Solubilization of rat liver inositol 1,4,5-trisphosphate receptor

High affinity Ins(1,4,5)P₃-binding sites of permeabilized hepatocytes are probably the ligand recognition sites of the receptors that mediate the effects of Ins91,4,5)P₃ on intracellular Ca²⁺ mobilization. We have now solubilized these sites from rat liver membranes in the zwitterionic detergent, CHAPS, and shown that the solubilized bind Ins(1,4,5)P₃ with an affinity (K_d = 7.26 ± 0.52 nM, Hill coefficient H = 1.05 ± 0.06) similar to that of the sites in native membranes (K_d = 6.02 ± 0.02). ATP and a range of inositol phosphates (Ins(2,4,5)P₃ Ins(4,5)P₂, and inositol 1,4,5-trisphosphorothioate) also bound with similar affinities to the native and solubilized sites. Solubilization of the liver InsP₃ receptor will allow its further characterization, purification, and comparison of its properties with those of InsP₃ receptors already purified from cerebellum and smooth muscle.     

Degradation of inositol 1,3,4,5-tetrakisphosphates by porcine brain cytosol yields inositol 1,3,4-trisphosphate and inositol 1,4,5-trisphosphate

Inositol 1,3,4,5-tetrakisphosphates (Ins(1,3,4,5)P4), 32P-labelled in positions 4 and 5 were prepared enzymatically, using [4-32P]-phosphatidylinositol 4-phosphate (PtdInsP) and [5-32P]phosphatidylinositol 4,5-bisphosphate (PtdInsP2) as substrates, respectively. Degradation studies of Ins(1,3,4,5)P4, using an enriched phosphatase preparation from porcine brain cytosol, led to the formation of two inositol trisphosphate isomers which were identified as inositol 1,3,4-trisphosphate (Ins(1,3,4)P3) and inositol 1,4,5-trisphosphate (Ins(1,4,5)P3). This novel degradation pathway of Ins(1,3,4,5)P4 to Ins(1,4,5)P3 provides an additional source for the generation of Ins(1,4,5)P3, involving a 3-phosphatase.     

The metabolism of inositol 1,3,4-trisphosphate to inositol 1,3-bisphosphate

We previously demonstrated a pathway for the metabolism of inositol 1,3,4-trisphosphate (Ins(1,3,4)P3) to inositol 3,4-bisphosphate (Ins(3,4)P2) in calf brain extracts. Inositol polyphosphate 1-phosphatase, a Mg2+-dependent, lithium ion-inhibited enzyme, specifically hydrolyzes Ins(1,3,4)P3 to Ins(3,4)P2 and Ins(1,4)P2 to Ins 4-P (Inhorn, R. C., Bansal, V. S., and Majerus, P. W. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 2170-2174). Now we have found an alternative pathway for the metabolism of Ins(1,3,4)P3 in crude calf brain extracts. Along this pathway, Ins(1,3,4)P3 is first converted to Ins(1,3)P2 which is further hydrolyzed to Ins 1-P. This pathway involves a 4-phosphatase and a 3-phosphatase which do not require Mg2+ and are not inhibited by lithium ions. A similar 4-phosphatase also degrades Ins(3,4)P2 to Ins 3-P. Three different inositol bisphosphates formed from calf brain supernatant are each further metabolized by a separate enzyme. The three inositol monophosphates, i.e. Ins 1-P, Ins 3-P, and Ins 4-P, are converted to inositol by inositol monophosphate phosphatase (Ackermann, K. E., Gish, B. G., Honchar, M. P., and Sherman, W. R. (1987) Biochem. J. 242, 517-524).     

