A concept for the mechanism of prostacyclin and thromboxane A2 biosynthesis.
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Cytochrome P-450 Enzyme System↗
Biomedical subjects
V Ullrich
Publications and source records attributed to V Ullrich.
Thromboxane synthase. From isolation to function.
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Animals↗
Involvement of glutathione peroxidase activity in the stimulation of 5-lipoxygenase activity by glutathione-depleting agents in human polymorphonuclear leukocytes.
We recently demonstrated activation of 5-lipoxygenase activity in human polymorphonuclear leukocytes (PMN) on preincubation of the cells with glutathione-depleting agents, namely 1-chloro-2,4-dinitrobenzene (Dnp-C1) and azodicarboxylic acid bis[dimethylamide] (diamide). In this paper we show that Dnp-C1, but not diamide, impairs the reduction of added organic peroxides in whole PMN. Also, since co-incubation of fatty acid hydroperoxides with arachidonate caused activation of 5-lipoxygenase, we propose that Dnp-C1 increases the peroxide level in PMN which is required for the onset of lipoxygenase activity. This could be substantiated in PMN homogenates by a glutathione-dependent depression of arachidonate 5-lipoxygenation. At higher arachidonate concentrations and in the presence of Ca2+ the glutathione effect was not observed but additional glutathione peroxidase also blocked this maximally stimulated 5-lipoxygenase. Together with other experiments, it became obvious that the formation of leukotrienes, but also of 15-lipoxygenase products, requires a sharply defined threshold level of fatty acid hydroperoxides which are generated by the lipoxygenases and counteracted by glutathione-dependent peroxidase(s). Dnp-C1 influences this equilibrium by removing glutathione and thereby inhibiting glutathione-dependent peroxidase activity. From our data we conclude that it is the physiological function of the peroxidase activity in PMN to determine an efficiently regulated threshold level of hydroperoxide products, below which no activation of 5-lipoxygenase or 15-lipoxygenase can occur.
Arachidonate 15-Lipoxygenase↗
The sulfhydryl reagent thimerosal elicits human platelet aggregation by mobilization of intracellular calcium and secondary prostaglandin endoperoxide formation.
The effect of the sulfhydryl (SH) group inhibitor ethylmercurithiosalicylate (thimerosal) on the function of human platelets was investigated. In contrast to known SH reagents such as p-chloromercuribenzoate or N-ethylmaleimide, thimerosal elicited both aggregation and [3H]serotonin release of washed human platelets at low micromolar concentrations (greater than or equal to 2 microM). Only a significant higher dose (greater than or equal to 15 microM) was effective when platelets were pretreated with the cyclooxygenase inhibitor aspirin, indicating an amplification of the proaggregatory effect of thimerosal by secondary prostaglandin (PG) endoperoxide and/or thromboxane (TX) formation. Consistent with this notion, thimerosal induced endogenous platelet arachidonic acid (20:4) metabolism which could be attributed to enhanced 20:4 liberation, presumably by activation of phospholipase A2. The latter effect was mediated by mobilization of intracellular calcium (Ca2+), and was not affected by removal of extracellular Ca2+. In the presence of aspirin, the thimerosal-induced Ca2+ elevation was completely reversed by dithiothreitol (DTT) which implicates SH groups in intracellular Ca2+ transport. In contrast to previous observations with other SH reagents, thimerosal had no effect on the inositoltrisphosphate (IP3)-mediated release or the sequestration (and/or extrusion) of intracellular Ca2+ following stimulation with thrombin, indicating an action on an as yet undefined CA2+ transport system.
Aspirin↗
On the mechanism of prostacyclin and thromboxane A2 biosynthesis.
