Rat liver contains a potent endogenous inhibitor of inositol 1,3,4,5-tetrakisphosphate 3-phosphatase

Targeted deletion of Minpp1 provides new insight into the activity of multiple inositol polyphosphate phosphatase in vivo

Multiple inositol polyphosphate phosphatase (Minpp1) metabolizes inositol 1,3,4,5,6-pentakisphosphate (InsP(5)) and inositol hexakisphosphate (InsP(6)) with high affinity in vitro. However, Minpp1 is compartmentalized in the endoplasmic reticulum (ER) lumen, where access of enzyme to these predominantly cytosolic substrates in vivo has not previously been demonstrated. To gain insight into the physiological activity of Minpp1, Minpp1-deficient mice were generated by homologous recombination. Tissue extracts from Minpp1-deficient mice lacked detectable Minpp1 mRNA expression and Minpp1 enzyme activity. Unexpectedly, Minpp1-deficient mice were viable, fertile, and without obvious defects. Although Minpp1 expression is upregulated during chondrocyte hypertrophy, normal chondrocyte differentiation and bone development were observed in Minpp1-deficient mice. Biochemical analyses demonstrate that InsP(5) and InsP(6) are in vivo substrates for ER-based Minpp1, as levels of these polyphosphates in Minpp1-deficient embryonic fibroblasts were 30 to 45% higher than in wild-type cells. This increase was reversed by reintroducing exogenous Minpp1 into the ER. Thus, ER-based Minpp1 plays a significant role in the maintenance of steady-state levels of InsP(5) and InsP(6). These polyphosphates could be reduced below their natural levels by aberrant expression in the cytosol of a truncated Minpp1 lacking its ER-targeting N terminus. This was accompanied by slowed cellular proliferation, indicating that maintenance of cellular InsP(5) and InsP(6) is essential to normal cell growth. Yet, depletion of cellular inositol polyphosphates during erythropoiesis emerges as an additional physiological activity of Minpp1; loss of this enzyme activity in erythrocytes from Minpp1-deficient mice was accompanied by upregulation of a novel, substitutive inositol polyphosphate phosphatase.

Comparison of the activities of a multiple inositol polyphosphate phosphatase obtained from several sources: a search for heterogeneity in this enzyme

A multiple inositol polyphosphate phosphatase (formerly known as inositol 1,3,4,5-tetrakisphosphate 3-phosphatase) was purified approx. 22,000-fold from rat liver. The final preparation migrated on SDS/PAGE as a doublet with a mean apparent molecular mass of 47 kDa. Upon size-exclusion chromatography, the enzyme was eluted with an apparent molecular mass of 36 kDa. This enzyme was approximately evenly distributed between the 'rough' and 'smooth' subfractions of endoplasmic reticulum. There was a 20-fold range of specific activities of this phosphatase in CHAPS-solubilized particulate fractions prepared from the following rat tissues: liver, heart, kidney, testis and brain. However, each of these extracts contained different amounts of endogenous inhibitors of enzyme activity. After removal of these inhibitors by MonoQ anion-exchange chromatography, there was only a 2.5-fold range of specific activities; kidney contained the most and brain contained the least. We prepared and characterized polyclonal antiserum to the hepatic phosphatase, which immunoprecipitated 85-100% of both particulate and soluble phosphatase activities. The antiserum also immunoprecipitated, with equivalent efficacy, CHAPS-solubilized phosphatase activities from heart, kidney, testis, brain and erythrocytes (all prepared from rat). Our data strengthen the case that the function of the mammalian phosphatase is unrelated to the metabolism of Ca(2+)-mobilizing cellular signals. The CHAPS-solubilized phosphatase from turkey erythrocytes was not immunoprecipitated by the polyclonal antiserum, and is therefore an isoform that is structurally distinct, and possibly functionally unique.

The metabolism of D-myo-inositol 1,4,5-trisphosphate and D-myo-inositol 1,3,4,5-tetrakisphosphate by porcine skeletal muscle