Hormonal regulation of inositol 1,4,5-trisphosphate receptor in rat liver

Inositol 1,4,5-trisphosphate (IP3) is a second messenger which induces Ca2+ release from an intracellular store. We have investigated the properties of the [32P]IP3 binding sites in rat liver. Two specific [32P]IP3 receptors with KD of 2.3 and 88 nM and respective capacities of 33 fmol/mg protein and 195 fmol/mg protein have been detected in a crude membrane fraction prepared from rat liver homogenate. The pretreatment of the liver with IP3-dependent hormones increased two-fold the capacity of the high affinity site. This effect was partly reversed by dibutyryl cyclic AMP. Permeabilized hepatocytes also displayed two [32P]IP3 binding sites with KD of 1.5 and 84 nM and respective capacities of 8 and 300 fmol/10(6) cells. We have measured the [32P]IP3 binding and the IP3-induced 45Ca2+ release in the same batch of permeabilized hepatocytes. In a low Mg2+ medium, the EC50 for 45Ca2+ release was in close correlation with the KD for the low affinity site. These data suggest that an equilibrium between two states of the IP3 receptor is regulated by hormone action and the low affinity state is responsible for the intracellular Ca2+ release.     

The pathway of myo-inositol 1,3,4-trisphosphate phosphorylation in liver. Identification of myo-inositol 1,3,4-trisphosphate 6-kinase, myo-inositol 1,3,4-trisphosphate 5-kinase, and myo-inositol 1,3,4,6-tetrakisphosphate 5-kinase

Inositol 1,3,4-trisphosphate (Ins(1,3,4)P3) metabolism has been studied in liver homogenates and in 100,000 x g supernatant and particulate fractions. When liver homogenates were incubated in an "intracellular" medium containing 5 mM MgATP, equal proportions of Ins(1,3,4)P3 were dephosphorylated and phosphorylated. Two inositol tetrakisphosphate (InsP4) products and an inositol pentakisphosphate (InsP5) were detected. The InsP4 isomers were unequivocally identified as inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4) and inositol 1,3,4,6-tetrakisphosphate (Ins(1,3,4,6)P4) by high performance liquid chromatography separation of inositol phosphates, periodate oxidation, alkaline hydrolysis, and stereo-specific polyol dehydrogenase. Ins(1,3,4)P3 5-kinase is a novel enzyme activity and accounted for 16% of the total Ins(1,3,4)P3 phosphorylation. Ins(1,3,4,6)P4 was also shown to be further phosphorylated to inositol 1,3,4,5,6-pentakisphosphate (Ins(1,3,4,5,6)P5) by a kinase not previously known to occur in liver. About 75% of Ins(1,3,4)P3 kinase activities were soluble and were partly purified by anion-exchange fast protein liquid chromatography. The two Ins(1,3,4)P3 kinase activities eluted as a single peak that was well resolved from Ins(1,3,4)P3 phosphatase, Ins(1,3,4,6)P4 5-kinase, and Ins(1,3,4,5)P4 5-phosphatase activities. A further novel observation was that 10 microM Ins(1,3,4,5)P4 inhibited Ins(1,3,4)P3 kinase activities by 60%.     

Akt kinase phosphorylation of inositol 1,4,5-trisphosphate receptors

A consensus RXRXX(S/T) substrate motif for Akt kinase is conserved in the C-terminal tail of all three inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) isoforms. We have shown that IP3R can be phosphorylated by Akt kinase in vitro and in vivo. Endogenous IP3Rs in Chinese hamster ovary T-cells were phosphorylated in response to Akt activation by insulin. LnCAP cells, a prostate cancer cell line with constitutively active Akt kinase, also showed a constitutive phosphorylation of endogenous type I IP3Rs. In all cases, the IP3R phosphorylation was diminished by the addition of LY294002, an inhibitor of phosphatidylinositol 3-kinase. Mutation of IP3R serine 2681 in the Akt substrate motif to alanine (S2681A) or glutamate (S2681E) prevented IP3R phosphorylation in COS cells transfected with constitutively active Akt kinase. Analysis of the Ca2+ flux properties of these IP3R mutants expressed in COS cell microsomes or in DT40 triple knock-out (TKO) cells did not reveal any modification of channel function. However, staurosporine-induced caspase-3 activation in DT40 TKO cells stably expressing the S2681A mutant was markedly enhanced when compared with wild-type or S2681E IP3Rs. We conclude that IP3 receptors are in vivo substrates for Akt kinase and that phosphorylation of the IP3R may provide one mechanism to restrain the apoptotic effects of calcium.     

