Metabolism of synthetic inositol trisphosphate analogs

Synthesis of inositol phosphate-based competitive antagonists of inositol 1,4,5-trisphosphate receptors

Inositol 1,4,5-trisphosphate receptors (IP3Rs) are intracellular Ca(2+) channels that are widely expressed in animal cells, where they mediate the release of Ca(2+) from intracellular stores evoked by extracellular stimuli. A diverse array of synthetic agonists of IP3Rs has defined structure-activity relationships, but existing antagonists have severe limitations. We combined analyses of Ca(2+) release with equilibrium competition binding to IP3R to show that (1,3,4,6)IP4 is a full agonist of IP3R1 with lower affinity than (1,4,5)IP3. Systematic manipulation of this meso-compound via a versatile synthetic scheme provided a family of dimeric analogs of 2-O-butyryl-(1,3,4,6)IP4 and (1,3,4,5,6)IP5 that compete with (1,4,5)IP3 for binding to IP3R without evoking Ca(2+) release. These novel analogs are the first inositol phosphate-based competitive antagonists of IP3Rs with affinities comparable to that of the only commonly used competitive antagonist, heparin, the utility of which is limited by off-target effects.

Stimulation of inositol 1,4,5-trisphosphate (IP3) receptor subtypes by adenophostin A and its analogues

Inositol 1,4,5-trisphosphate receptors (IP3R) are intracellular Ca(2+) channels. Most animal cells express mixtures of the three IP3R subtypes encoded by vertebrate genomes. Adenophostin A (AdA) is the most potent naturally occurring agonist of IP3R and it shares with IP3 the essential features of all IP3R agonists, namely structures equivalent to the 4,5-bisphosphate and 6-hydroxyl of IP3. The two essential phosphate groups contribute to closure of the clam-like IP3-binding core (IBC), and thereby IP3R activation, by binding to each of its sides (the α- and β-domains). Regulation of the three subtypes of IP3R by AdA and its analogues has not been examined in cells expressing defined homogenous populations of IP3R. We measured Ca(2+) release evoked by synthetic adenophostin A (AdA) and its analogues in permeabilized DT40 cells devoid of native IP3R and stably expressing single subtypes of mammalian IP3R. The determinants of high-affinity binding of AdA and its analogues were indistinguishable for each IP3R subtype. The results are consistent with a cation-π interaction between the adenine of AdA and a conserved arginine within the IBC α-domain contributing to closure of the IBC. The two complementary contacts between AdA and the α-domain (cation-π interaction and 3″-phosphate) allow activation of IP3R by an analogue of AdA (3″-dephospho-AdA) that lacks a phosphate group equivalent to the essential 5-phosphate of IP3. These data provide the first structure-activity analyses of key AdA analogues using homogenous populations of all mammalian IP3R subtypes. They demonstrate that differences in the Ca(2+) signals evoked by AdA analogues are unlikely to be due to selective regulation of IP3R subtypes.

Benzene polyphosphates as tools for cell signalling: inhibition of inositol 1,4,5-trisphosphate 5-phosphatase and interaction with the PH domain of protein kinase Balpha

Novel benzene polyphosphates were synthesised as inositol polyphosphate mimics and evaluated against type-I inositol 1,4,5-trisphosphate 5-phosphatase, which only binds soluble inositol polyphosphates, and against the PH domain of protein kinase Balpha (PKBalpha), which can bind both soluble inositol polyphosphates and inositol phospholipids. The most potent trisphosphate 5-phosphatase inhibitor is benzene 1,2,4-trisphosphate (2, IC(50) of 14 microM), a potential mimic of D-myo-inositol 1,4,5-trisphosphate, whereas the most potent tetrakisphosphate Ins(1,4,5)P(3) 5-phosphatase inhibitor is benzene 1,2,4,5-tetrakisphosphate, with an IC(50) of 4 microM. Biphenyl 2,3',4,5',6-pentakisphosphate (4) was the most potent inhibitor evaluated against type I Ins(1,4,5)P(3) 5-phosphatase (IC(50) of 1 microM). All new benzene polyphosphates are resistant to dephosphorylation by type I Ins(1,4,5)P(3) 5-phosphatase. Unexpectedly, all benzene polyphosphates studied bind to the PH domain of PKBalpha with apparent higher affinity than to type I Ins(1,4,5)P(3) 5-phosphatase. The most potent ligand for the PKBalpha PH domain, measured by inhibition of biotinylated diC(8)-PtdIns(3,4)P(2) binding, is biphenyl 2,3',4,5',6-pentakisphosphate (4, K(i)=27 nm). The approximately 80-fold enhancement of binding relative to parent benzene trisphosphate is explained by the involvement of a cation-pi interaction. These new molecular tools will be of potential use in structural and cell signalling studies.

3-Hydroxybenzene 1,2,4-Trisphosphate, a Novel Second Messenger Mimic and unusual Substrate for Type-I myo-Inositol 1,4,5-Trisphosphate 5-Phosphatase: Synthesis and Physicochemistry

3-Hydroxybenzene 1,2,4-trisphosphate 4 is a new myo-inositol 1,4,5-trisphosphate analogue based on the core structure of benzene 1,2,4-trisphosphate 2 with an additional hydroxyl group at position-3, and is the first noninositol based compound to be a substrate for inositol 1,4,5-trisphosphate 5-phosphatase. In physicochemical studies on 2, when three equivalents of protons were added, the (31)P NMR spectrum displayed monophasic behaviour in which phosphate-1 and phosphate-2 behaved independently in most of the studied pH range. For compound 4, phosphate-2 and phosphate-4 interacted with the 3-OH group, which does not titrate at physiological pH, displaying complex biphasic behaviour which demonstrated co-operativity between these groups. Phosphate-1 and phosphate-2 strongly interacted with each other and phosphate-4 experienced repulsion because of the interaction of the 3-OH group. Benzene 1,2,4-trisphosphate 2 is resistant to inositol 1,4,5-trisphosphate type I 5-phosphatase catalysed dephosphorylation. However, surprisingly, 3-hydroxybenzene 1,2,4-trisphosphate 4 was dephosphorylated by this 5-phosphatase to give the symmetrical 2,3-dihydroxybenzene 1,4-bisphosphate 16. The extra hydroxyl group is shown to form a hydrogen bond with the vicinal phosphate groups at -15 degrees C, and (1)H NMR titration of the ring and hydroxyl protons in 4 shows the OH proton to be strongly stabilized as soon as the phosphate groups are deprotonated. The effect of the phenolic 3-OH group in compound 4 confirms a critical role for the 6-OH group of the natural messenger in the dephosphorylation mechanism that persists even in radically modified analogues.

