Identification of haptoglobin as an endogenous inhibitor of prostaglandin H synthetase in the cytosol of sheep vesicular gland cells

It was shown that sheep vesicular gland cytosol inhibits the peroxidase activity of prostaglandin H synthetase (PGHS). The degree of enzyme inactivation depends on cytosol concentration and incubation time. It was found that cytosol contains a glycoprotein, haptoglobin, which is one of the cytosolic basic components responsible for its ability to inhibit PGHS. Haptoglobin is supposed to participate in endogenous regulation of the PGHS activity in sheep vesicular glands.     

The effect of haptoglobin on prostaglandin H synthase from ram vesicular glands

It was shown that the ability of sheep and horse haptoglobins differing in their immunological properties to inhibit PGH synthetase is about the same. It was found that haptoglobin inhibits the PGH synthetase-catalyzed enzymatic reaction, the inhibiting effect being non-competitive with respect to the electron donor, adrenaline. The degree of PGH synthetase inhibition by haptoglobin depends on the glycoprotein concentration, incubation time and enzyme activity.     

Inhibition of prostaglandin synthase activity of sheep seminal vesicular gland by human serum haptoglobin

Serum haptoglobin was added to the reaction mixture of prostaglandin synthase (EC 1.14.99.1) and its inhibitory effect was studied [1-14C]Arachidonic acid was used as substrate and the enzyme activity was estimated by monitoring the radioactivity of the products after thin layer chromatography. With or without addition of hemoglobin to the reaction mixture, both the purified haptoglobin 1-1 and 2-2 showed inhibitory activity. In the presence of 5 microM hematin, however, inhibitory activity haptoglobin was not observed. Inhibition of prostaglandin synthesis in the system depended on the molar ratio of haptoglobin to hemoglobin in the reaction mixture. These results demonstrate that haptoglobin inhibits prostaglandin synthase by restricting available heme group for the enzyme activity through complexing with hemoglobin. However, haptoglobin did not inhibit completely the stimulatory effect of free hemoglobin. Relevant significant of this effect was discussed.     

Properties of solubilized prostaglandin synthetase from sheep vesicular glands.

Prostaglandin synthetase activity associated with the microsomal fraction from sheep vesicular glands has been solubilized by treatment with the non-ionic detergents Tween 20, Lubrol Px and Lubrol Wx. Approx.8 fold purification from microsomes is obtained and over 90% of the activity is recovered in the detergent solubilized fraction. The solubilized synthetase activity is stable at pH 5.0 but is gradually lost at pH 8.0; it is also heat and acid labile. The relative amounts of prostaglandins E₂, D₂ and F_2α formed by the microsomal-bound synthetase and by the solubilized synthetase are similar. Also similar are the pH optima (7.9–8.5) of the two synthetase preparations. The solubilization process appears to yield a fully active enzymatic preparation which could be employed for further purification and characterization of the prostaglandin synthetase complex.     

Purification and characterisation of prostaglandin endoperoxide synthetase from sheep vesicular glands.

The membrane-bound prostaglandin endoperoxide synthetase was purified until homogeneity, starting from sheep vesicular glands. The enzyme was obtained as a complex with Tween-20, containing 0.69 mg detergent per mg protein. No residual phospholipid could be detected. Prostaglandin endoperoxide synthetase appeared to be a glycoprotein, containing mannose and N-acetyl-glucosamine. No haemin or metal atoms were present. A molecular weight of 126 000 was found for the apoprotein by ultracentrifugation in 0.1% Tween solutions. The polypeptide chain without carbohydrate had a molecular weight of 69 000 as determined by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. The pure enzyme displays both cyclooxygenase and peroxidase activity, thus converting arachidonic acid into prostaglandin H2. The isolated synthetase requires haemin, which possibly acts as an easily dissociable prosthetic group, and a suitable hydrogen donor to protect the enzyme from peroxide inactivation and which is consumed in stoichiometric amounts to reduce the intermediate hydroperoxy group.     

Isolation of an activator for prostaglandin hydroperoxidase from bovine vesicular gland cytosol and its identification as uric acid.

The enzymatic conversion of prostaglandin G₁ to H₁ was stimulated by an activator present in the cytosol of bovine vesicular gland. The activator was purified by Sephadex G-25 gel filtration and Dowex 1 column chromatography. The purified activator was identified to be uric acid by thin layer chromatography, ultraviolet and infrared absorption spectroscopy and combined gas chromatography-mass spectroscopy. Among various purine compounds tested, only uric acid and 2,8-dihydroxyadenine were active.     

