Partial purification of inositol polyphosphate 1-phosphomonoesterase with characterization of its substrates and products by nuclear magnetic resonance spectroscopy

Conformational studies of myo-inositol phosphates

The discovery of the second messenger role of myo-inositol 1,4,5-trisdihydrogenphosphate [Ins(1,4,5)P3] has triggered tremendous interest in investigating the structure, metabolism, and biological roles of inositol phosphates. Although the conformation of phytic acid [(myo-inositol hexakisdihydrogenphosphate), Ins P6] has been the subject of much study, the conformations of lower inositol phosphates such as inositol-pentakis-, tetrakis-, and tris-dihydrogenphosphates have not been investigated. We investigated, by 1H NMR spectroscopy, the conformations of inositol phosphates (Ins P5, Ins P4, Ins P3, Ins P2, and Ins P1) and monitored the influence of pH on conformational preferences. Ins P6 adopts the sterically stable 1ax/5eq (one phosphate in the axial position and five phosphates in the equatorial position) conformation in the pH range 0.5-9.0, and the sterically hindered 5ax/1eq (five phosphates in the axial position and one phosphate in the equatorial position) conformation above pH 9.5. At pH 9.5, both conformations are in dynamic equilibrium. Ins(1,2,3,4,6)P5 and Ins(1,2,3,5,6)P5 adopt the 1ax/5eq form in the pH range 1.0-9.0; in the pH range 9.5-13.0, the 1ax/5eq and 5ax/1eq conformations are in dynamic equilibrium. In contrast to Ins P6 and Ins P5, all the lower inositol phosphates (Ins P4 to Ins P1) investigated adopt the 1ax/5eq conformation over the entire pH range, 1.0-13.0. Preference for the 5ax/1eq conformation by Ins P6 and Ins P5 is probably due to decreased electrostatic repulsion between negatively charged vicinal equatorial phosphates in the 1ax/5eq conformation and stabilization of the sterically hindered 5ax/1eq conformation by hydrogen bonding and/or sodium counter-ions bonding between the syn-oriented phosphates. On the basis of conformations adopted by the inositol phosphates (Ins P6 to Ins P1) at different pH, we conclude that the presence of four or five equatorial phosphates on the inositol ring induces a change in the conformation from the sterically unhindered 1ax/5eq structure to the sterically hindered 5ax/1eq conformation, at high pH. This investigation illustrates that the conformational preferences of inositol phosphates at different pH is unique to the particular isomer and does not parallel the behaviour of phytic acid.

Smooth muscle stretch-activated phospholipase C activity

Rabbit aortic muscles were stretched from a holding length of 0.6 maximum length (Lmax) to lengths as great as 1.0 Lmax and the new length maintained. When muscles were stretched to 1.0 Lmax, inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and inositol 1,4-bisphosphate [Ins(1,4)P2] contents were increased at 375 ms (uncorrected for freezing time) poststretch to 209 +/- 27 and 139.8 +/- 12% (SE), respectively, of control values. Increases in Ins(1,4,5)P3 and Ins(1,4)P2 contents reached an apparent maximum at approximately 500 ms, i.e., to 243.7 +/- 15.8 and 180.9 +/- 16.2% of control, and were decreased to near control levels at 1,700 ms poststretch. The stretch threshold for phospholipase C (PLC) activation was 0.85 Lmax. The latency to onset of PLC activation, correcting for the time for freezing, was 275 to 375 ms. Maximal PLC activity was 91 pmol.s-1.100 nmol total lipid P(i)-1, which corresponded to 10% of total phosphatidylinositol bisphosphate being hydrolyzed per second. The mechanism of stretch-activated PLC activity involved influx of Ca2+ via gadolinium-sensitive ion channels, but not via nifedipine-sensitive ion channels.

Biochemical and molecular properties of lithium-sensitive myo-inositol monophosphatase

Myo-inositol monophosphatase is a pivotal enzyme of the inositol second messenger system which is specifically inhibited by therapeutic levels of lithium salts, implicating inhibition of this enzyme as a potential site of its action in bipolar disease. This enzyme has a native molecular weight of 59,000, and has traditionally been found in the cytosolic fraction, although a membrane-bound form has also been identified. Possessing two identical subunits, this enzyme hydrolyzes those monophosphates which are equatorially located within the inositol ring, and several nucleoside monophosphates phosphorylated at the 2-position. Each subunit of the native enzyme contains an active site with unusually large caverns as revealed by crystallographic studies, which may explain the accommodation of these structurally unrelated substrates. We have suggested that the uncompetitive inhibition of this phosphatase by lithium ions may prevent the formation of an enzyme-bound non-isomeric (meso) intermediate, Mg(2+)-inositol 1,3 or 4,6 cyclic monophosphate when this enzyme hydrolyzes its respective isomeric substrates.