Pretreatment with D-myo-inositol trisphosphate reduces infarct size in rabbit hearts: role of inositol trisphosphate receptors and gap junctions in triggering protection

Pretreatment with D-myo-inositol-1,4,5-trisphosphate hexasodium (D-myo-IP(3)), the sodium salt of the second messenger inositol 1,4,5-trisphosphate (IP(3)), is cardioprotective and triggers a reduction of infarct size comparable in magnitude to that obtained with ischemic preconditioning. However, this observation is enigmatic; whereas IP(3) signaling is conventionally initiated by receptor binding, IP(3) receptors are typically considered to be intracellular, and D-myo-IP(3) is membrane-impermeable. We propose that this paradox is explained by the presence of poorly characterized external IP(3) receptors and hypothesize that: 1) infarct size reduction with D-myo-IP(3) is receptor-mediated; and 2) communication via gap junctions and/or hemichannels is required to initiate this protection. To investigate the role of receptor binding, isolated buffer-perfused rabbit hearts underwent 30 min of coronary occlusion (CO) and 2 h of reflow. Prior to CO, hearts received no treatment (controls), D-myo-IP(3), L-myo-IP(3) (enantiomer not recognized by the IP(3) receptor), D-myo-IP(3) + the IP(3) receptor inhibitor xestospongin C (XeC), or XeC alone. Infarct size, assessed by tetrazolium staining, was reduced with D-myo-IP(3) treatment, whereas hearts that received L-myo-IP(3) or D-myo-IP(3) + XeC showed no protection. To evaluate the contribution of gap junctions/hemichannels, additional control and D-myo-IP(3)-treated cohorts received a 5-min infusion of heptanol or Gap 27, two structurally distinct gap junction inhibitors, administered at doses confirmed to attenuate intercellular transmission of a gap junction-permeable fluorescent dye. There was no infarct-sparing effect of D-myo-IP(3) in inhibitor-treated hearts. These data support the concepts that infarct size reduction with D-myo-IP(3) is triggered by receptor binding and that communication via gap junctions/hemichannels is involved in initiating this protection.

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.

Differential stereoselectivity of D- and L-myo-inositol 1,2,4, 5-tetrakisphosphate binding to the inositol 1,4,5-trisphosphate receptor and 3-kinase

D- and L-myo-inositol 1,2,4,5-tetrakisphosphate (Ins(1,2,4,5)P(4)) were investigated for their ability to bind to the D-myo-inositol 1, 4,5-trisphosphate (Ins(1,4,5)P(3)) receptor in a bovine adrenal cortical membrane fraction, to mobilize intracellular Ca(2+) stores in Xenopus oocytes, and to bind to the rat brain Ins(1,4,5)P(3) 3-kinase overexpressed and purified in E. coli. In competitive binding experiments with the Ins(1,4,5)P(3) receptor, D-Ins(1,2,4, 5)P(4) effectively displaced [(3)H]Ins(1,4,5)P(3) in a concentration-dependent manner with a potency comparable to that of D-Ins(1,4,5)P(3), while L-Ins(1,2,4,5)P(4) was approximately 50-fold less effective than D-Ins(1,4,5)P(3) and D-Ins(1,2,4,5)P(4). The DL-Ins(1,2,4,5)P(4) racemate bound to the Ins(1,4,5)P(3) receptor with an apparent intermediate efficiency. Injection of D-Ins(1,2,4, 5)P(4) into oocytes evoked a chloride current dependent on intracellular Ca(2+) mobilization in which the agonists ranked in a similar order of potency as in the Ins(1,4,5)P(3) receptor binding. On the other hand, D-Ins(1,2,4,5)P(4) only inhibited the binding of [(3)H]Ins(1,4,5)P(3) to 3-kinase very weakly with a markedly reduced potency compared to D-Ins(1,4,5)P(3), indicating that D-Ins(1,2,4, 5)P(4) is not an effective competitor in the phosphorylation of [(3)H]-Ins(1,4,5)P(3) by 3-kinase. The results, therefore, clearly indicate that D-Ins(1,2,4,5)P(4) is as effective as D-Ins(1,4,5)P(3) in the binding to the receptor but not 3-kinase, and access of Ins(1, 2,4,5)P(4) over the Ins(1,4,5)P(3) receptor calls for stringent stereospecificity with D-Ins(1,2,4,5)P(4) being the active form in DL-Ins(1,2,4,5)P(4)-mediated Ca(2+) mobilization.

myo-inositol 1,4,6-trisphosphorothioate and myo-inositol 1,3, 6-trisphosphorothioate: partial agonists with very low intrinsic activity at the platelet myo-inositol 1,4,5-trisphosphate receptor