Identification of XAF1 as an endogenous AKT inhibitor

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Effect of low and high methional concentrations on prostaglandin biosynthesis in microsomes from bovine and sheep vesicular glands

The effect of methional on prostaglandin biosynthesis from 5,8,11, 14-eicosatetraenoic acid was studied with microsomes from both bovine vesicular glands (BVG) and sheep vesicular glands (SVG). Ethylene was identified when methional was added to the fatty acid-microsome incubation systems showing that oxygen centered radicals such as hydroxyl radical were generated during incubation. A low methional level, 1 mM, enhanced the rate of prostaglandin biosynthesis in both BVG and SVG. A high methional level, 10 mM, inhibited prostaglandin biosynthesis in both BVG alone and SVG solubilized with 1% Tween 20. The inhibitory effect of 10 mM methional was reversed by lyophilization. These data suggest that oxygen centered radicals are used in prostaglandin biosynthesis even though they inactivate the enzyme complex.     

Identification of a pharmacologically distinct prostaglandin H synthase in cultured epithelial cells.

Nonsteroidal anti-inflammatory drugs inhibit the action of prostaglandin H synthase (PGH synthase), and this effect may constitute the basis for therapeutic and idiosyncratic responses to these agents. We found that aspirin treatment of cultured ovine tracheal epithelial cells blocked PGH synthase-catalyzed formation of PG as expected but also caused a dose-dependent increase in 15-hydroxyeicosatetraenoic acid (15-HETE) production from arachidonic acid. In contrast, aspirin caused only inhibition of PG production without enhancing 15-HETE formation in ovine seminal vesicle and other tissues. The 15-HETE formed by aspirin-treated ovine tracheal epithelial cells was generated by a PGH synthase-dependent mechanism because: (i) the 15-HETE forming activity was just as sensitive as PG forming activity to selective inhibition by indomethacin; (ii) both 15-HETE and PG forming activities were quantitatively immunoprecipitated (depleted from supernatants and recovered in immune complex pellets) by a specific anti-PGH synthase antiserum. Additional immunoprecipitation experiments indicated that anti-PGH synthase monoclonal antibodies (cyo-1 and cyo-5) raised against the aspirin-inhibited form of the enzyme (contained in seminal vesicle) did not recognize the aspirin-stimulated 15-HETE-forming PGH synthase (contained in cultured epithelial cells). Thus, sequential immunoprecipitation of cultured epithelial cell material first with excess cyo-1 followed by anti-PGH synthase antiserum indicated that two isoforms of PGH synthase were expressed in these cells. SDS-polyacrylamide gel electrophoresis of immunoprecipitated PGH synthase from cultured epithelial cells revealed distinct protein bands for each form of the enzyme (M(r) = 70,000 and 72,000). The identification of a distinct PGH synthase which may be modified by aspirin so that selective oxygenation of fatty acid substrate is enhanced (while PG formation is inhibited) indicates that isozymes of PGH synthase exist which are pharmacologically distinct.     

Observations on the substrate specificity of prostaglandin hydroxylases of monkey seminal vesicles and sheep vesicular glands

Prostaglandin (PG) 19-hydroxylase of monkey seminal vesicles metabolizes PGE1 and PGE2 to their 19-hydroxy metabolites, while PGE2 20-hydroxylase of ram vesicular glands metabolizes PGE2 to 20-hydroxy-PGE2. The purpose of the present study was determine whether PGF2 alpha is a substrate of these enzymes. Deuterated 20-hydroxy-PGF2 alpha was employed as an internal standard to study the hydroxylation of PGF2 alpha (0.2 mM) by microsomes of monkey (Macaca fascicularis) seminal vesicles in the presence of NADPH, and the biosynthesis was compared with the hydroxylation of PGE2 under identical conditions. 19-Hydroxy-PGF2 alpha was formed at a rate of 3.5% of the formation of 19-hydroxy-PGE2. Microsomes of ram vesicular glands also hydroxylated PGE2 more efficiently than PGF2 alpha, which was converted to both 20-hydroxy-PGF2 alpha and 19-hydroxy-PGF2 alpha at a combined rate of 5% of the biosynthesis of 20-hydroxy-PGE2 under the same conditions. 20-Hydroxy-PGF2 alpha was demonstrated in ram semen, but the concentration was low (0.1 microM) in comparison with 20-hydroxy-PGE2 (24 microM). The two genital PG hydroxylases thus metabolize PGF2 alpha much less efficiently than PGE2. This finding may suggest that 19-hydroxy- and 20-hydroxy-PGF2 alpha in seminal fluids also could be formed by other mechanisms, e.g., from 19-hydroxy- and 20-hydroxy-PGE2 by the PGE 9-keto reductase enzyme.     

