inositol 1,2,4, 5-tetrakisphosphate (3), scyllo-inositol 1,2,4,5-tetrakisphosphate (4), D-myo-inositol 2,4,5-trisphosphate (5), and D-chiro-inositol 1, 3,4-trisphosphate (6). The protonation processes of 7 have also been investigated by 31P NMR titration experiments.

Rapid kinetic measurements of 45Ca2+ mobilization reveal that Ins(2,4,5)P3 is a partial agonist at hepatic InsP3 receptors

Ins(2,4,5)P3, a metabolically stable analogue of Ins(1,4,5)P3, is widely used in analyses of Ca2+ signalling pathways, but its utility depends upon it faithfully mimicking the effects of the natural messenger, Ins(1,4,5)P3, at InsP3 receptors. To compare the kinetics of InsP3-evoked 45Ca2+ mobilization, Ins(1,4,5)P3- and Ins(2,4,5)P3-stimulated 45Ca2+ release from the intracellular stores of permeabilized rat hepatocytes was measured using rapid superfusion. Both Ins(1,4,5)P3 and Ins(2,4,5)P3 caused concentration-dependent increases in the rate of 45Ca2+ efflux, which accelerated towards a peak and then abruptly switched to a bi-exponentially decaying release rate. However, the peak rate of 45Ca2+ mobilization evoked by maximal concentrations of Ins(2,4,5)P3 was only 65+/-3% (n = 3) of that evoked by Ins(1,4,5)P3. Furthermore, Ins(2,4,5)P3 inhibited the peak rate of 45Ca2+ efflux evoked by Ins(1,4,5)P3. These results indicate that Ins(2,4,5)P3 is a partial agonist at hepatic Ins(1,4,5)P3 receptors. Additionally, responses to Ins(2,4,5)P3 were less positively cooperative [Hill coefficient (h) = 1.9+/-0.3] than were those to Ins(1,4,5)P3 (h = 3.0+/-0.2) and the kinetics of termination of 45Ca2+ mobilization were slower. The lesser efficacy of Ins(2,4,5)P3 may account for the lower cooperativity in the responses it evokes, the slower inactivation of InsP3 receptors and the characteristic patterns of Ca2+ spiking it evokes in intact cells.

The preparation of intermediates for the synthesis of 1D-myo-inositol 1,4,5- and 2,4,5-trisphosphates, 1,4-bisphosphate 5-phosphorothioate, and 4,5-bisphosphate 1-phosphorothioate from 1D-3,6-di-O-benzyl-1,2-O-isopropylidene-myo-inositol

The preparation of 1D-1,6-di-O-benzyl-2,5-di-O-p-methoxybenzyl-myo-inositol is described. This compound and 1D-3,6-di-O-benzyl-1,2-O-isopropylidene-myo-inositol were converted into 1D-1,3,6-tri-O-benzyl-myo-inositol which was phosphorylated to give an intermediate for the synthesis of 1D-myo-inositol 2,4,5-trisphosphate. 1D-3,6-Di-O-benzyl-1,2-O-isopropylidene-myo-inositol was converted into 1D-2,3,6-tri-O-benzyl-myo-inositol (an intermediate for the synthesis of 1D-myo-inositol 1,4,5-trisphosphate) and 1D-2,3,6-tri-O-benzyl-1-O-p-methoxybenzyl-myo-inositol (an intermediate for the synthesis of the 1-phosphorothioate analogue of 1D-myo-inositol 1,4,5-trisphosphate). 1D-3,6-Di-O-benzyl-1,2-O-isopropylidene-myo-inositol was also converted into 1D-2,3,6-tri-O-benzyl-5-O-p-methoxybenzyl[and -5-O(cis-prop-1-enyl)]-myo- inositol both of which are intermediates for the synthesis of the 5-phosphorothioate analogue of 1D-myo-inositol 1,4,5-trisphosphate. The synthesis of 1D-2,3,6-tri-O-benzyl-myo-inositol 1,4-bis(dibenzyl phosphate) 5-(dibenzyl phosphorothioate) from 1D-2,3,6-tri-O-benzyl-myo-inositol 1,4-bis(dibenzyl phosphate) is described.