In soluble and particulate extracts from muscle D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and D-myo-inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] are metabolised stepwise to inositol. Ins(1,4,5)P3 is rapidly dephosphorylated to D-myo-inositol 1,4-bisphosphate then to D-myo-inositol 4-phosphate and finally inositol. In soluble extracts Ins(1,3,4,5)P4 is dephosphorylated to D-myo-inositol 1,3,4-trisphosphate then sequentially to D-myo-inositol 3,4-bisphosphate, D-myo-inositol 3-phosphate and inositol, while in particulate extracts D-myo-inositol 1,3-bisphosphate is the predominant inositol bisphosphate formed. Dephosphorylation of these inositol polyphosphates is Mg2+ dependent and inhibited by D-2,3-bisphosphoglyceric acid. Ins(1,4,5)P3 is also phosphorylated to form Ins(1,3,4,5)P4 in soluble extracts by Ins(1,4,5)P3 3-kinase. Ins(1,4,5)P3 3-kinase activity is Mg2+ and ATP dependent and is stimulated by Ca2+ and calmodulin. Particulate (sarcotubular) inositol polyphosphate 5-phosphatase (5-phosphatase) is found in membranes which are intimately involved in excitation-contraction coupling and the generation of the primary Ca2+ signal of muscle cells. Particulate 5-phosphatase had the highest specific activity in the transverse-tubule membrane, when compared to the terminal cisternae and longitudinal-tubule membranes of the sarcoplasmic reticulum. Particulate Ins(1,3,4,5)P4-3-phosphatase activity was also detected after fractionation of solubilised sarcotubular membranes by DEAE-Sephacel. Particulate 5-phosphatase activity was purified 25,600-fold to a specific activity of 25.6 mumol Ins(1,4,5)P3 hydrolysed.min-1.mg protein-1, after DEAE-Sephacel and novel affinity chromatography using D-2,3-bisphosphoglycerate/agarose and Sepharose-4B-immobilised Ins(1,4,5)P3-analog matrices. Purified particulate 5-phosphatase had apparent Km of 46.3 microM and 1.9 microM and Vmax of 115 and 0.046 mumol substrate hydrolysed.min-1.mg protein-1, for Ins(1,4,5)P3 and Ins(1,3,4,5)P4, respectively. In contrast, purified soluble type I 5-phosphatase had apparent Km of 8.9 microM and 1.1 microM and Vmax of 3.55 and 0.13 mumol substrate hydrolysed.min-1.mg protein-1, for Ins(1,4,5P3 and Ins(1,3,4,5)P4, respectively. As in other cells, muscle 5-phosphatases have a lower affinity, but a higher capacity to metabolise Ins(1,4,5)P3 than Ins(1,3,4,5)P4. Soluble type I 5-phosphatase may have a functional role in the metabolism of both inositol polyphosphates, while particulate 5-phosphatase may primarily metabolise Ins(1,4,5)P3. Purified Ins(1,4,5)P3 3-kinase had an apparent Km of 0.42 microM and a Vmax of 4.12 nmol Ins(1,4,5)P3 phosphorylated.min-1.mg protein-1. The profile of inositol polyphosphate metabolism in muscle is similar to that reported in other tissues.(ABSTRACT TRUNCATED AT 400 WORDS)

Inositol polyphosphate metabolism and inositol lipids in a green alga, Chlamydomonas eugametos

Swimming suspensions of Chlamydomonas eugametos were pelleted and homogenized, and the metabolism of inositol polyphosphates by cellular homogenates or supernatants was investigated. Ins(1,4,5)P3 was dephosphorylated under physiological conditions to yield a single InsP2, Ins(1,4]2. In the presence of ATP it was phosphorylated to give Ins(1,3,4,5)P3 as the only InsP4. The Ins(1,4,5)P3 3-kinase activity was predominantly soluble, was not detectably affected by calmodulin or Ca2+, and had a Km for Ins(1,4,5)P3 of 50 microM (two orders of magnitude higher than its mammalian counterpart). Ins(1,3,4,5)P4 was dephosphorylated by the cellular supernatants to Ins(1,3,4)P3 and Ins(1,4,5)P3, and could be phosphorylated to Ins(1,3,4,5,6)P4. No Ins(1,3,4)P3 6-kinase activity could be detected, and experiments with [3H]Ins(1,4,[32P]5)P3 revealed that Ins(1,3,4,5,6)P5 is formed from Ins(1,4,5)P3 with little loss of the 5-phosphate, i.e. the predominant route of synthesis is probably by a direct 6-phosphorylation of Ins(1,3,4,5)P4. Similar experiments with an (NH4)2SO4 fraction of turkey erythrocyte cytosol gave essentially the same result, i.e. direct phosphorylation of Ins(1,3,4,5)P4 in the 6 position is the predominant route of synthesis of InsP5 from that InsP4 in vitro. No InsP6 formation was detected in any of these experiments, but labelling of intact C. eugametos with [3H]inositol revealed that the cells do synthesize InsP6. The lipids of C. eugametos cells contain PtdIns, PtdIns(4)P and PtdIns(4,5)P2 [Irvine, Letcher, Lander, Drøbak, Dawson & Musgrave (1989) Plant Physiol. 64, 888-892]. Further examination of 32P-labelled lipids revealed that about 20% of the PtdInsP was the PtdIns(3)P isomer, and about 1% or less of the PtdInsP2 was the PtdIns(3,4)P2 isomer. The overall inositide metabolism of C. eugametos resembles that of a mammalian cell more closely than it does that of a plant cell or slime mould, and this suggests firstly that the known metabolism of inositol polyphosphates arose at an early time in eukaryotic evolution, and secondly that Chlamydomonas might prove a useful organism for genetic and comparative studies of inositide enzymology.