The regulation of the phosphorylation of inositol 1,3,4-trisphosphate in cell-free preparations and its relevance to the formation of inositol 1,3,4,6-tetrakisphosphate in agonist-stimulated rat parotid acinar cells

High performance liquid chromatography analysis of supernatants from acid-quenched [3H]inositol-labeled parotid acinar cells revealed an inositol pentakisphosphate and three inositol tetrakisphosphates. Two of the latter were identified as the 1,3,4,5 and 1,3,4,6 isomers, whereas the third was probably a mixture of unknown proportions of the 3,4,5,6/1,4,5,6 enantiomeric pair. Methacholine (100 microM) produced a 40-50-fold increase in the levels of inositol trisphosphate (mainly the 1,3,4 isomer) and inositol 1,3,4,5-tetrakisphosphate, but inositol 1,3,4,6-tetrakisphosphate only increased 5-fold. Levels of inositol 3,4,5,6/1,4,5,6-tetrakisphosphate and inositol pentakisphosphate were unaffected by agonist stimulation. Thus, in parotid cells, an agonist-induced increase in both inositol trisphosphate and inositol 1,3,4,6-tetrakisphosphate formation does not result in an increase in the rate of formation of inositol pentakisphosphate. Following the addition of 100 microM atropine to methacholine-stimulated parotid cells, the levels of [3H]inositol 1,3,4,5-tetrakisphosphate fell rapidly, returning to basal levels within 5 min. Inositol trisphosphate was metabolized more slowly and was still elevated 20-fold above basal 5 min after the addition of atropine. Inositol 1,3,4,6-tetrakisphosphate was metabolized much more slowly (t1/2 approximately 15 min). Inositol 1,3,4-trisphosphate metabolism was examined in parotid homogenates as well as in 100,000 x g cytosolic and particulate fractions. Inositol 1,3,4-trisphosphate was both dephosphorylated and phosphorylated. Two inositol tetrakisphosphate products were formed, namely the 1,3,4,6 and 1,3,4,5 isomers. Over 90% of both kinase and phosphatase activities were found in the cytosolic fractions. The ratio of activities of kinase to phosphatase decreased as the levels of inositol 1,3,4-trisphosphate substrate were increased from 1 nM to 10 microM. These data led to the conclusion that the kinetic parameters of the inositol 1,3,4-trisphosphate kinases and phosphatases are such that in stimulated cells, dephosphorylation of inositol 1,3,4-trisphosphate is greatly favored. Inositol 1,3,4-trisphosphate kinase activity was potently inhibited by inositol 3,4,5,6-tetrakisphosphate (IC50 = 0.1-0.2 microM), which leads us to propose that inositol 3,4,5,6-tetrakisphosphate is an endogenous inhibitor of the kinase.     

Inositol 1,4,5-trisphosphate and inositol 1,2-cyclic 4,5-trisphosphate are minor components of total mass of inositol trisphosphate in thrombin-stimulated platelets. Rapid formation of inositol 1,3,4-trisphosphate

Stimulation of human platelets by thrombin leads to rises of both inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) and inositol 1,3,4-trisphosphate (Ins(1,3,4)P3) within 10 s. The mass of Ins(1,4,5)P3 was measured in platelet extracts after conversion to [3-32P]Ins(1,3,4,5)P4 with Ins(1,4,5)P3 3-kinase and [gamma-32P]ATP. Basal levels were equivalent to 0.2 microM and rose to 1 microM within 10 s of stimulation by thrombin. The mass of Ins(1,3,4)P3 was more than 10-fold greater than that of Ins(1,4,5)P3 between 10 and 60 s of thrombin stimulation. These results indicate that the majority of InsP3 liberated by phospholipase C in stimulated platelets must be the non-cyclic Ins(1,4,5)P3 in order to allow rapid phosphorylation by Ins(1,4,5)P3 3-kinase to Ins(1,3,4,5)P4 and then dephosphorylation to Ins(1,3,4)P3 by 5-phosphomonoesterase. A significant proportion of the InsP3 extracted from thrombin-stimulated platelets under neutral conditions is resistant to Ins(1,4,5)P3 3-kinase but susceptible after acid treatment, implying the presence of inositol 1,2-cyclic 4,5-trisphosphate (Ins(1,2cyc4,5)P3. The relative proportion of Ins(1,2cyc4,5)P3 increases with time. We suggest that such gradual accumulation is attributable to the relative insensitivity of this compound to hydrolytic and phosphorylating enzymes. Therefore, early Ca2+ mobilization in platelets is more likely to be effected by Ins(1,4,5)P3 than by Ins(1,2cyc4,5)P3.     