Racemic mixtures and enantiomerically pure D-isomers of both myo-inositol 1,3,6-trisphosphorothioate [Ins(1,3,6)PS(3)] and myo-inositol 1,4,6-trisphosphorothioate [Ins(1,4,6)PS(3)], prepared by total synthesis, were examined in Ca(2+) flux and binding assays. Both D-Ins(1,3,6)PS(3) and D-Ins(1,4,6)PS(3) were shown to be low intrinsic activity partial agonists at the platelet myo-inositol 1,4, 5-trisphosphate [Ins(1,4,5)P(3)] receptor, releasing less than 20% of the Ins(1,4,5)P(3)-sensitive Ca(2+) store. D-Ins(1,4,6)PS(3) displaced specifically bound [(3)H]Ins(1,4,5)P(3) from rat cerebellar membranes, although displacement was some 34-fold weaker than by D-Ins(1,4,5)P(3). D-Ins(1,4,6)PS(3) displaced [(3)H]Ins(1,4, 5)P(3) from cerebellar membranes with roughly twice the affinity of DL-Ins(1,4,6)PS(3) (IC(50) value = 1.4 +/- 0.35 microM compared with 2.15 +/- 0.13 microM), whereas D-Ins(1,3,6)PS(3) displaced [(3)H]Ins(1,4,5)P(3) with roughly twice the affinity of DL-Ins(1,3, 6)PS(3) (IC(50) value = 17.5 +/- 5.8 microM compared with 34 +/- 10 microM), confirming that the activity of both these phosphorothioates resides in their D-enantiomers. Increasing concentrations of either D-Ins(1,3,6)PS(3) or D-Ins(1,4,6)PS(3) were able to partially antagonize Ca(2+) release induced by submaximal concentrations of Ins(1,4,5)P(3), an inhibition that could be overcome by increasing the concentration of Ins(1,4,5)P(3), suggesting competition for binding at the Ins(1,4,5)P(3)-R. The only low-efficacy partial agonists at the Ins(1,4,5)P(3)-R discovered to date have been phosphorothioates; the novel D-Ins(1,3,6)PS(3) and D-Ins(1,4,6)PS(3) can now be added to this small group of analogs. However, D-Ins(1,4,6)PS(3) has a relatively high affinity for the Ins(1,4,5)P(3)-R but maintains the lowest efficacy of all the partial agonists thus far identified. As such, it may be a useful tool for pharmacological intervention in the polyphosphoinositide pathway and an important lead compound for the development of further Ins(1,4,5)P(3)-R antagonists.

Use of phosphorofluoridate analogues of D-myo-inositol 1,4,5-trisphosphate to assess the involvement of ionic interactions in its recognition by the receptor and metabolising enzymes

D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] analogues fluoridated at 4- or 5-phosphate or both were analysed to assess the involvement of ionic interactions between the phosphates of Ins(1,4,5)P3 and the proteins that recognize it, such as metabolic enzymes and the InsP3 receptor. These analogues were effective in inhibiting type I Ins(1,4,5)P3 5-phosphatase activity with much the same potency as Ins(1,4,5)P3, although the enzyme showed a lower Km value as pH values increased. In contrast, the analogues were less potent ligands than Ins(1,4,5)P3 in both the assay of [3H]Ins(1,4,5)P3 binding to the receptors and the phosphorylation of [3H]Ins(1,4,5)P3 catalysed by Ins(1,4,5)P3 3-kinase. These results suggest that ionic interactions with the dianionic 4- and 5-phosphates of Ins(1,4,5)P3 are involved in recognition by the receptor and the kinase, but not by the phosphatase.

New developments in the molecular pharmacology of the myo-inositol 1,4,5-trisphosphate receptor

Receptor-mediated activation of phospholipase C to generate inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] is a ubiquitous signalling pathway in mammalian systems. A family of three IP3 receptor subtype monomers form functional tetramers, which act as effectors for Ins(1,4,5)P3, providing a ligand-gated channel that allows Ca2+ ions to move between cellular compartments. As IP3 receptors are located principally, although not exclusively, in the endoplasmic reticular membrane, Ins(1,4,5)P3 is considered to be a second messenger that mobilizes Ca2+ from intracellular stores. Ca2+ store mobilization by Ins(1,4,5)P3 can be shown to contribute to a variety of physiological and pathophysiological phenomena, and therefore the IP3 receptor represents a novel, potential pharmacological target. In this article, Rob Wilcox and colleagues review recent developments in IP3 receptor pharmacology, with particular emphasis on ligand molecular recognition by this receptor-channel complex. The potential for designing non-inositol phosphate-based agonists and antagonists is also discussed.

Syntheses of D- and L-myo-inositol 1,2,4,5-tetrakisphosphate and stereoselectivity of the I(1,4,5)P3 receptor binding

D- and L-myo-Inositol 1,2,4,5-tetrakisphosphate [D- & L-I(1,2,4,5)P4], which are analogues of D-myo-Inositol 1,4,5-trisphosphate [D-I(1,4,5)P3], a calcium mobilizing second messenger, were synthesized via resolution of the camphanate ester of a myo-inositol derivative, and the binding affinities to I(1,4,5)P3 receptor were measured.

A role for hydrolysis of inositol 1,4,5-trisphosphate in terminating the response to inositol 1,4,5-trisphosphate and to a flash of light in Limulus ventral photoreceptors

Injection of inositol 1,4,5-trisphosphate (Ins 1,4,5-P3) into Limulus ventral photoreceptors produces excitation similar to that produced by light. One process which might contribute to rapid termination of the responses to Ins 1,4,5-P3 and to light is the hydrolysis of Ins 1,4,5-P3 by an InsP3-5-phosphatase to form inositol 1,4-bisphosphate. Inositol 2,4,5-trisphosphate (Ins 2,4,5-P3) is known to be less hydrolysable by the InsP3-5-phosphatase than is Ins 1,4,5-P3. Whereas ventral photoreceptors respond to an injection of Ins 1,4,5-P3 with a single wave of depolarization, the response to Ins 2,4,5-P3 is a burst of waves of depolarization. Our hypothesis is that it is the resistance to hydrolysis by the InsP3-5-phosphatase which accounts for the burst of waves produced by Ins 2,4,5-P3. To test this idea we injected ventral photoreceptors with Ins 1,4,5-P3 in the presence of the non-specific phosphatase inhibitors, vanadate and fluoride, which prolong the response to a flash of light in ventral photoreceptors (D.W. Corson, A. Fein, W.W. Walthall, J. Gen. Physiol. 82 (1983) 659-677). In the presence of fluoride or vanadate the response to Ins 1,4,5-P3 was composed of a burst of waves rather than a single wave of depolarization. We conclude that hydrolysis of Ins 1,4,5-P3 by the InsP3-5-phosphatase plays a role in terminating the ventral photoreceptors response to Ins 1,4,5-P3 and also to light.

