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The roles of phospholipase D and a GTP-binding protein in guanosine 5'-[gamma-thio]triphosphate-stimulated hydrolysis of phosphatidylcholine in rat liver plasma membranes.

ABSTRACT: 1. Guanosine 5'-[gamma-thio]triphosphate (GTP[S]) stimulated by 50% the rate of release of [3H]choline and [3H]phosphorylcholine in rat liver plasma membranes labelled with [3H]choline. About 70% of the radioactivity released in the presence of GTP[S] was [3H]choline and 30% was [3H]phosphorylcholine. 2. The hydrolysis of phosphorylcholine to choline and the conversion of choline to phosphorylcholine did not contribute to the formation of [3H]choline and [3H]phosphorylcholine respectively. 3. The release of [3H]choline from membranes was inhibited by low concentrations of SDS or Triton X-100. Considerably higher concentrations of the detergents were required to inhibit the release of [3H]phosphorylcholine. 4. Guanosine 5'-[beta gamma-imido]triphosphate and guanosine 5'-[alpha beta-methylene]triphosphate, but not adenosine 5'-[gamma-thio]-triphosphate, stimulated [3H]choline release to the same extent as did GTP[S]. The GTP[S]-stimulated [3H]choline release was inhibited by guanosine 5'-[beta-thio]diphosphate, GDP and GTP but not by GMP. 5. It is concluded that, in rat liver plasma membranes, (a) GTP[S]-stimulated hydrolysis of phosphatidylcholine is catalysed predominantly by phospholipase D with some contribution from phospholipase C, and (b) the stimulation of phosphatidylcholine hydrolysis by GTP[s] occurs via a GTP-binding regulatory protein.

SUBMITTER: Hurst KM

PROVIDER: S-EPMC1149772 | biostudies-other | 1990 Dec

REPOSITORIES: biostudies-other

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Project description:Phosphatidylinositol (PtdIns), phosphatidylinositol 4-phosphate (PtdIns4P) and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] of turkey erythrocytes were labelled by using either [32P]Pi or [3H]inositol. Although there was little basal release of inositol phosphates from membranes purified from labelled cells, in the presence of guanosine 5'-[gamma-thio]triphosphate (GTP[S]) the rate of accumulation of inositol bis-, tris- and tetrakis-phosphate (InsP2, InsP3 and InsP4) was increased 20-50-fold. The enhanced rate of accumulation of 3H-labelled inositol phosphates was linear for up to 20 min; owing to decreases in 32P specific radioactivity of phosphoinositides during incubation of membranes with unlabelled ATP, the accumulation of 32P-labelled inositol phosphates was linear for only 5 min. In the absence of ATP and a nucleotide-regenerating system, no InsP4 was formed, and the overall inositol phosphate response to GTP[S] was decreased. Analyses of phosphoinositides during incubation with ATP indicated that interconversions of PtdIns to PtdIns4P and PtdIns4P to PtdIns(4,5)P2 occurred to maintain PtdIns(4,5)P2 concentrations; GTP[S]-induced inositol phosphate formation was accompanied by a corresponding decrease in 32P- and 3H-labelled PtdIns, PtdIns4P and PtdIns(4,5)P2. In the absence of ATP, only GTP[S]-induced decreases in PtdIns(4,5)P2 occurred. Since inositol monophosphate was not formed under any condition, PtdIns is not a substrate for the phospholipase C. The production of InsP2 was decreased markedly, but not blocked, under conditions where Ins(1,4,5)P3 5-phosphomonoesterase activity in the preparation was inhibited. Thus the predominant substrate of the GTP[S]-activated phospholipase C of turkey erythrocyte membranes is PtdIns(4,5)P2. Ins(1,4,5)P3 was the major product of this reaction; only a small amount of Ins(1:2-cyclic, 4,5)P3 was released. The effects of ATP on inositol phosphate formation apparently involve the contributions of two phenomena. First, the P2-receptor agonist 2-methylthioadenosine triphosphate (2MeSATP) greatly increased inositol phosphate formation and decreased [3H]PtdIns4P and [3H]PtdIns(4,5)P2 in the presence of a low (0.1 microM) concentration of GTP[S]. ATP over the concentration range 0-100 microM produced effects in the presence of 0.1 microM-GTP[S] essentially identical with those observed with 2MeSATP, suggesting that the effects of low concentrations of ATP are also explained by a stimulation of P2-receptors. Higher concentrations of ATP also increase inositol phosphate formation, apparently by supporting the synthesis of substrate phospholipids.(ABSTRACT TRUNCATED AT 400 WORDS)

Project description:Rabbit platelets were labelled with [3H]inositol and a membrane fraction was isolated in the presence of ATP, MgCl2 and EGTA. Incubation of samples for 10 min with 0.1 microM-Ca2+free released [3H]inositol phosphates equivalent to about 2.0% of the membrane [3H]phosphoinositides. Addition of 10 microM-guanosine 5'-[gamma-thio]triphosphate (GTP[S]) caused an additional formation of [3H]inositol phosphates equivalent to 6.6% of the [3H]phosphoinositides. A half-maximal effect was observed with 0.4 microM-GTP[S]. The [3H]inositol phosphates that accumulated consisted of 10% [3H]inositol monophosphate, 88% [3H]inositol bisphosphate ([3H]IP2) and 2% [3H]inositol trisphosphate ([3H]IP3). Omission of ATP and MgCl2 led to depletion of membrane [3H]polyphosphoinositides and marked decreases in the formation of [3H]inositol phosphates. Thrombin (2 units/ml) or GTP (4-100 microM) alone weakly stimulated [3H]IP2 formation, but together they acted synergistically to exert an effect comparable with that of 10 microM-GTP[S]. The action of thrombin was also potentiated by 0.1 microM-GTP[S]. Guanosine 5'-[beta-thio]diphosphate not only inhibited the effects of GTP[S], GTP and GTP with thrombin, but also blocked the action of thrombin alone, suggesting that this depended on residual GTP. Incubation with either GTP[S] or thrombin and GTP decreased membrane [3H]phosphatidylinositol 4-phosphate ([H]PIP) and prevented an increase in [3H]phosphatidylinositol 4,5-bisphosphate ([3H]PIP2) observed in controls. Addition of unlabelled IP3 to trap [3H]IP3 before it was degraded to [3H]IP2 showed that only about 20% of the additional [3H]inositol phosphates that accumulated with GTP[S] or thrombin and GTP were derived from the action of phospholipase C on [3H]PIP2. The results provide further evidence that guanine-nucleotide-binding protein mediates signal transduction between the thrombin receptor and phospholipase C, and suggest that PIP may be a major substrate of this enzyme in the platelet.

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The roles of phospholipase D and a GTP-binding protein in guanosine 5'-[gamma-thio]triphosphate-stimulated hydrolysis of phosphatidylcholine in rat liver plasma membranes.

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