Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:3.1.4.3 (phospholipase C)
 18,461 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Prolyl 4-hydroxylase (EC 1.14.11.2) catalyzes the formation of 4-hydroxyproline in collagens by the hydroxylation of proline residues in X-Pro-Gly sequences. The reaction requires Fe2+, 2-oxoglutarate, O2, and ascorbate and involves an oxidative decarboxylation of 2-oxoglutarate. Ascorbate is not consumed during most catalytic cycles, but the enzyme also catalyzes decarboxylation of 2-oxoglutarate without subsequent hydroxylation, and ascorbate is required as a specific alternative oxygen acceptor in such uncoupled reaction cycles. A number of compounds inhibit prolyl 4-hydroxylase competitively with respect to some of its cosubstrates or the peptide substrate, and recently many suicide inactivators have also been described. Such inhibitors and inactivators are of considerable interest, because the prolyl 4-hydroxylase reaction would seem a particularly suitable target for chemical regulation of the excessive collagen formation found in patients with various fibrotic diseases. The active prolyl 4-hydroxylase is an alpha 2 beta 2 tetramer, consisting of two different types of inactive monomer and probably containing two catalytic sites per tetramer. The large catalytic site may be cooperatively built up of both the alpha and beta subunits, but the alpha subunit appears to contribute the major part. The beta subunit has been found to be identical to the enzyme protein disulfide isomerase and a major cellular thyroid hormone-binding protein and shows partial homology with a phosphoinositide-specific phospholipase C, thioredoxins, and the estrogen-binding domain of the estrogen receptor. The COOH-terminus of this beta subunit has the amino acid sequence Lys-Asp-Glu-Leu, which was recently suggested to be necessary for the retention of a polypeptide within the lumen of the endoplasmic reticulum. The alpha subunit does not have this COOH-terminal sequence, and thus one function of the beta subunit in the prolyl 4-hydroxylase tetramer appears to be to retain the enzyme within this cell organelle.
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PMID:Protein hydroxylation: prolyl 4-hydroxylase, an enzyme with four cosubstrates and a multifunctional subunit. 253 73

Platelets are discoid, anucleate cells with a large number of secretory granules. Physiological agonists (thrombin, collagen, ADP, adrenaline, thromboxane A2, serotonin, vasopressin) interact with specific receptors on the platelet surface which causes the platelet responses shape change, aggregation, secretion of substances from three types of granules and liberation of arachidonate from membrane phospholipids. Some secreted substances and conversion products of arachidonate are platelet agonists and enhance platelet stimulation (positive feedback). The shape change and aggregation responses are of central importance for platelet adhesion to the subendothelium and formation of platelet thrombi. Dense granule secretion and the storage of ADP, ATP, Ca2+ and serotonin, a-granule secretion of platelet-specific, cationic, coagulation and carbohydrate-containing proteins as well as secretion of glycosidases are also shown to be important for platelet participation in haemostasis and thrombosis. Signal transduction mechanisms (phospholipase C activation, polyphosphoinositide metabolism, Ca2+ mobilization) and arachidonate oxygenation are central processes for the physiological functions of platelets.
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PMID:Physiological functions of platelets. 253 34

The inhibitory effect of cyclic GMP on collagen-induced platelet activation was studied using 8-bromo cyclic GMP (8brcGMP) in washed rabbit platelets. Addition of collagen (1 micrograms/ml) to platelet suspension caused shape change and aggregation associated with thromboxane (TX) A2 formation. 8brcGMP (10-1000 microM) inhibited collagen-induced platelet aggregation and TXA2 formation in a concentration-dependent manner. 8brcGMP did not affect platelet cyclooxygenase pathways, but markedly inhibited collagen-induced arachidonic acid (AA) liberation from membrane phospholipids in [3H]AA-prelabeled platelets, indicating that the inhibitory effect of 8brcGMP on collagen-induced aggregation is due to an inhibition of AA liberation. In [32P]orthophosphate-labeled platelets, collagen stimulated phosphorylation of a 20,000 dalton (20-kD) and 40-kD proteins. 8BrcGMP stimulated phosphorylation of a specific protein having molecular weight of 46-kD and inhibited collagen-induced both 20- and 40-kD protein phosphorylation. Collagen could stimulate the AA liberation without activation of phospholipase C or Na+-H+ exchange, but could not in the absence of extracellular Ca2+. These findings suggest that cyclic GMP inhibits collagen-induced AA liberation which is mediated by an extracellular Ca2+-dependent phospholipase A2. However, cyclic GMP seems to inhibit the Ca2+-activated phospholipase A2 indirectly, since 8brcGMP had no effect on Ca2+ ionophore A23187-induced platelet aggregation or AA liberation. It is therefore suggested that cyclic GMP may regulate collagen-induced increase in an availability of extracellular Ca2+ which is responsible for phospholipase A2 activation in rabbit platelets.
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PMID:Inhibitory effect of 8-bromo cyclic GMP on an extracellular Ca2+-dependent arachidonic acid liberation in collagen-stimulated rabbit platelets. 254 81