Inositol 1,4,5-trisphosphate and inositol 1,3,4-trisphosphate formation in Ca2+-mobilizing-hormone-activated cells.

The inositol trisphosphate liberated on stimulation of guinea-pig hepatocytes, pancreatic acinar cells and dimethyl sulphoxide-differentiated human myelomonocytic HL-60 leukaemia cells is composed of two isomers, the 1,4,5-trisphosphate and the 1,3,4-trisphosphate. Inositol 1,4,5-trisphosphate was released rapidly, with no measurable latency on hormone stimulation, and, consistent with its proposed role as an intracellular messenger for Ca2+ mobilization, there was good temporal correlation between its formation and Ca2+-mediated events in these tissues. There was a definite latency before an increase in the formation of inositol 1,3,4-trisphosphate could be detected. In all of these tissues, however, it formed a substantial proportion of the total inositol trisphosphate by 1 min of stimulation. In guinea-pig hepatocytes, where inositol trisphosphate increases for at least 30 min after hormone application, inositol 1,3,4-trisphosphate made up about 90% of the total inositol trisphosphate by 5-10 min. In pancreatic acinar cells, pretreatment with 20 mM-Li+ caused an increase in hormone-induced inositol trisphosphate accumulation. This increase was accounted for by a rise in inositol 1,3,4-trisphosphate; inositol 1,4,5-trisphosphate was unaffected. This finding is consistent with the observation that Li+ has no effect on Ca2+-mediated responses in these cells. The role, if any, of inositol 1,3,4-trisphosphate in cellular function is unknown.     

Rapid formation of inositol 1,3,4,5-tetrakisphosphate and inositol 1,3,4-trisphosphate in rat parotid glands may both result indirectly from receptor-stimulated release of inositol 1,4,5-trisphosphate from phosphatidylinositol 4,5-bisphosphate.

Addition of 1 mM-carbachol to [3H]inositol-labelled rat parotid slices stimulated rapid formation of [3H]inositol 1,3,4,5-tetrakisphosphate, the accumulation of which reached a peak 20 s after stimulation, and then declined rapidly towards a new steady state. The initial rate of formation of inositol 1,3,4,5-tetrakisphosphate was slower than that for inositol 1,4,5-trisphosphate. The radioactivity in [3H]inositol 1,3,4,5-tetrakisphosphate fell quickly in carbachol-stimulated and then atropine-blocked parotid slices, suggesting that it is rapidly metabolized during stimulation. Parotid homogenates rapidly dephosphorylated inositol 1,4,5-trisphosphate, inositol 1,3,4,5-tetrakisphosphate and, less rapidly, inositol 1,3,4-trisphosphate. Inositol 1,3,4,5-tetrakisphosphate was specifically hydrolysed to a compound with the chromatographic properties of inositol 1,3,4-trisphosphate. The only 3H-labelled phospholipids that we could detect in parotid slices labelled with [3H]inositol for 90 min were phosphatidylinositol, phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate. Parotid homogenates synthesized inositol tetrakisphosphate from inositol 1,4,5-trisphosphate. This activity was dependent on the presence of ATP. We suggest that, during carbachol stimulation of parotid slices, the key event in inositol lipid metabolism is the activation of phosphatidylinositol 4,5-bisphosphate-specific phospholipase C. The inositol 1,4,5-trisphosphate thus liberated is metabolized in two distinct ways; by direct hydrolysis of the 5-phosphate to form inositol 1,4-bisphosphate and by phosphorylation to form inositol 1,3,4,5-tetrakisphosphate and hence, by hydrolysis of this tetrakisphosphate, to form inositol 1,3,4-trisphosphate.     