Effect of low and high methional concentrations on prostaglandin biosynthesis in microsomes from bovine and sheep vesicular glands.

The effect of methional on prostaglandin biosynthesis from 5,8,11,14-eicosatetraenoic acid was studied with microsomes from both bovine vesicular glands (BVG) and sheep vesicular glands (SVG). Ethylene was identified when methional was added to the fatty acid-microsome incubation systems showing that oxygen centered radicals such as hydroxyl radical were generated during incubation. A low methional level, 1 mM, enhanced the rate of prostaglandin biosynthesis in both BVG and SVG. A high methional level, 10 mM, inhibited prostaglandin biosynthesis in both BVG alone and SVG solubilized with 1% Tween 20. The inhibitory effect of 10 mM methional was reversed by lyophilization. These data suggest that oxygen centered radicals are used in prostaglandin biosynthesis even though they inactivate the enzyme complex.     

Immunohistochemical localization of prostaglandin H synthase in the sheep placenta from early pregnancy to term.

Prostaglandin H synthase (PGHS) activity within intrauterine tissues is considered to catalyze a critical step in prostaglandin (PG) biosynthesis at parturition. In sheep, the placenta is a major site of PG production throughout pregnancy, but little information is available concerning the cells that are responsible. Therefore we determined the distribution of immunoreactive (IR-) PGHS in ovine placental tissue obtained at different times of pregnancy using immunohistochemical techniques. In placentomes from early pregnancy (Days 30-54), IR-PGHS was present in maternal epithelial syncytium, but was not detectable in trophoblast cells. Between Day 54 and Day 100, the number of cells that stained positive for PGHS declined in the maternal epithelial layer in the body of the placenta, but IR-PGHS was present in maternal epithelial cells overlying the vascular cones of the placental hemophagous zone. It was also present in the chorionic fibroblasts, but remained undetectable from all classes of trophoblast cells. IR-PGHS was first detectable in the trophoblastic epithelium by Day 114. Between Day 119 and term the trophoblast mononuclear epithelial cells were intensely immunopositive for PGHS, although immunonegative binucleate cells were present. The maternal epithelium was immunonegative except during the last 7-10 days of pregnancy when PGHS immunostaining appeared in both basal and apical regions of the placenta. Thus, the cellular localization of IR-PGHS changes during ovine pregnancy, from predominantly maternal during the first half of gestation to undetectable and then to predominantly trophoblastic between Day 114 and term, suggesting a gestation-dependent change in sites of PG production during ovine pregnancy. Appearance of IR-PGHS in the trophoblast precedes activation of the fetal hypothalamic-pituitary-adrenal axis, generally considered to provide the trigger to the onset of parturition in sheep, and would therefore appear to be regulated through alternative pathways or mechanisms.     

Identification of SVIP as an endogenous inhibitor of endoplasmic reticulum-associated degradation

Misfolded proteins in the endoplasmic reticulum (ER) are eliminated by a process known as ER-associated degradation (ERAD), which starts with misfolded protein recognition, followed by ubiquitination, retrotranslocation to the cytosol, deglycosylation, and targeting to the proteasome for degradation. Actions of multisubunit protein machineries in the ER membrane integrate these steps. We hypothesized that regulation of the multisubunit machinery assembly is a mechanism by which ERAD activity is regulated. To test this hypothesis, we investigated the potential regulatory role of the small p97/VCP-interacting protein (SVIP) on the formation of the ERAD machinery that includes ubiquitin ligase gp78, AAA ATPase p97/VCP, and the putative channel Derlin1. We found that SVIP is anchored to microsomal membrane via myristoylation and co-fractionated with gp78, Derlin1, p97/VCP, and calnexin to the ER. Like gp78, SVIP also physically interacts with p97/VCP and Derlin1. Overexpression of SVIP blocks unassembled CD3delta from association with gp78 and p97/VCP, which is accompanied by decreases in CD3delta ubiquitination and degradation. Silencing SVIP expression markedly enhances the formation of gp78-p97/VCP-Derlin1 complex, which correlates with increased degradation of CD3delta and misfolded Z variant of alpha-1-antitrypsin, established substrates of gp78. These results suggest that SVIP is an endogenous inhibitor of ERAD that acts through regulating the assembly of the gp78-p97/VCP-Derlin1 complex.     