Chronic ethanol ingestion leads to hepatocellular injury and alcoholic liver disease (ALD) only if multiple factors combine to favor centrilobular hepatocellular hypoxia. It is hypothesized that these factors include a shift in the redox state, the induction of the microsomal ethanol oxidizing system (MEOS), a high blood alcohol level (BAL), a high polyunsaturated fat diet and episodic decreased O2 supply to the liver. The shift in the redox state favors a low cellular pH, decreased fatty acid oxidation and increased triglyceride formation. The increased MEOS activity increases O2 consumption and portal-central O2 gradient as well as favors acetaldehyde toxic effects including retention of hepatic lipids and export proteins causing cell swelling. The resultant increase in the concentration of acetaldehyde and lactate may stimulate fibrosis as they stimulate collagen synthesis in vitro. The resultant fatty liver narrows the sinusoids slowing sinusoid blood flow. The combination of events reduces available O2 leading to decreased levels of ATP and cellular pH making the liver vulnerable to episodes of systemic hypoxia. The role of membrane changes are reviewed, i.e., 1) membrane fluidity as related to changes in the species of phospholipids, 2) mitochondrial function as related to the changes in the lipid environment of the electron transport chain, and 3) linoleic acid-prostaglandin metabolism. Acute ethanol in vitro has been shown to affect liver cell metabolism regulation by triggering and increasing protein phosphorylation through the Ca2+-phospholipase C pathway. A high fat diet enhances the liver injury caused by chronic ethanol ingestion.
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PMID:Biochemical basis for alcohol-induced liver injury. 265 Sep 22

Defects in platelet cytoplasmic Ca++ mobilization have been postulated but not well demonstrated in patients with inherited platelet secretion defects. We describe studies in a 42-year-old white woman, referred for evaluation of easy bruising, and her 23-year-old son. In both subjects, aggregation and 14C-serotonin secretion responses in platelet-rich plasma (PRP) to adenosine diphosphate (ADP), epinephrine, platelet activating factor (PAF), arachidonic acid (AA), U46619, and ionophore A23187 were markedly impaired. Platelet ADP and adenosine triphosphate (ATP), contents and thromboxane synthesis induced by thrombin and AA were normal. In quin2-loaded platelets, the basal intracellular Ca++ concentration, [Ca++]i, was normal; however, peak [Ca++]i measured in the presence of 1 mmol/L external Ca++ was consistently diminished following activation with ADP (25 mumol/L), PAF (20 mumol/L), collagen (5 micrograms/mL), U46619 (1 mumol/L), and thrombin (0.05 to 0.5 U/mL). In aequorin-loaded platelets, the peak [Ca++]i studied following thrombin (0.05 and 0.5 U/mL) stimulation was diminished. Myosin light chain phosphorylation following thrombin (0.05 to 0.5 U/mL) stimulation was comparable with that in the normal controls, while with ADP (25 mumol/L) it was more strikingly impaired in the propositus. We provide direct evidence that at least in some patients with inherited platelet secretion defects, agonist-induced Ca++ mobilization is impaired. This may be related to defects in phospholipase C activation. These patients provide a unique opportunity to obtain new insights into Ca++ mobilization in platelets.
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PMID:Impaired cytoplasmic ionized calcium mobilization in inherited platelet secretion defects. 275 41

Administration of ethanol to human platelets resulted in a rapid shape change which was maximal within 30 s. Ethanol did not cause aggregation or secretion of ATP at any time and inhibited aggregation induced by collagen. In platelets that were loaded with the intracellular calcium indicator fura2, ethanol induced a rapid mobilization of calcium from internal, thrombin-sensitive pools. Cytosolic calcium increased to a maximum within 5 s and decreased slowly over the ensuing 5 min to near basal levels. The mobilization of calcium by ethanol coincided with the rapid formation of phosphatidic acid and a decrease in the level of phosphatidylinositol 4,5-bisphosphate, as measured in 32P-labeled platelets. In platelets labeled with myo-[2-3H]inositol, ethanol caused a 20-30% increase in the levels of inositol (1,4,5)-trisphosphate and inositol bisphosphate within 10 s. Ethanol also induced the transient phosphorylation of myosin light chain (20 kDa) and a 40 kDa protein, a known substrate for protein kinase C. The results indicate that ethanol activates phosphoinositide-specific phospholipase C in human platelets. The subsequent mobilization of intracellular calcium and activation of protein kinase C can account for the shape change induced by ethanol.
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PMID:Ethanol stimulates shape change in human platelets by activation of phosphoinositide-specific phospholipase C. 282 32