The present research describes studies which address the mechanism of prostacyclin (PGI2) and thromboxane A2 (TXA2) biosynthesis. In addition to prostaglandin H1 (PGH1), PGG2, PGH2, and PGH3, also 8-iso-PGH2, 13(S)-hydroxy-PGH2, and 15-keto-PGH2 were applied to determine the substrate specificities and kinetics of prostacyclin and thromboxane synthase in more detail. Human platelet thromboxane synthase converted PGH1, 8-iso-PGH2, 13(S)-hydroxy-PGH2 and 15-keto-PGH2 into the corresponding heptadecanoic acid (C17) plus malondialdehyde, whereas the thromboxane derivative was formed only from PGG2, PGH2, and PGH3 together with the corresponding C17 metabolite and malondialdehyde in a 1:1:1 ratio. In contrast, PGG2, PGH2, 13(S)-hydroxy-PGH2, 15-keto-PGH2 and PGH3 were almost completely isomerized to the corresponding prostacyclin derivative by bovine aortic prostacyclin synthase, whereas PGH1 and 8-iso-PGH2 only produced the corresponding C17 hydroxy acid plus malondialdehyde. Isotope-labeling experiments with [5,6,8,9,11,12,14,15-2H]PGH2 revealed complete retention of label and no isotope effect in the course of thromboxane biosynthesis, but the loss of one 2H atom at C-6 with an isotope effect of 1.20 during PGI2 formation. Prostacyclin and thromboxane synthase bind both 9,11-epoxymethano-PGF2 alpha and 11,9-epoxymethano-PGF2 alpha at the heme iron, but according to their difference spectra in opposite ways with respect to the 9- and 11-position. In agreement with published model studies, a cage radical mechanism is proposed for both enzymes according to which the initial radical process is terminated through oxidation of carbon-centered radicals by the iron-sulfur catalytic site, followed by ionic rearrangement to PGI2 or TXA2. Various Fe(III) model compounds as well as liver microsomes or cytochrome P-450CAM can also form small amounts of PGI2 and TXA2, but mainly yield 12(S)-hydroxy-5,8,10-heptadecatrienoic acid plus malondialdehyde probably by a radical fragmentation pathway.
Animals↗
Physiological targets of superoxide anion and hydrogen peroxide in reperfusion injury.
Current dogma associates reperfusion injury with the introduction of reactive oxygen species (ROS) into the ischemic tissue. The sources of ROS under discussion are xanthine oxidase in the endothelium of small vessels and/or invaded polymorphonuclear leukocytes (PMN). The beneficial effects of both superoxide dismutase and catalase suggest an involvement of superoxide anions and hydrogen peroxide in this pathophysiological process, without describing the targets of their action. In our work we demonstrate that these two ROS effectively interact with two enzymes. Superoxide anions inhibit soluble guanylate cyclase. Its product, cGMP, is considered to antagonize platelet activation and to cause smooth muscle relaxation. Thus O2- can intensify platelet aggregability and small vessel occlusion. Similar effects are elicited by H2O2, which shifts the dose response curve of several agonists towards smaller concentrations by activating cyclooxygenase. This enzyme provides the substrate for thromboxane synthase which generates TxA2, the most potent physiologically occurring platelet aggregating and smooth muscle contacting agonist. These results lead us to the suggestion that the influence of the oxidative burst of PMN in the phenomenon of reperfusion injury should be reconsidered.
Arachidonic Acid↗
Studies on the interaction of minoxidil with prostacyclin synthase in vitro.
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Animals↗
The omega-hydroxylation of arachidonic acid by human polymorphonuclear leukocytes.
Incubations of [1-14C]arachidonic acid with unstimulated human polymorphonuclear leukocytes resulted in the formation of four new metabolites in a previously described reverse-phase HPLC system. Three of these metabolites were largely suppressed in a CO/O2 (80/20, by vol.) atmosphere indicating a cytochrome-P450-dependent monooxygenase reaction. In agreement with this assumption is their NADPH/O2-dependent formation in the microsomal fraction. One metabolite was identified by gas chromatography/mass spectrometry analysis as omega-hydroxy-arachidonic acid and the two others were secondary products identified as omega-carboxy-arachidonic acid and 5,20-dihydroxy-E,Z,Z,Z-6,8,11,14-eicosatetraenoic acid. Since the affinity for arachidonate of the omega-monooxygenase was quite low and the presence of LTB4 suppressed the omega-hydroxylation of arachidonate, we conclude that the known LTB4 omega-monooxygenase is responsible for the formation of omega-hydroxy-arachidonate. It is unlikely, however, that significant concentrations of these metabolites are formed by activated polymorphonuclear leukocytes in vivo. The fourth metabolite remains tightly associated with the leukocytes but has not been further characterized.
Arachidonic Acid↗
The suppression of granulocyte functions by lipophilic antioxidants.