Myo-inositol trisphosphate-mediated calcium release from internal stores of Entamoeba histolytica

Calcium mobilisation from internal stores of the parasitic protozoan Entamoeba histolytica was studied by fluorescence measurements of the calcium indicator quin 2 and 45Ca2+ incorporation studies in saponin-permeabilised amoebae. Prior energy-dependent calcium sequestration was found to be necessary for subsequent release of calcium by inositol 1,4,5-trisphosphate (Ins(1,4,5)P3). Both Ins(1,4,5)P3 and inositol 2,4,5-trisphosphate (Ins(2,4,5)P3) could release calcium equally well from permeabilised E. histolytica with similar EC50 (concentration which produced half maximal release) values for calcium release. Ins(1,4,5)P3-mediated calcium release occurred from a vesicular store, was sensitive to prior treatment by heparin and was attenuated by prior addition of a lower concentration of Ins(1,4,5)P3. cAMP failed to influence inositol trisphosphate induced calcium release, indicating the absence of control mechanisms through cAMP-dependent phosphorylation. GTP neither induced calcium release nor could potentiate inositol trisphosphate mediated calcium mobilisation. A saturating concentration of Ins(1,4,5)P3 could release 50% of radiolabelled calcium sequestered by energy-dependent mechanisms in E. histolytica. The energy-dependent calcium sequestration was inhibited by vanadate and the calcium antagonist Diltiazem but not by dicyclohexylcarbodiimide (DCCD), suggesting the involvement of an endoplasmic reticulum-like structure in calcium storage. Binding studies showed specific association of [3H]Ins(1,4,5)P3 to crude membrane fractions of E. histolytica, which was significantly inhibited by heparin in a dose-dependent manner. IC50 (concentration which produced half-maximal inhibition) values for displacement of radiolabelled Ins(1,4,5)P3 binding by unlabelled Ins(1,4,5)P3 and Ins(2,4,5)P3 were estimated to be 0.99 microM for both isomers. Our results suggested that Ins(1,4,5)P3-mediated calcium release from internal stores of E. histolytica most probably occurred in an inositol trisphosphate receptor-dependent manner.

New tetherable derivatives of myo-inositol 2,4,5- and 1,3,4-trisphosphates

(+/-)-myo-Inositol 1-(3-aminopropyl hydrogen phosphate) 3,4-bis(disodium phosphate) (5) and (+/-)-myo-inositol 2-(3-aminopropyl hydrogen phosphate) 4,5-bis(disodium phosphate) (11) have been synthesized by conventional procedures. Each derivative has been immobilized on a polymeric resin in order to give a bioaffinity matrix.

Syntheses of D-myo-inositol 1,4,5-trisphosphate affinity ligands

A mixture of 2,3,6-tri-O-benzoyl-4,5-di-O-benzyl-D-myo-inositol and 1,3,6-tri-O-benzoyl-4,5-di-O-benzyl-D-myo-inositol, obtained during our synthesis of D-myo-inositol 1,4,5-trisphosphate [C.E. Ballou and W. Tegge, Proc. Natl. Acad. Sci. U.S.A., 86 (1989) 94-98], was separated after tetrahydropyranylation of the free hydroxyl group in each. 2,3,6-Tri-O-benzoyl-4,5-di-O-benzyl-1-O- (tetrahydro-2-pyranyl)-D-myo-inositol was debenzylated and the two free hydroxyl groups were phosphorylated by a dibenzyl phosphoramidite procedure. The tetrahydropyranyl group was then removed, and phosphorylation at position 1 with benzyl 3-(benzyloxycarbonylamino)propyl di-N-isopropylphosphoramidite, followed by oxidation and deprotection, provided 1-[3-aminopropoxy(hydroxy)phosphinyl]-D-myo-inositol 4,5-bisphosphate. This compound was coupled to activated agarose to prepare an affinity matrix for the isolation of D-myo-inositol 1,4,5-trisphosphate-binding proteins, and it was coupled to 4-azido-2-hydroxybenzoic acid to give a product that was labeled with 125I to prepare a photoactivable derivatizing reagent. The new derivatives retain significant biological activity as assessed by their ability to stimulate the release of stored Ca2+ from the endoplasmic reticulum of permeabilized rat basophilic leukemia cells.

The Ca2+ release activities of D-myo-inositol 1,4,5-trisphosphate analogs are quantized

Comparison is made between several synthetic stereo and positional isomers of D-myo-inositol 1,4,5-trisphosphate (D-myo-1,4,5-IP3) with respect to their ability to mobilize calcium from the internal stores of saponin-permeabilized rat basophilic leukemia cells. D- and L-myo-Inositol 1,4,5-trisphosphates, D- and L-myo-inositol 2,4,5-trisphosphates, D- and L-chiro-inositol 1,3,4-trisphosphates, D,L-trans-1,2-cyclohexane-diol bisphosphate, D,L-myo-inositol 4,5-bisphosphate, L-glycerol 1,2-bisphosphate, glycerol 1,3-bisphosphate and D,L-(1R,3R,4R)-1-phosphoryloxymethyl-trans-3,4-cyclohexanediol bisphosphate were tested. The analogs, each of which contains a vicinal trans-1,2-diol-bisphosphate motif, displayed potencies that were distributed over a 10(4)-fold range of concentration and fell into 4 distinct classes of activity.