Evidence for phosphatidylinositol hydrolysis in pancreatic islets stimulated with carbamoylcholine. Kinetic analysis of inositol polyphosphate metabolism

Anion-exchange h.p.l.c. was used initially to analyse the products formed after addition of either [3H]Ins(1,3,4,5)P4 or [3H]Ins(1,4,5)P3 to homogenates of pancreatic islets. Metabolic routes similar to those of other tissues were established: dephosphorylation of Ins(1,4,5)P3 to Ins(1,4)P2 and then Ins4P; and sequential degradation of Ins(1,3,4,5)P4 to Ins(1,3,4)P3, Ins(3,4)P2 and Ins(3 or 1)P. In addition, there was a limited conversion of Ins(1,3,4)P3 into Ins(1,3)P2. After stimulation of [3H]inositol-prelabelled islets with the muscarinic-receptor agonist carbamoylcholine (carbachol), there was a rapid (10 s) increase in Ins(1,4,5)P3, Ins(1,3,4)P3, Ins(1,4)P2 and Ins4P. In the presence of 10 mM-LiCl, Ins1P was also significantly increased (P less than 0.05) by 5 s, before any increase in Ins4P (10 s), Ins(1,3)P2 (60 s) or Ins(3,4)P2. When carbachol was displaced with atropine, after 1 h pre-stimulation, the maximal decreases in Ins(1,4,5)P3 and Ins1P from the stimulated steady state (5 s) clearly preceded those of the other metabolites. These declines were used to calculate the turnover times and rate of metabolic flux through the various inositol phosphates. These experiments confirmed the relatively minor importance of the Ins(1,3)P2 pathway (less than 10% of the total flux) and demonstrated that Ins(1,4,5)P3 removal was evenly distributed through the Ins(1,4)P2 and Ins(1,3,4,5)P4 routes. They also established that flux through Ins1P was 8-fold greater than that through Ins(1,4,5)P3, indicating that the former could not have been derived from PtdInsP2 hydrolysis. Similarly, in islets pretreated with neomycin, which binds to PtdInsP2 with greater affinity than to PtdIns, the increase in Ins1P caused by 1 min stimulation with carbachol was not affected, despite virtual abolition of the increase in Ins4P, and an overall inhibition of PtdInsP2 hydrolysis by 67%. The results indicate that, in addition to PtdInsP2 breakdown, carbachol also promotes a rapid PtdIns hydrolysis which becomes increasingly predominant with prolonged stimulation.

The inositol phosphate pathway as a mediator in the proliferative response of rat calvarial bone cells to cyclical biaxial mechanical strain

Isolated newborn rat calvarial bone cells grown in monolayer on polyurethane membranes in specially constructed culture chambers and subjected to a cyclical biaxial mechanical strain of 0.17% at a frequency of 1 Hz for 30 min demonstrated a 16% increase in DNA synthesis during the subsequent 24 h. The metabolites of the inositol phosphate pathway, shown to be an important second messenger in many cell types, were shown to be elevated using high-performance liquid chromatography to separate and quantitate the various inositol polyphosphates. Inositol 1,4,5-trisphosphate, inositol 1,4-bisphosphate, and inositol 1,3,4,5-tetrakisphosphate reached peak accumulations after 20 s of mechanical strain. Inositol 1,3,4-trisphosphate reached a peak accumulation after 2 min, and inositol 1,2,3,4,5,6 phosphate reached a peak accumulation after 60 min of mechanical strain. Neomycin, an inhibitor of phospholipase C, a membrane-bound enzyme that hydrolyzes phosphatidyl inositol 4,5-bisphosphate to start the inositol phosphate cascade, completely inhibited accumulation of the above inositol phosphates during mechanical straining of the bone cells. Neomycin also completely abolished the increase in DNA synthesis that was seen after a mechanical strain of 0.17%. It is concluded from this study that the inositol phosphate pathway is activated by mechanical strain in bone cells and that this pathway is an important and primary mediator in the transduction of mechanical strain into cellular proliferation in these cells.

Calcium-sensitivity of inositol 1,4,5-trisphosphate metabolism in exocrine cells from the avian salt gland