Binding and activity of the nine possible regioisomers of myo-inositol tetrakisphosphate at the inositol 1,4,5-trisphosphate receptor

All 9 racemic regioisomers (15 enantiomerically) of myo-inositol tetrakisphosphates (IP4s): DL-Ins(1,2,4,5)P4 [A], DL-Ins(1,2,4,6)P4 [B], Ins(1,2,3,5)P4 [C], Ins(1,3,4,6)P4 [D], Ins(2,4,5,6)P4 [E], DL-Ins(1,3,4,5)P4 [F], DL-Ins(1,2,5,6)P4 [G], DL-Ins(1,2,3,4)P4 [H] and DL-Ins(1,4,5,6)P4 [I] [Chung S-K., Chang Y-T. Synthesis of all possible regioisomers of myo-inositol tetrakisphosphate. J Chem Soc Chem Commun 1995; 11-13] were investigated for their ability to bind to the D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] receptor in bovine adrenal cortical membranes, and for their ability to mobilize 45Ca2+ from Ins(1,4,5)P3-sensitive Ca2+ stores in permeabilized Chinese hamster ovary (CHO) cells. DL-Ins(1,2,4,5)P4 (Ki = 11 nM) bound to Ins(1,4,5)P3 receptors with an affinity only 2-fold lower than Ins(1,4,5)P3 (Ki = 6 nM). Ins(1,2,3,5)P4, Ins(1,3,4,6)P4, Ins(2,4,5,6)P4, DL-Ins(1,3,4,5)P4, DL-Ins(1,2,3,4)P4 and DL-Ins(1,4,5,6)P4 bound with affinities of between 0.4-0.7 microM. DL-Ins(1,2,4,6)P4 and DL-Ins(1,2,5,6)P4 bound to the Ins(1,4,5)P3 receptor with low affinity (approximately 2-3 microM). All but one of the IP4s mediated release of 45Ca2+ from stores of permeabilized CHO cells with a similar rank order of potency as that for Ins(1,4,5)P3 receptor binding, being between 16-fold and 50-fold less potent at releasing 45Ca2+ compared with their apparent binding affinities to the Ins(1,4,5)P3 receptor. The notable exception was Ins(1,2,3,5)P4, which showed an approximately 200-fold lower potency compared with its affinity for the Ins(1,4,5)P3 receptor. Ins(1,2,3,5)P4 may be a useful lead compound for the rational design of novel synthetic Ins(1,4,5)P3 analogues possessing structure-activity profiles with relatively high binding affinity, but low intrinsic efficacy, and hence partial agonists and antagonists at the Ins(1,4,5)P3 receptor.

Synthesis of D- and L-myo-Inositol 1,4,6-Trisphosphate, Regioisomers of a Ubiquitous Second Messenger

A regioisomer of the second messenger D-myo-inositol 1,4,5-trisphosphate [D-Ins(1,4,5)P(3), 1], DL-myo-inositol 1,4,6-trisphosphate [DL-Ins(1,4,6)P(3), 4ab], together with the chiral antipodes D-Ins(1,4,6)P(3)(4a) and L-Ins(1,4,6)P(3)(4b), was synthesized from myo-inositol. The racemic diol 6, after removal of the trans-ketal of fully protected 5 was p-methoxybenzylated to give the 6-O-alkylated derivative 9, as the major product in 52% yield. Gentle acidic hydrolysis of 9, followed by benzylation of the resulting triol, gave the fully protected compound 11ab. Isomerization of the two allyl groups followed by acidic hydrolysis of the resulting cis-prop-1-enyl moieties and the p-methoxybenzyl group gave the triol 13ab. Phosphorylation of 13ab followed by deprotection of the resulting compound, 14ab, with sodium in liquid ammonia and purification by ion exchange chromatography provided 4ab in 60% yield. The intermediate 9 was converted into the cis-diol 16ab in two steps. Selective acylation at the equatorial hydroxyl group using (S)-(+)-O-acetylmandelic acid in the presence of DCC and DMAP provided two diastereoisomers, 18 and 19, which were separated by flash chromatography. Further transformations provided the corresponding D- and L-1,4,6 triols, 13a and 13b, respectively, and phosphorylation, followed by deprotection of the fully blocked products as for the racemic 4ab, gave 4a and 4b, respectively. The absolute configuration of fully protected 11a was determined by transformation to the known compound L-1,2,4,5-tetra-O-benzyl-myo-inositol (22). Compound 4a was a full agonist at the platelet Ins(1,4,5)P(3) receptor for Ca(2+) release, but 4b was devoid of activity.

Pharmacological modulators of the inositol 1,4,5-trisphosphate receptor

Elevation of cytosolic calcium concentrations, induced by many neurotransmitters, plays a crucial role in neuronal function. Some neurotransmitters produce the second messenger InsP3 which activates an intracellular calcium channel (InsP3 receptor) usually located in the endoplasmic reticulum. This article undertakes a comprehensive survey of most pharmacological modulators of the InsP3 receptor so far reported. This review discusses in detail competitive antagonists, non-competitive antagonists and thiol reactive reagents, highlighting their modes of action and in some cases indicating drawbacks in their use.

D-myo-inositol 1,4,5-trisphosphate analogues substituted at the 3-hydroxyl group

D-myo-Inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) analogues derived at 3-OH with a bulky substituent were chemically synthesized and structural features of vicinity surrounding the 3-OH of Ins(1,4,5)P3, recognized by metabolic enzymes and by the receptor were explored. 3-Benzoyl-, 3-methylbenzoyl- and 3-para-aminobenzoyl-Ins(1,4,5)P3 inhibited the dephosphorylation of [3H]Ins(1,4,5)P3 by the 5-phosphatase present in erythrocyte ghosts, but the potency varied. The inhibitory potency for the former two compounds was slightly lower than that for Ins(1,4,5)P3, while that for the latter compound was higher. Transfer of the amino group to the meta-position of the benzoyl group led to a less potent analogue. In an assay of [3H]Ins(1,4,5)P3 3-kinase at a low Ca2+ concentration, catalyzed by rat brain cytosol, 3-meta-aminobenzoyl-Ins(1,4,5)P3 was the most potent among compounds examined, including Ins(1,4,5)P3 in inhibiting the phosphorylation, whereas both 3-benzoyl- and 3-methylbenzoyl-Ins(1,4,5)P3 at concentrations up to 30 microM, were without effect. All analogues examined were effective in inhibiting [3H]Ins(1,4,5)P3 binding to purified Ins(1,4,5)P3 receptor, but all 3-derived analogues were less potent and 3-benzoyl-Ins(1,4,5)P3 was the least potent. It would thus appear that the space in the vicinity surrounding the 3-hydroxyl group of Ins(1,4,5)P3 is sterically restrictive with regard to recognition by metabolic enzymes and the receptor, whereas the amino group providing arms for either the electrostatic interaction or the hydrogen bond, makes the analogues more potent.