Identification of bikunin as an endogenous inhibitor of dynorphin convertase in human cerebrospinal fluid

Dynorphin-converting enzymes constitute a group of peptidases capable of converting dynorphins to enkephalins. Through the action of these enzymes, the dynorphin-related peptides bind to delta-opioid instead of kappa-opioid receptors, leading to a change in the biological function of the neuropeptides. In this article, we describe the identification of the protein bikunin as an endogenous, competitive inhibitor of a dynorphin-converting enzyme in human cerebrospinal fluid. This protein is present together with its target enzyme in the same body fluids. The K(M) value of the convertase was found to be 9 microm, and the K(i) value of the inhibitor was 1.7 nm. The finding indicates that bikunin may play a significant role as a regulatory mechanism of neuropeptides, where one bioactive peptide is converted to a shorter sequence, which in turn, can affect the action of its longer form.     

Regulation of prostaglandin H synthase activity in dog urothelial cells.

TPA regulation of prostaglandin H synthase activity in primary and subcultured dog urothelial cells was investigated. Previous studies have demonstrated an early (0-2 hr) increase in PGE2 synthesis mediated by TPA which is dependent upon release of endogenous arachidonic acid by a phospholipase-mediated pathway. In this study, prostaglandin H synthase activity was assessed directly with microsomes and indirectly after addition of exogenous arachidonic acid at a maximum effective concentration (100 microM) to media. PGE2 synthesis, measured by radioimmunoassay, served as an index of prostaglandin H synthase activity. After a 24-hr incubation with 0.1 microM TPA or 1.0 microM A23187, arachidonic acid elicited significantly more PGE2 synthesis in agonist-treated cells than it did in control cells in primary culture. Microsomes from 24-hr TPA-treated cells exhibited significantly more prostaglandin H synthase activity than did those from control cells. In addition, the PGE2 content of overnight media was approximately 10-fold greater in TPA-treated cells than in control cells. The late (24 hr) response was more sensitive to lower concentrations of TPA than was the earlier (0-2 hr) response. TPA at 0.1 microM was a maximum effective dose for both responses. The 24-hr response was blocked by cycloheximide and staurosporine, inhibitors of protein synthesis and protein kinase C, respectively. Pretreatment of cells with aspirin, an irreversible inhibitor of prostaglandin H synthase, prior to addition of TPA did not prevent the late TPA-mediated increase in PGE2 synthesis. Subcultured cells exhibited both an early and a late TPA response. Only the early response was inhibited by aspirin pretreatment. Results suggest that the late response with TPA is caused by de novo synthesis of prostaglandin H synthase. Thus, primary and subcultured dog urothelial cells possess two distinct mechanisms for regulating signal transduction by arachidonic acid metabolism. This study provides a basis for assessing these mechanisms of signal transduction in urothelial cell lines and transformed cells.     

Identification of an endogenous inhibitor of arachidonate metabolism in human epidermoid carcinoma A431 cells