The generation of inositol phosphates upon muscarinic-receptor activation was studied in [3H]inositol-loaded exocrine cells from the nasal salt glands of the duck Anas platyrhynchos, and the metabolism of different inositol phosphates in vitro was studied in tissue homogenates, with particular reference to the possible interaction of changes in intracellular [Ca2+] ([Ca2+]i) with the metabolic processes. In intact cells, there was a rapid (within 15 s) generation of Ins(1,4,5)P3 and Ins(1,3,4,5)P4, followed by an accumulation of their breakdown products, Ins(1,3,4)P3 and inositol bis- and monophosphates. Ca(2+)-sensitivity of the Ins(1,4,5)P3 3-kinase was demonstrated in tissue homogenates, with the rate of phosphorylation increasing 2-fold at free Ca2+ concentrations greater than 1 microM. However, addition of calmodulin or the presence of the calmodulin inhibitor W-7 (up to 100 microM) had no effect. 3-Kinase activity increased proportionally with the initial Ins(1,4,5)P3 concentration up to 1 microM, but a 10-fold higher substrate concentration produced only a doubling in the phosphorylation rate. Ins(1,3,4,5)P4 was dephosphorylated to Ins(1,3,4)P3, which accumulated in the homogenate assays as well as in intact cells. Depending on its concentration, Ins(1,3,4)P3 was phosphorylated [in part to Ins(1,3,4,6)P4] or dephosphorylated. To investigate the Ca(2+)-sensitivity of the 3-kinase in intact cells, excess quin2 was used to buffer the receptor-mediated transient changes in [Ca2+]i in [3H]inositol-loaded cells. These experiments revealed that increasing [Ca2+]i from less than 100 to approx. 400 nM (i.e. within the physiological range) has no effect on the partitioning of Ins(1,4,5)P3 metabolism (phosphorylation versus dephosphorylation) and on the accumulation of Ins(1,4,5)P3 and Ins(1,3,4,5)P4. This indicates that activation of the 3-kinase by physiologically relevant Ca2+ concentrations may not play a major role in the generation of Ins(1,3,4,5)P4 signals upon receptor activation in these cells. The latter are mainly achieved by the receptor-mediated increase in Ins(1,4,5)P3 in the cell and its phosphorylation by the 3-kinase in a substrate-concentration-dependent manner.

Inositol 1,4,5-trisphosphate compartmentalization in tracheal smooth muscle

Pool sizes of inositol phosphate species in myo-[3H]inositol-labeled porcine tracheal smooth muscle were determined under three conditions: (a) unstimulated; (b) stimulated with carbachol; (c) atropine-relaxed from a carbachol contraction. In unstimulated muscle, the inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) content was 14 pmol/100 nmol lipid P1. This is equivalent to a mean [Ins(1,4,5)P3] of about 3 microM (in total cellular water), a level about 30-fold in excess of that required for Ca2+ release from Ins(1,4,5)P3-sensitive sarcoplasmic reticulum (SR). Pool sizes of breakdown products of Ins(1,4,5)P3 were relatively small or absent in unstimulated muscle, suggesting that, under this condition, Ins(1,4,5)P3 was sequestered and had limited access to Ins(1,4,5)P3 5-phosphatase and/or 3-kinase. During carbachol stimulation, the Ins(1,4,5)P3 pool did not increase while those of other mono-, di-, and trisphosphate isomers increased over 10-fold. Subsequent atropine-induced relaxation resulted in a partial depletion (40%) of total tissue Ins(1,4,5)P3. Decreases in Ins(1,4,5)P3 were paralleled by decreases in Ins(1,4)P2 and Ins(1,3,4)P3. During contraction a portion of total tissue Ins(1,4,5)P3 has access to Ins(1,4,5)P3 3-kinase and 5-phosphatase and to Ins(1,4,5)P3-sensitive SR, though during antagonist-induced relaxation access to Ins(1,4,5)P3-sensitive SR for Ca2+ release is restricted. Data are consistent with a mechanism by which a large pool of Ins(1,4,5)P3 present in the unstimulated state in a sequestered compartment can contribute in activated muscle to increases in [Ins(1,4,5)P3] in a nonsequestered compartment, controlling SR Ca2+ release.

Regulation of the metabolism of 1,2-diacylglycerols and inositol phosphates that respond to receptor activation

This review assimilates information on the regulation of the metabolism of those inositol phosphates and diacylglycerols that respond to receptor activation. Particular emphasis is placed on the regulation of specific enzymes, the occurrence of isoenzymes, and metabolic compartmentalization; the overall aim is to demonstrate the significance of these activities in relation to the physiological impact of the various cell signalling processes.

Hydrolysis of inositol phosphates by plant cell extracts

A gel-filtered soluble fraction prepared from suspension-cultured Nicotiana tabacum cells hydrolysed inositol mono-, bis- and tris-phosphates. At a concentration of 7.5 microM the rates of hydrolysis followed the sequence Ins(1,4,5)P3 greater than Ins(1,4)P2 greater than Ins(4)P congruent to Ins(1)P. The major products of Ins(1,4,5)P3 hydrolysis identified by h.p.l.c. were Ins(1,4)P2 and Ins(4,5)P2. Ins(1,4)P2 was hydrolysed exclusively to Ins(4)P. The inclusion of Ca2+ in the incubation buffer markedly stimulated the hydrolysis of all the inositol phosphate substrates. Under identical conditions, Ca2+ inhibited the hydrolysis of inositol phosphates by soluble extracts prepared from rat brain. Half-maximal stimulation of Ins(1,4)P2 hydrolysis was obtained at free [Ca2+] of 0.6 and 1.2 microM when the Mg2+ concentration in the incubations was 0.3 and 1.0 mM respectively. This effect of Ca2+ was exerted solely by increasing the Vmax. of hydrolysis without affecting the Km for Ins(1,4)P2. Again, in contrast with brain, the hydrolysis of inositol bis- or mono-phosphates was insensitive to high concentrations of Li+. We conclude that plants contain specific Li+-insensitive inositol phosphate phosphatases that are regulated by low concentrations of Ca2+ in a manner which is different from that observed in mammalian tissues.