Phosphorylation of inositol 1,4,5-trisphosphate analogues by 3-kinase and dephosphorylation of inositol 1,3,4,5-tetrakisphosphate analogues by 5-phosphatase

A series of 32P-labeled D-myo-inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] analogues was enzymically prepared from the corresponding D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] analogues using recombinant rat brain Ins(1,4,5)P3 3-kinase and [gamma-32P]ATP. Ins(1,4,5)P3 analogues with bulky groups at the 2-OH position, substitutions of phosphates by thiophosphates and D-6-deoxy-myo-Ins(1,4,5)P3 were tested. Using [3H]Ins(1,4,5)P3 and ATP gamma S, a [3H]Ins(1,3,4,5)P4 analogue with a thiophosphate at the D-3 position was prepared. The D-4 and/or D-5 phosphate group seemed to be important for 3-kinase activity, while the OH group at position 6 was not crucial. The addition of bulky groups at the 2-OH position did not prevent phosphorylation. The labeled Ins(1,3,4,5)P4 analogues were purified and their degradation by type-I Ins(1,4,5)P3/Ins(1,3,4,5)P4 5-phosphatase was compared with the degradation of Ins(1,3,4,5)P4. Substitution of the phosphate group at positions 1 or 3 by a thiophosphate, or the addition of bulky groups at the 2-OH position did not prevent degradation. D-6-Deoxy-myo-inositol 1,3,4,5-tetrakisphosphate could not be degraded by the 5-phosphatase, indicating the importance of the 6-OH group for 5-phosphatase action. D-6-Deoxy-myo-inositol 1,3,4,5-tetrakisphosphate could be an important tool in elucidating the cellular functions of Ins(1,3,4,5)P4.

Modification at C2 of myo-inositol 1,4,5-trisphosphate produces inositol trisphosphates and tetrakisphosphates with potent biological activities

Novel 2-position-modified D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] analogues, DL-2-deoxy-2-fluoro-myo-inositol 1,4,5-trisphosphate [DL-2F-Ins(1,4,5)P3], DL-myo-inositol 1,2,4,5-tetrakisphosphate [DL-Ins(1,2,4,5)P4], DL-scyllo-inositol 1,2,4-trisphosphate [DL-sc-Ins(1,2,4)P3], scyllo-inositol 1,2,4,5-tetrakisphosphate [sc-Ins(1,2,4,5)P4] and scyllo-inositol 1,2,4,5-tetrakisphosphorothioate [sc-Ins(1,2,4,5)PS4] were investigated for their ability to bind to the Ins(1,4,5)P3 receptor, mobilise intracellular Ca2+ stores and interact with metabolic enzymes. With the exception of sc-Ins(1,2,4,5)PS4, all the Ins(1,4,5)P3 analogues potently displaced [3H]Ins(1,4,5)P3 from its receptor in bovine adrenal cortex and were apparently potent full agonists at the Ca2+ mobilising Ins(1,4,5)P3 receptor of SH-SY5Y cells, giving respective IC50 and EC50 values of: sc-Ins(1,2,4,5)P4 (IC50 14 nM, EC50 77 nM), DL-2F-Ins(1,4,5)P3 (IC50 25 nM, EC50 105 nM), DL-Ins(1,2,4,5)P4 (IC50 26 nM, EC50 163 nM), DL-sc-Ins(1,2,4)P3 (IC50 52 nM, EC50 171 nM), compared to Ins(1,4,5)P3 (IC50 4 nM, EC50 52 nM). sc-Ins(1,2,4,5)P4 was equipotent to Ins(1,4,5)P3 for Ca2+ release making it the most potent inositol tetrakisphosphate and indeed Ins(1,4,5)P3 analogue yet characterised. In contrast, although sc-Ins(1,2,4,5)P4 (IC50 425 nM, EC50 1603 nM) was a significantly weaker ligand and agonist than Ins(1,4,5)P3, it was a partial agonist of high intrinsic activity with maximally effective concentrations releasing only about 80% of Ins(1,4,5)P3-sensitive Ca2+ stores of SH-SY5Y cells. Ins(1,4,5)P3 and sc-Ins(1,2,4,5)P4 were readily metabolised by Ins(1,4,5)P3 3-kinase and 5-phosphatase activities, DL-2F-Ins(1,4,5)P3 and DL-sc-Ins(1,2,4)P3 were resistant to 5-phosphatase, while sc-Ins(1,2,4,5)PS4 and DL-Ins(1,2,4,5)P4 were resistant to both 3-kinase and 5-phosphatase activity and were potent inhibitors of the 5-phosphatase enzyme (Ki = 300 nM and 2.9 microM, respectively). These results demonstrate that modification of the 2-position of Ins(1,4,5)P3, even with an anionic group, does not critically affect Ins(1,4,5)P3 binding interaction or Ca2+ release, suggesting that the 2-OH of Ins(1,4,5)P3 fails to interact significantly with the binding site of its receptor. However, modification remote from the crucial vicinal 4,5-bisphosphate can affect analogue efficacy in Ca2+ release.