Eicosanoids, which include prostaglandins, thromboxanes, and leukotrienes, are produced from arachidonic acid by three main pathways in cells, including cyclooxygenases and lipoxygenases, and cytochrome P450 enzymes. Accumulated evidence indicates that a certain peroxide tone is required for the initiation of reaction by lipoxygenases and cyclooxygenases. An endogenous inhibitor of arachidonate oxygenation was suspected in the cytosolic fraction of human epidermoid carcinoma A431 cells. After a series of studies, the existence of this inhibitor was confirmed, while it was purified and characterized. By amino acid sequence analysis, the inhibitor in A431 cells was subsequently identified as a phospholipid hydroperoxide glutathione peroxidase (PHGPx). Depletion of cellular glutathione in cells by diethyl maleate or by dibuthionine-sulfoximine results in an increase in enzyme activities of 12(S)-lipoxygenase and cyclooxygenase, suggesting that glutathione-depleting agents abolish the enzyme activity of PHGPx in cells. Stable transfectants of A431 cells with overexpression and depletion of PHGPx have been constructed, respectively. Reduction of arachidonate metabolism through 12(S)-lipoxygenase and cyclooxygenase 1 and that of the arsenite-induced generation of reactive oxygen species are observed in cells overexpressing PHGPx. On the other hand, enhancement of arachidonate metabolism and the arsenite-induced generation of reactive oxygen species is detected in PHGPx-depleted cells. In conclusion, the endogenous inhibitor of arachidonate metabolism present in A431 cells is a PHGPx, which plays a functional role in the down-regulation of arachidonate oxygenation catalyzed by 12(S)-lipoxygenase and cyclooxygenase 1 through the reduction of the level of intracellular lipid hydroperoxides. The latter acts as the peroxide tone for arachidonate metabolism in A431 cells.     

Determination of the structure of the carbohydrate chains of prostaglandin endoperoxide synthase from sheep

Prostaglandin endoperoxide synthase was isolated from sheep seminal vesicles. Sugar analysis of the glycoprotein revealed the presence of mannose and N-acetylglucosamine only. The carbohydrate moiety was released from the polypeptide backbone by hydrazinolysis. After re-N-acetylation and reduction, the resulting mixture of oligosaccharide-alditols was fractionated on Bio-Gel P-4 and their structures were investigated by 500-MHz1H-NMR spectroscopy. The carbohydrate chains turned out to be of the oligomannoside type containing six to nine mannose residues. The largest and most abundant compound was established to be: (formula; see text) For the smaller structures heterogeneity occurs with respect to the outer alpha(1----2)-linked mannose residues. Furthermore, a small amount of Man6GlcNAc-ol (artefact of the hydrazinolysis procedure) was detected by 1H-NMR spectroscopy and fast atom bombardment mass spectrometry.     

Kinetic and physicochemical characteristics of an endogenous inhibitor to progesterone--receptor binding in rat placental cytosol.

This study describes the kinetic behaviour and physicochemical aspects of an endogenous inhibitor of progesterone--receptor binding in trophoblast cytosol from day-12 embryos. The progesterone cytosol receptor was partially purified and isolated from the inhibitor as the 0--50%-satd. (NH4)2SO4 fraction. The inhibitory substance was shown to reside in the 50--70%-satd. (NH4)2SO4 fraction. Equilibration of the inhibitor preparation with the receptor fraction increased the Kapp.D of the ligand--receptor binding reaction in a concentration-dependent manner (26 +/- 3-fold increase in Kapp.D per mg of protein of the (NH4)2SO4 fraction, n = 16). However, the inhibitor did not alter the concentration of binding sites. Studies of other physicochemical aspects of the inhibitor showed it to be non-diffusible, excluded from Sephadex G-25, stable at 35 degrees C for 30 min, but irreversibly denatured at 70 degrees C for 30 min. The Stokes' radius was estimated by gel chromatography to be 2.8 +/- 0.11 nm (n = 5). Inhibitory activity was destroyed by HgCl2, suggesting that disulphide bridges play an essential role in the biological activity of this molecule. The inhibitor is a macromolecule which does not bind progesterone and differs from albumin. The kinetic mechanism by which the inhibitor enhanced Kapp.D was investigated by measuring association and dissociation rate constants and the energy of activation (Ea) for each reaction. The association rate (k+1) for progesterone and receptor was (1.3 +/- 0.2) x 10(4) M-1 . s-1 but declined to (0.4 +/- 0.1) x 10(4) M-1 . s-1 (n = 5) when exposed to the inhibitor (P less than 0.01). The dissociation rate (k-1) was (3.2 +/- 0.6) x 10(-5) s-1 for progesterone--receptor complex and was unchanged by the inhibitor. The Ea for the association of complex was 33.6 +/- 4.2 kJ/mol and was increased to 63.0 +/- 8.4 kJ/mol by the inhibitor (P less than 0.05). The Ea of dissociation was unaltered. Thus, an inhibitor is present in trophoblast cytosol which specifically enhances Kapp.D without altering availability of binding sites. The mode of action of inhibitor is to increase the energy of activation for association of complex without influencing the dissociation reaction.