Carbon monoxide (CO) inhibits human platelet aggregation triggered with threshold levels of agonists like arachidonate, ADP, collagen, thrombin, or the prostaglandin endoperoxide analogue U46619. This inhibition is counteracted by illumination with light above 400 nm indicating the involvement of a ferrous hemoprotein. An earlier suggestion that the mechanism of CO inhibition involves the cytochrome P450 protein thromboxane A2 synthase was ruled out as well as the involvement of the iron containing enzymes like cyclooxygenase or 12-lipoxygenase. In the presence of CO, no arachidonate was released from phospholipids, no increase of intracellular calcium levels was observed, and phospholipase C was not activated suggesting that the transducing mechanisms from the receptors to phospholipase C was effected in the presence of CO. cAMP levels were also unchanged but cGMP levels showed an increase of about 30%. By comparison with the guanylate cyclase stimulator nitroprusside, it was shown that such levels could block aggregation. In a 10,000 X g supernatant, CO enhanced guanylate cyclase activity 4-fold, supporting the view that CO acts by increasing platelet cGMP levels. With respect to the mechanism of guanylate cyclase action, the binding of CO to the regulatory subunit of guanylate cyclase must be responsible for the observed activation. It is concluded that cGMP is an important feedback regulator of the Pl response and that already a 25% increase in its steady state levels can cause inhibition of platelet aggregation.
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PMID:Inhibition of platelet aggregation by carbon monoxide is mediated by activation of guanylate cyclase. 289 93

These studies were undertaken to examine the effects and the mechanism of action of flurazepam and diazepam on human platelet activation. One minute preincubation with flurazepam (3-300 microM) or diazepam (3-300 microM) inhibited platelet aggregation, serotonin secretion and prostaglandin synthesis induced by ADP (1-5 microM), epinephrine (1-5 microM), and arachidonic acid (600-1000 microM). However, 357% higher concentration of diazepam (265 microM) as compared to flurazepam (58 microM), was required to inhibit arachidonic acid induced production of malondialdehyde (MDA) by 50%. In addition, flurazepam and not diazepam inhibited the release of arachidonic acid from platelet phospholipids in a concentration dependent manner. In other experiments flurazepam but not diazepam also blocked aggregation and secretion induced by U46619 (2 microM), a stable analog of prostaglandin H2. Platelet aggregation and serotonin secretion induced by collagen (40-300 micrograms/ml) was inhibited by flurazepam with an IC-50 of 153 microM and 136 microM respectively, whereas higher than 300 microM diazepam was required to inhibit collagen-induced aggregation and secretion by 50%. Flurazepam and diazepam both exhibited their most potent antiplatelet effects against phospholipase C-induced aggregation which is mediated by prostaglandin-independent mechanisms. Only 15 microM and 11 microM flurazepam and 31 microM and 27 microM diazepam were needed to inhibit PLC-induced aggregation and secretion of serotonin by 50% respectively. Effects of these benzodiazepines on platelet cyclic AMP and cyclic GMP were also examined. Neither flurazepam nor diazepam caused any significant change in cyclic AMP or cyclic GMP levels in platelets. These findings suggest that: (a) flurazepam, as compared to diazepam, is 106% - 357% more effective in inhibiting platelet aggregation and serotonin secretion induced by arachidonic acid, collagen and phospholipase C; (b) flurazepam inhibits platelet activation by inhibiting the release of arachidonic acid, its conversion into prostaglandins and by blocking the action of prostaglandins on platelets; (c) diazepam does not inhibit thrombin-induced release of arachidonic acid, conversion of exogenously added arachidonic acid into MDA, or the action of prostaglandins; (d) both flurazepam and diazepam inhibit PLC-mediated activation of platelets; and (e) neither diazepam nor flurazepam achieve their antiplatelet actions by affecting platelet cyclic nucleotide levels.
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PMID:Benzodiazepines inhibit human platelet activation: comparison of the mechanism of antiplatelet actions of flurazepam and diazepam. 299 62

Saponin (5 to 25 micrograms/ml) produced a concentration-dependent decrease in the cellular content of total ATP and [32P]ATP in 32P-labeled human platelets. In platelets whose ATP had been profoundly decreased by saponin, Ca2+ produced phosphomonoesteratic cleavage of the polyphosphoinositides with a concomitant accumulation of phosphatidylinositol. Collagen still induced secretion of serotonin in platelets that had been treated with saponin in the presence or absence of Ca2+. This effect of collagen occurred in the absence of the formation of cyclooxygenase metabolites. In platelet permeabilized with saponin, agonist-induced secretion and aggregation seems to be unrelated to protein phosphorylation, breakdown of the inositol phospholipids by phospholipase C and formation of cyclooxygenase metabolites.
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PMID:ATP depletion in human platelets caused by permeabilization with saponin does not prevent serotonin secretion induced by collagen. 299 17

The early breakdown of phosphatidylinositol 4,5-bisphosphate in human platelets stimulated by a threshold concentration of either collagen or thrombin was inhibited by 5 mM NaF through its inhibition of phospholipase C activity. However, 5 mM NaF did not inhibit Ca2+ mobilization due to the stimuli from internal stores, but it did inhibit the influx of extracellular Ca2+ through its suppression of thromboxane A2 formation.
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PMID:Dissociation of Ca2+ mobilization from breakdown of phosphatidylinositol 4,5-bisphosphate in activated human platelets. 301 69


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