Calcium release activity and metabolism of inositol 1,4,5-trisphosphate in T cells. Modulation by novel inositol 1,4,5-trisphosphate 5-phosphatase inhibitors

Stimulation of the T cell antigen receptor/CD3 complex is followed by phospholipase C activation, phosphoinositol lipid metabolism and ultimately by a rapid rise in both myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and myo-inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] as well as cytosolic free calcium concentration. A 5-phosphatase plays a pivotal role in the subsequent metabolism of Ins(1,4,5)P3 and Ins(1,3,4,5)P4. Synthetic routes have been developed which have enabled the synthesis of both natural and unnatural inositol phosphates and this approach has yielded several compounds which have been shown to act as inhibitors of Ins(1,4,5)P3 5-phosphatase. These compounds offer considerable potential for investigation of the complex metabolism and function of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 in T cell activation and proliferation. We now report the time course and temperature sensitivity of Ins(1,4,5)P3-induced 45Ca2+ release in the permeabilised leukaemic T cell line Jurkat. Furthermore, we demonstrate that the metabolism of Ins(1,4,5)P3 in the presence of two novel 5-phosphatase inhibitors, namely L-myo-inositol 1,4,5-trisphosphorothioate [L-Ins(1,4,5)PS3] and myo-inositol 1,3,5-trisphosphorothioate [Ins(1,3,5)PS3], can be inhibited with concomitant elevation of the heparin-sensitive Ins(1,4,5)P3-induced release of 45Ca2+. These novel 5-phosphatase inhibitors provide a starting point for development of cell-permeable analogues which may be able to modulate cell function in intact cells and may be used as manipulative tools with which to elucidate the function of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 with respect to T cell activation.

Studies on the properties of myo-inositol-1,4,5-trisphosphate 5-phosphatase and myo-inositol monophosphatase in bovine iris sphincter smooth muscle: effects of okadaic acid and protein phosphorylation

In bovine iris sphincter, myo-inositol 1,4,5-trisphosphate (IP3) 5-phosphatase and myo-inositol 1-phosphate (IP1) monophosphatase are mainly localized in the microsomal and soluble fractions, respectively. Studies on the properties of these enzymes can be summarized as follows. (1) The microsomal IP3 5-phosphatase hydrolyzed IP3 to myo-inositol 1,4-bisphosphate with an apparent Km of 28 microM and Vmax of 32 nmol/min per mg protein. The IP1 monophosphatase in the soluble fraction hydrolyzed IP1 into free inositol with an apparent Km of 89 microM and Vmax of 7 nmol/min per mg protein. (2) IP3 5-phosphatase and IP1 monophosphatase had optimal pH values at 8.0 and 7.0, respectively. (3) Both enzymes required Mg2+ and their highest specific activities were at a cation concentration of 2 mM. (4) Ca2+ (> 0.5 microM) exerted an inhibitory effect on IP3 5-phosphatase activity, and marked inhibition (47%) was observed at a concentration of 10 microM. Higher concentrations of the cation (> 100 microM) were required to inhibit IP1 monophosphatase. (5) IP1 monophosphatase, but not IP3 5-phosphatase, was inhibited by Li+. Li+ had no effect on the contractile response in this smooth muscle. (6) Both enzymes were inhibited by ATP and by the thiol-blocking agent, disulfiram. In addition, thimerosal, a thiol reagent, also inhibited the IP3 5-phosphatase activity. (7) Protein phosphorylation of the microsomal and soluble fractions with PKA or PKC had no effect on the activities of these enzymes. (8) Okadaic acid, a protein phosphatase inhibitor, had no effect on the activity of IP3 5-phosphatase. However, in the intact iris sphincter the toxin significantly reduced the carbachol-induced IP3 production, 1,2-diacylglycerol formation, measured as phosphatidic acid, and caused muscle relaxation.

Second messenger specificity of the inositol trisphosphate receptor: reappraisal based on novel inositol phosphates

To further understand how the second messenger D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] interacts with its intracellular receptor, we injected 47 highly purified inositol phosphate (InsP) positional isomers in Xenopus oocytes and compared their potency in releasing intracellular Ca2+. The potency of the Ca(2+)-releasing InsPs spanned four orders of magnitude. Seven compounds, including the novel inositol 1,2,4,5-tetrakisphosphate [D/L-Ins (1,2,4,5)P4] and D/L-Ins(1,4,6)P3, had a very high potency. All of these highly active InsPs shared the following structure: two D-trans-equatorial phosphates (eq-P) and one equatorial hydroxyl (eq-OH) attached to ring carbons D-4, D-5, and D-6 (or to the structurally equivalent D-1, D-6, and D-5 carbons). This permissive structure was not sufficient for Ca2+ release, because it was also found in two inactive compounds, Ins(1,6)P2 and Ins(1,3,6)P3. To be active, InsPs also required the structural equivalent of a D-3 eq-OH and/or a D-1 eq-P. Together, our data reveal how the structure of the InsP molecule affects its ability to release Ca2+.

Myo-inositol 1,3,4,5-tetrakisphosphate can independently mobilise intracellular calcium, via the inositol 1,4,5-trisphosphate receptor: studies with myo-inositol 1,4,5-trisphosphate-3-phosphorothioate and myo-inositol hexakisphosphate

Myo-inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] acts as a full agonist for Ca2+ release in saponin-permeabilised SH-SY5Y neuroblastoma cells. Studies were conducted in the presence of myo-inositol hexakisphosphate (InsP6, 10 microM), to inhibit the Ins(1,3,4,5)P(4)-3-phosphatase catalysed back conversion of Ins(1,3,4,5)P4 to Ins(1,4,5)P3. HPLC analysis confirmed that Ins(1,3,4,5)P4 releases the entire content of Ins(1,4,5)P3-sensitive intracellular Ca2+ stores, independent of 3-phosphatase activity. Further we utilised racemic myo-inositol 1,4,5-trisphosphate-3-phosphorothioate [DL-Ins(1,3,4,5)P(4)-3S], a novel intrinsically Ins(1,3,4,5)P(4)-3-phosphatase resistant Ins(1,3,4,5)P4 analogue. DL-Ins(1,3,4,5)P(4)-3S specifically displaced [3H]Ins(1,4,5)P3 from bovine adrenal cortex Ins(1,4,5)P3 binding sites (IC50 = 889 nM, compared to Ins(1,4,5)P3, IC50 = 4.4 nM and Ins(1,3,4,5)P4, IC50 = 152 nM). DL-Ins(1,3,4,5)P(4)-3S was a full agonist for Ca2+ release (EC50 = 4.7 microM), being 90- and 2-fold less potent than Ins(1,4,5)P3 and Ins(1,3,4,5)P4 (with InsP6), respectively. DL-Ins(1,3,4,5)P(4)-3S will be an important tool for identification of potentially exclusive Ins(1,3,4,5)P4 second messenger functions, since its resistance to 3-phosphatase action precludes the inconvenient artefact of steady state Ins(1,4,5)P3 generation.