Formation of inositol 1,4,5-trisphosphate and inositol 1,3,4-trisphosphate from inositol 1,3,4,5-tetrakisphosphate and their pathways of degradation in RBL-2H3 cells

Previous studies with antigen-stimulated rat basophilic leukemia (RBL-2H3) cells indicated the formation of multiple isomers of each of the various categories of inositol phosphates. The identities of the different isomers have been elucidated by selective labeling of [3H]inositol 1,3,4,5-tetrakisphosphate with [32P]phosphate in the 3'-or 4',5'-positions and by following the metabolism of different radiolabeled inositol phosphates in extracts of RBL-2H3 cells. We report here that inositol 1,3,4,5-tetrakisphosphate, when incubated with the membrane fraction of extracts of RBL-2H3 cells, was converted to inositol 1,4,5-trisphosphate and inositol 1,3,4-trisphosphate. Further dephosphorylation of the inositol polyphosphates proceeded rapidly in whole extracts of cells, although the process was significantly retarded when ATP (2 mM) levels were maintained by an ATP-regenerating system. The degradation of inositol 1,4,5-trisphosphate proceeded with the sequential formation of inositol 1,4-bisphosphate, the inositol 4-monophosphate (with smaller amounts of the 1-monophosphate), and finally inositol. Inositol 1,3,4-trisphosphate, on the other hand, was converted to inositol 1,3-bisphosphate and inositol 3,4-bisphosphate and subsequently to inositol 4-monophosphate and inositol 1-monophosphate (stereoisomeric forms were undetermined). The possible implications of the apparent interconversion between inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate in regulating histamine secretion in the RBL-2H3 cells are discussed.     

Calcium modulates the generation of inositol 1,3,4-trisphosphate in human platelets by the activation of inositol 1,4,5-trisphosphate 3-kinase.

Histamine (0.5 mM) stimulated the cyclic AMP content of cell suspensions containing greater than 80% parietal cells. Epidermal growth factor (EGF) inhibited this stimulatory effect of histamine, but had no effect on basal cyclic AMP content. The half-maximally effective concentration of EGF for inhibition of histamine-stimulated cyclic AMP was 3.9 nM. The equivalent measurement for the inhibition of histamine-stimulated aminopyrine accumulation was 3.0 nM. Aminopyrine accumulation was measured because it provides an index of the secretory activity of the cell. The cyclic AMP phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) prevented the inhibitory effect of EGF on cyclic AMP content. This effect of IBMX was not caused by its ability to raise cellular cyclic AMP content in the presence of histamine. Prevention by IBMX of the inhibitory action of EGF on histamine-stimulated aminopyrine accumulation had been shown previously [Shaw, Hatt, Anderson & Hanson (1987) Biochem. J. 244, 699-704]. EGF stimulated prostaglandin E2 (PGE2) production in the cell fraction containing greater than 80% parietal cells, with the half-maximally effective concentration being 7.5 nM. EGF was ineffective in stimulating PGE2 production if the cell fraction was depleted of parietal cells (12%), or if 0.5 mM-histamine was added to the enriched parietal-cell fraction. In conclusion, EGF may inhibit histamine-stimulated acid secretion by decreasing the cyclic AMP content of parietal cells. This effect could be mediated by an increase in cyclic AMP phosphodiesterase activity, but it is unlikely to involve an effect of EGF on parietal-cell prostaglandin production.     

Purification and characterization of inositol-1,3,4-trisphosphate 5/6-kinase from rat liver using an inositol hexakisphosphate affinity column.