Inositol 1,3,4,5,6-pentakisphosphate and inositol hexakisphosphate are inhibitors of the soluble inositol 1,3,4,5-tetrakisphosphate 3-phosphatase and the inositol 1,4,5-trisphosphate/1,3,4,5-tetrakisphosphate 5-phosphatase from pig brain

The influence of highly phosphorylated inositol phosphates on the Ins(1,3,4,5)P4 3-phosphatase enriched from the soluble fraction of pig brain was tested, using [5-32P]Ins(1,3,4,5)P4 as substrate. Both Ins(1,3,4,5,6)P5 and InsP6 were very potent inhibitors of the Ins(1,3,4,5)P4 3-phosphatase. The Ki values were approximately 60 nM and approximately 3 nM for Ins(1,3,4,5,6)P5 and InsP6 respectively. Ins(1,3,4,5,6)P5 and InsP6 also inhibited the Ins(1,4,5)P3/Ins(1,3,4,5)P4 5-phosphatase. Using Ins(1,3,4,5)P4 as substrate, the Ki values were about 35 microM and 15 microM for Ins(1,3,4,5,6)P5 and InsP6 respectively. The concentrations which led to a 50% inhibition of Ins(1,4,5)P3 (0.5 microM) degradation by the 5-phosphatase were about 20 and 10 microM for the pentakis- and hexakis-phosphate respectively. As the intracellular concentrations of Ins(1,3,4,5,6)P5 and InsP6 are high (up to 60 microM) compared with those of the inositol trisphosphates and tetrakisphosphates, it is possible that the highly phosphorylated inositol phosphates act as regulators in the metabolism of Ca(2+)-mobilizing inositol phosphates.

Electroporation can cause artefacts due to solubilization of cations from the electrode plates. Aluminum ions enhance conversion of inositol 1,3,4,5-tetrakisphosphate into inositol 1,4,5-trisphosphate in electroporated L1210 cells

1. In electroporated L1210 cells, Ins(1,3,4,5)P4 causes Ca2+ release, owing to its conversion into Ins(1,4,5)P3, but this does not happen in cells permeabilized by digitonin treatment [Cullen, Irvine, Drøbak & Dawson (1989) Biochem. J. 259, 931-933]. 2. If the assay medium is subjected to electroporation by using a commercially available electroporation apparatus and then the cells are added and permeabilized with digitonin, the cells behave as if they had been electroporated. 3. Electroporation causes the release of high concentrations of Al3+ into the experimental medium, and addition of these concentrations of Al3+ into the experimental medium mimics the effect of electroporation on the conversion of Ins(1,3,4,5)P4 into Ins(1,4,5)P3. 4. It is concluded that the difference between electroporated and digitonin-permeabilized L1210 cells in this experimental system can be attributed to dissolution of Al3+ from the electroporation cuvette. Al3+ contamination may thus be a serious problem when using this apparatus.

Synthetic inositol 1,3,4,5-tetrakisphosphate analogues

Inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] analogues were synthesized and their effects on [3H]Ins(1,3,4,5)P4 5-phosphatase, [3H]Ins(1,3,4,5)P4 3-phosphatase and [3H]inositol 1,4,5-trisphosphate [3H]Ins(1,4,5)P3] 5-phosphatase activities were examined. The Ins(1,3,4,5)P4 analogue with the aminobenzoyl group at the 2-position of Ins(1,3,4,5)P4 inhibited the hydrolysis of 5-phosphate of [3H]Ins(1,3,4,5)P4 catalysed by erythrocyte ghosts, with a lower Ki value than seen with Ins(1,3,4,5)P4, whereas the analogue with the aminocyclohexanecarbonyl group at the same position had a higher Ki value. The Ins(1,4,5)P3 analogues that we had previously synthesized were also capable of inhibiting this process, with the same tendency as Ins(1,3,4,5)P4 analogues. Such differences in the potency among Ins(1,3,4,5)P4 and Ins(1,4,5)P3 analogues were applicable to other phosphatase activities, namely [3H]Ins(1,3,4,5)P4 3-phosphatase and [3H]Ins(1,4,5)P3 5-phosphatase. These results suggest that the active sites of these enzymes may catalyse the dephosphorylation in a similar fashion.

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.