Interaction of benzene 1,2,4-trisphosphate with inositol 1,4,5-trisphosphate receptor and metabolizing enzymes

In a wide variety of cells, inositol 1,4,5-trisphosphate (InsP3) is an important second messenger involved in the regulation of intracellular Ca2+ concentration. InsP3 interacts with specific receptors and triggers the release of sequestered Ca2+ from an internal store. We have synthesized a structural analogue of InsP3 by phosphorylation of the free hydroxyl groups of 1,2,4-benzenetriol with dibenzylphosphorochloridate. The product benzene 1,2,4-trisphosphate (BzP3) was shown to interact with InsP3 receptor and InsP3 metabolizing enzymes of bovine adrenal cortex. BzP3 competitively blocked InsP3 binding to adrenal cortex microsomes with a half-maximal efficiency at 34 microM. This affinity was about 10,000 times lower than that of InsP3 for its receptor. The Ca2+ releasing activity of BzP3 on the same microsomal preparation was monitored with the fluorescent indicator fura-2. BzP3 had no agonistic effect on this activity but it was able to inhibit InsP3-induced Ca2+ release in a dose-dependent manner. The activity of InsP3 phosphatase was also studied. BzP3 inhibited the activity of the phosphatase with a half-maximal efficiency of 32 microM. BzP3 was also able to inhibit the activity of the cytosolic InsP3 kinase with a half-maximal efficiency of 6.1 microM. These results show that BzP3 is interacting with the three specific recognition sites for InsP3 in the bovine adrenal cortex. The inhibitory effect of this compound is relatively more potent on the metabolizing enzymes than on the Ca(2+)-mobilizing receptor.

The role of the 2- and 3-hydroxyl groups of 1D-myo-inositol 1,4,5-trisphosphate in the mobilisation of calcium from permeabilised human 1321N1 astrocytoma cells

The functional significance of the 2- and 3-hydroxyl groups of 1 D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] was probed by using Ins(1,4,5)P3 analogues variously modified at positions 2 and 3 or elsewhere. The intrinsic activities of these compounds were compared to that of Ins(1,4,5)P3, using the calcium-mobilising receptor of the 1321N1 human astrocytoma cell line. The ligand-binding affinities were also determined using membrane preparations from rat cerebellum and bovine adrenal cortex. The results show that HO-2 and HO-3 of Ins(1,4,5)P3 have a relatively insignificant role in receptor binding and calcium release. However, the possibility of a regulatory role for the 3-position of Ins(1,4,5)P3 in these processes is proposed.

3-position modification of myo-inositol 1,4,5-trisphosphate: consequences for intracellular Ca2+ mobilisation and enzyme recognition

The ability of the novel D-myo-inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) analogues, L-chiro-inositol 2,3,5-trisphosphate (L-ch-Ins(2,3,5)P3) and D-3-deoxy-3-fluoro-myo-inositol 1,4,5-trisphosphate (3F-Ins(1,4,5)P3), to bind to the Ins(1,4,5)P3 receptor, mobilise intracellular Ca2+ stores and interact with metabolic enzymes has been investigated. L-ch-Ins(2,3,5)P3 and 3F-Ins(1,4,5)P3 were bound by the Ins(1,4,5)P3 receptor from bovine adrenal cortex with relatively high affinity (Ki values 60.4 and 8.0 nM respectively) but with lower affinity than Ins(1,4,5)P3 (KD = 5.9 nM). Both analogues were apparent full agonists at the Ca2+ mobilising receptor in SH-SY5Y cells, but were less potent than Ins(1,4,5)P3 (EC50 L-ch-Ins(2,3,5)P3 = 1.4 microM, 3F-Ins(1,4,5)P3 = 0.37 microM and Ins(1,4,5)P3 = 0.12 microM). L-ch-Ins(2,3,5)P3 and 3F-Ins(1,4,5)P3 were resistant to Ins(1,4,5)P3 3-kinase, and were potent inhibitors of the enzyme (Ki values 7.1 and 8.6 microM respectively). 3F-Ins(1,4,5)P3 was hydrolysed by Ins(1,4,5)P3 5-phosphatase at a similar rate to Ins(1,4,5)P3, but inhibited dephosphorylation of [3H]Ins(1,4,5)P3 with high potency (apparent Ki = 3.9 microM) L-ch-Ins(2,3,5)P3 was also recognised by the enzyme with high affinity (Ki = 7.7 microM) but was resistant to hydrolysis. These results suggest that the environment around C-3 is of major importance for recognition not only by Ins(1,4,5)P3 3-kinase but also by Ins(1,4,5)P3 5-phosphatase.

Muscarinic receptors, phosphoinositide metabolism and intracellular calcium in neuronal cells

1. We have utilised SH-SY5Y human neuroblastoma cells and primary cultures of rat neonatal cerebellar granule cells, both expressing M3 muscarinic receptors, to examine agonist driven polyphosphoinositide hydrolysis and alterations in intracellular calcium. 2. Stimulation of SH-SY5Y cells leads to a biphasic increase in intracellular calcium, the initial peak being due to the release of calcium from an intracellular store and the second maintained phase being due to calcium entry across the plasma membrane. The channel involved does not appear to be voltage sensitive, to involve a pertussis toxin sensitive G protein, or be opened by inositol polyphosphates. 3. Muscarinic receptor stimulation also leads to increased inositol polyphosphate formation in SH-SY5Y cells. Ins(1,4,5)P3 mass formation was biphasic in profile whereas Ins(1,3,4,5)P4 mass formation was slower and monophasic in profile. These data are consistent with substantial activity of 5-phosphatase (dephosphorylating Ins(1,4,5)P3 to Ins(1,4)P2) and 3-kinase (phosphorylating Ins(1,4,5)P3 to Ins(1,3,4,5)P4) in SH-SY5Y cells. 4. In order to better understand the role of Ins(1,4,5)P3 and its metabolites in calcium homeostasis we have examined the ability of a variety of natural and synthetic analogues to release intracellular sequestered calcium. The Ins(1,4,5)P3 calcium mobilizing receptor displays a remarkable degree of stereo- and positional selectivity with the most potent agonist to date being Ins(1,4,5)P3 (EC50 = 0.09 microM). 5. As an alternative to the continuous SH-SY5Y neuroblastoma (tumour derived) cell line we have used the primary cultured cerebellar granule cell. These cells also display a biphasic increase in Ins(1,4,5)P3 mass and a subsequent release of intracellular stored calcium. In our hands carbachol appears to increase calcium influx, a response which is only visible in the absence of magnesium.