The metabolism of inositol 1,3,4-trisphosphate is a pivotal branch point of inositol phosphate turnover; its dephosphorylation replenishes cellular inositol pools, its phosphorylation at the 6-position supports the synthesis of inositol pentakisphosphate, and its phosphorylation at the 5-position produces inositol 1,3,4,5-tetrakisphosphate (Shears, S.B. (1989) J. Biol. Chem. 264, 19879-19886). In order to increase understanding of the control of inositol-1,3,4-trisphosphate kinase activity, the enzyme was highly purified from rat liver by precipitation with polyethylene glycol, MonoQ ion-exchange chromatography, heparin-agarose affinity chromatography, and a novel affinity chromatography procedure that utilized Affi-Gel resin to which InsP6 was coupled (Marecek, J.F., and Prestwich, G.D. (1991) Tetrahedron Lett. 32, 1863-1866). The final purification was about 26,000-fold, with a 6% yield. This final preparation performed both 5- and 6-kinase activities in the ratio of approximately 1:5. The affinity of the enzyme for inositol 1,3,4-trisphosphate was 0.04 microM, the highest yet determined for an inositol phosphate kinase. Both inositol 1,3,4,5-tetrakisphosphate and inositol 1,3,4,6-tetrakisphosphate were competitive inhibitors of the kinase (Ki values of 2-4 microM). The enzyme was determined to have a molecular mass of 36 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Kinase activity was unaffected by Ca2+/calmodulin, protein kinase A, or protein kinase C.     

Specificity determinants in inositol polyphosphate synthesis: crystal structure of inositol 1,3,4-trisphosphate 5/6-kinase

Inositol hexakisphosphate and other inositol high polyphosphates have diverse and critical roles in eukaryotic regulatory pathways. Inositol 1,3,4-trisphosphate 5/6-kinase catalyzes the rate-limiting step in inositol high polyphosphate synthesis in animals. This multifunctional enzyme also has inositol 3,4,5,6-tetrakisphosphate 1-kinase and other activities. The structure of an archetypal family member, from Entamoeba histolytica, has been determined to 1.2 A resolution in binary and ternary complexes with nucleotide, substrate, and product. The structure reveals an ATP-grasp fold. The inositol ring faces ATP edge-on such that the 5- and 6-hydroxyl groups are nearly equidistant from the ATP gamma-phosphate in catalytically productive phosphoacceptor positions and explains the unusual dual site specificity of this kinase. Inositol tris- and tetrakisphosphates interact via three phosphate binding subsites and one solvent-exposed site that could in principle be occupied by 18 different substrates, explaining the mechanisms for the multiple specificities and catalytic activities of this enzyme.     

Different localization of inositol 1,4,5-trisphosphate and ryanodine binding sites in rat liver.

The distribution of inositol 1,4,5-trisphosphate and ryanodine binding sites between plasma membrane, microsomal, and mitochondrial fractions of rat liver were compared. IP3 bound mostly to the plasma membrane fraction (Kd = 6 nM; Bmax = 802 fmol/mg protein). Some IP3 binding sites were also present in the microsomal and mitochondrial fractions (Kd = 2.5 and 2.9 nM; Bmax = 35 and 23 fmol/mg protein respectively). The possibility that these binding sites are due to contamination of the fractions with plasma membrane cannot be excluded. Binding of IP3 to the plasma membrane was inhibited by heparin but not by either caffeine or tetracaine. High-affinity ryanodine binding sites were present mostly in the microsomal fraction (Kd = 13 nM; Bmax = 301 fmol/mg protein). Lower affinity binding sites were also found to be present in the mitochondrial and plasma membrane fractions. Binding of ryanodine to the microsomal fraction was inhibited by both caffeine and tetracaine but not by heparin. These data demonstrate that IP3 and ryanodine binding sites are present in different cellular compartments in the liver. These differences in the localization of the binding sites might be indicative of their functional differences.     

Detection of inositol 1,4,5-trisphosphate kinase in retina. A direct demonstration of phosphorylation of inositol 1,4,5-trisphosphate by ATP.