GnRH-induced Ca2+ oscillations and rhythmic hyperpolarizations of pituitary gonadotropes

Secretion of gonadotropic hormones from pituitary gonadotropes in response to gonadotropin-releasing hormone (GnRH) is essential for regulation of reproductive potential. Gonadotropes from male rats exhibited an unusual form of cellular excitation that resulted from periodic membrane hyperpolarization. GnRH induced an oscillatory release of intracellular Ca2+ via a guanosine triphosphate (GTP) binding protein-coupled phosphoinositide pathway and hyperpolarized the gonadotrope periodically by opening apamin-sensitive Ca(2+)-activated K+ (SK) channels. Each hyperpolarization was terminated by firing of a few action potentials that may result from removal of inactivation from voltage-gated Na+ and Ca2+ channels.

Synthetic D- and L-enantiomers of 2,2-difluoro-2-deoxy-myo-inositol 1,4,5-trisphosphate interact differently with myo-inositol 1,4,5-trisphosphate binding proteins: identification of a potent small molecule 3-kinase inhibitor

The ability of two enantiomeric fluoro-analogues of D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] to mobilize intracellular Ca2+ stores in SH-SY5Y neuroblastoma cells has been investigated. (-)-D-2,2-difluoro-2-deoxy-myo-Ins(1,4,5)P3 [D-2,2-F2-Ins(1,4,5)P3] was a full agonist [EC50 0.21 microM] and slightly less potent than D-Ins(1,4,5)P3 [EC50 0.13 microM]. (+)-L-2,2-F2Ins(1,4,5)P3 was a very poor agonist, confirming the stereospecificity of the Ins(1,4,5)P3 receptor. D-2,2-F2-Ins(1,4,5)P3 mobilized Ca2+ with broadly similar kinetics to Ins(1,4,5)P3 and was a substrate for Ins(1,4,5)P3 3-kinase inhibiting Ins(1,4,5)P3 phosphorylation (apparent Ki = 10.2 microM) but was recognised less well than Ins(1,4,5)P3. L-2,2-F2-Ins(1,4,5)P3 was a potent competitive inhibitor of 3-kinase (Ki = 11.9 microM). Whereas D-2,2-F2-Ins(1,4,5)P3 was a good substrate for Ins(1,4,5)P3 5-phosphatase, L-2,2-F2Ins(1,4,5)P3 was a relatively potent inhibitor (Ki = 19.0 microM).

Interaction of polyanions with the recognition sites for inositol 1,4,5-trisphosphate in the bovine adrenal cortex

Inositol 1,4,5-trisphosphate (InsP3) serves as a second messenger for Ca2+ mobilization in a wide variety of cells. InsP3 activates a specific receptor/channel located on an internal Ca2+ store. Because heparin has already been shown to block the action of InsP3, we have looked at the influence of other polyanions (dextran sulfate and polyvinyl sulfate) on the action and metabolism of InsP3 in the bovine adrenal cortex. Polyvinyl sulfate blocked InsP3 binding to adrenal cortex microsomes with a half-maximal efficiency of 250 nM. Scatchard analyses revealed that this effect was not competitive. The Ca2+ releasing activity of InsP3 on the same microsomal preparation was monitored with the fluorescent indicator, fura-2. Polyvinyl sulfate blocked this activity with a half-maximal efficiency of 80 nM. The effect of polyvinyl sulfate could not be overcome by supramaximal doses of InsP3, suggesting a non-competitive inhibitory effect. The activity of InsP3 phosphatase from bovine adrenal cortex microsomes was also studied. Polyvinyl sulfate inhibited the activity of the phosphatase with a half-maximal efficiency of 5 microM. Lineweaver-Burk plots revealed that this effect was not competitive. Polyvinyl sulfate was able to inhibit the activity of InsP3 kinase from bovine adrenal cortex cytosol. The half-maximal dose was 15 nM and the Lineweaver-Burk analysis showed that the inhibition was not competitive. The effect of dextran sulfate 5000 (DS-5000) on these activities was also studied. DS-5000 inhibited in a competitive manner the binding of InsP3 to its receptor (IC50 of 34 microM), the release of Ca2+ induced by InsP3 (IC50 of 6.5 microM) and the activity of InsP3 phosphatase (IC50 of 57 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

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

Partial benzoylation of the 3,4-dibenzyl ethers of D- and L-chiro-inositol provided the 1,2,5-tri-O-benzoyl-3,4-di-O-benzyl-chiro-inositols. Inversion of the free axial hydroxyl group gave a mixture of chiral 1,3,4- and 1,2,4-tri-O-benzoyl-5,6-di-O-benzyl-myo-inositols [W. Tegge and C. E. Ballou, Proc. Natl. Acad. Sci. U.S.A., 86 (1989) 94-98]. Catalytic hydrogenolysis cleaved the benzyl ether groups of the 1,3,4-tri-O-benzoyl-5,6-di-O-benzyl-myo-inositols (D- and L-) to yield the 1,3,4-tri-O-benzoyl-myo-inositols, which were phosphorylated by a dibenzyl phosphoramidite method. Removal of all blocking groups gave the pure enantiomeric myo-inositol 2,4,5-trisphosphates. Syntheses of the chiro-inositol 1,3,4-trisphosphates, which are analogs of the myo-inositol 1,4,5-trisphosphates having an axial phosphate group at position 1, or analogs of the myo-inositol 2,4,5-triphosphates having an axial hydroxyl at position 1, were also devised starting with the 1,2,5-tri-O-benzoyl-3,4-di-O-benzyl-chiro-inositols. In a calcium-release assay with saponin-permeabilized rat basophilic leukemia cells, the D isomers of both of these analogs had EC50 values of 4 microM, compared with a value of 0.17 microM for D-myo-inositol 1,4,5-trisphosphate, whereas the L isomers had EC50 values of about 100 microM.

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.