The metabolism of myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] consists of two pathways: dephosphorylation by 5-phosphomonoesterase(s) produces inositol 1,4-bisphosphate, and phosphorylation by Ins(1,4,5)P3 3-kinase yields inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4]. The requirements for Ins(1,4,5)P3 kinase activity in retina were characterized. Apparent Km values for ATP and Ins(1,4,5)P3 are 1.4 mM and 1.3 microM respectively. A direct demonstration of phosphorylation of Ins(1,4,5)P3 by [gamma-32P]ATP was achieved. Characterization of the 32P-labelled product revealed that it had the expected chromatographic and electrophoretic properties of Ins(1,3,4,5)P4.     

Characterization of D-myo-inositol 1,4,5-trisphosphate phosphatase in rat liver plasma membranes

D-myo-Inositol 1,4,5-trisphosphate has been previously demonstrated to act as a second messenger for the hormonal mobilization of intracellular calcium in rat liver. In this study, the breakdown of D-myo-inositol 1,4,5-trisphosphate by a phosphatase activity was characterized. Using partially purified subcellular fractions, it was found that D-myo-inositol 1,4,5-trisphosphate phosphatase (I-P3ase) specific activity was highest in the plasma membrane fraction, while D-myo-inositol 1,4-bisphosphate phosphatase specific activity was highest in the cytosolic and microsomal fractions. The plasma membrane I-P3ase was Mg2+-dependent with optimal activity observed at 0.5-1.5 mM free Mg2+. The enzyme had a neutral pH optimum, suggesting that it was neither an acid nor alkaline phosphatase. Neither LiCl nor NaF inhibited the I-P3ase activity. However, both L-cysteine and dithiothreitol stimulated the activity 2-fold. Spermine (2.0 mM) inhibited the I-P3ase activity by 50%, while putrescine and spermidine had little or no effect.     

The pathway of myo-inositol 1,3,4-trisphosphate dephosphorylation in liver.

We studied the dephosphorylation pathway for Ins(1,3,4)P3 (inositol 1,3,4-trisphosphate) by liver homogenates and soluble and particulate subfractions incubated in media resembling physiological ionic strength and pH. Ins(1,3,4)P3 was dephosphorylated to two InsP2 (inositol bisphosphate) isomers, one of which is Ins(3,4)P2 [Shears, Parry, Tang, Irvine, Michell & Kirk (1987) Biochem. J. 246, 139-147]. The second InsP2 is the 1,3 isomer. Ins(3,4)P2 is dephosphorylated to inositol 3-phosphate by an enzyme activity located in both soluble and particulate fractions. The phosphatase(s) that attacks Ins(1,3)P2 was largely soluble, but we have not determined which phosphate(s) is removed. When the initial substrate concentration was 1 nM, the rate of dephosphorylation of Ins(1,4)P2 greater than Ins(1,3)P2 greater than Ins(3,4)P2. None of these bisphosphates was phosphorylated when incubated with liver homogenates and 5 mM-ATP, but their rates of dephosphorylation were then decreased.     

Alterations in binding of inositol 1,4,5-trisphosphate to subcellular structures of rat liver during chronic endotoxemia

Inositol 1,4,5-trisphosphate (IP3) binding to, and Ca2+ uptake and release by plasma membrane- and endoplasmic reticulum-enriched fractions of rat liver were measured after continuous Escherichia coli endotoxin (ET) administration in vivo. IP3 binding to both fractions was significantly reduced by ET treatment. This was associated with decreased Ca2+ uptake and impaired IP3-dependent Ca2+ release. A decrease of 5'-nucleotidase specific activity of plasma membrane-enriched fraction was also observed in ET treated rats. The results suggest that previously observed impairments in the ability of hepatocytes to mobilize Ca2+, to activate glycogen phosphorylase and to respond--when saponin permeabilized--by Ca2+ release upon IP3 addition during chronic endotoxemia are due to alterations in both IP3 binding to the subcellular fractions that are imputed to be targets of IP3, and a decrease in the size of IP3-sensitive pool of releasable Ca2+.