The effects of several antioxidants on the three major functions of human neutrophils--oxidative burst, secretion and leukotriene formation--were investigated with special emphasis on the lipophilicity. The most striking differences were obtained when ascorbate and the lipophilic ester ascorbyl palmitate were compared. As expected, the luminol- and lucigenin-dependent chemiluminescence was inhibited by all antioxidants to a different degree. Ascorbyl palmitate was able to block the biphasic luminol-dependent response completely with IC50 values of 10 and 25 microM for the first and second phase, respectively. In contrast, ascorbate only blocked efficiently the first phase of the response. The secretion of elastase was inhibited by ascorbyl palmitate dose-dependently with an IC50 value of around 200 microM, whereas ascorbate was completely inactive. Electron microscopy supported the assumption that inhibition was due to a block in degranulation and not to enzyme inactivation. This was further supported by a parallel, although somewhat lower, inhibition of other secretory enzymes like myeloperoxidase, beta-glucuronidase or lysozyme. Cells treated with the Ca2+-ionophore A23187 responded by LTB4-synthesis which was also inhibited by ascorbyl palmitate. A very efficient inhibition was observed in cell homogenates with an IC50 value of 1.5 microM. No inhibition by ascorbate was detected in both systems. Concomitant with the inhibition of 5-lipoxygenase the activity of 15-lipoxygenase increased. We conclude that cellular reductants may control neutrophil functions and that the inhibition by ascorbyl palmitate of the three processes relevant for inflammatory responses could be of therapeutic importance.
Acridines↗
12(S)-Hydroxy-5,8,10 (Z,E,E)-heptadecatrienoic acid (HHT) is preferentially metabolized to its 12-keto derivative by human erythrocytes in vitro.
The metabolism of [1-14C]-labelled 12 (S)-hydroxy-5,8,10 (Z,E,E)-heptadecatrienoic acid (HHT) by crude 15-hydroxyprostaglandin dehydrogenase (PGDH) fractions from swine kidney and human erythrocytes has been investigated. HPLC radiochromatography analysis revealed that HHT was extensively converted into three metabolites by swine kidney cytosol in the presence of NAD+. They were identified by combined GLC mass spectrometry as 12-keto-5,8,10 (Z,E,E)-heptadecatrienoic acid (KHT), 12-keto-5,8 (Z,E)-heptadecadienoic acid and 12 (RS)-hydroxy-5,8 (Z,E)-heptadecadienoic acid, respectively. In contrast, HHT was metabolized only to the 12-keto derivative by human erythrocyte cytosol supplemented with NADP+, and HHT turnover was found to be enhanced severalfold when compared to prostaglandins E2 (PGE2) or F2 alpha. Since PGE2 was also converted only into 15-keto-PGE2, and no metabolism of KHT was detected with NADPH, there is probably no 15-ketoprostaglandin delta 13-reductase activity in human erythrocytes. Biosynthetic KHT (0.5-5 microM) inhibited the aggregation of human platelets to almost all agonists, probably by raising intracellular cAMP. KHT (between 0.01 and 1 microM) also induced the chemotaxis of human polymorphonuclear leukocytes. Among other still unrecognized effects, these biological activities of KHT may be of physiological significance with respect to its presumably exclusive formation in the blood. The potential use of KHT for monitoring thromboxane synthase activity in vivo is discussed.
Animals↗
Thromboxane synthase catalyses hydroxylations of prostaglandin H2 analogs in the presence of iodosylbenzene.
Human platelet microsomes supplemented with iodosylbenzene converted the stable prostaglandin H2 analog 15(S)-hydroxy-11 alpha,9 alpha-epoxymethano-5(Z),13(E)-prostadienoic acid (U46619) into three metabolites (17.5% yield) which were not formed in the presence of specific thromboxane synthase inhibitors. The same three products were also formed among others by incubation of U46619 with liver microsomes from phenobarbital-pretreated rats with NADPH/O2 or with iodosylbenzene. The NADPH-supported metabolism of U46619 was suppressed in the presence of carbon monoxide. Combined gas chromatography/negative-ion chemical ionization mass spectrometry analysis revealed for all three compounds the incorporation of one oxygen atom which according to the electron impact fragmentation pattern had to be introduced either at the 9-methylene group or at the cyclopentane ring. The identification of these metabolites as 9 beta,15(S)-dihydroxy-11 alpha,9 alpha-epoxymethano-5(Z),13(E)-prostadienoic acid and the R and S isomer of 15(S)-hydroxy-11 alpha,9 alpha-(C-hydroxy-epoxymethano)-5(Z),13(E)-prostadienoic acid is only tentative since no reference compounds were available, but clearly thromboxane synthase was acting as an oxene transferase in this reaction. In contrast to U46619, its 9,11-epoxymethano isomer U44069 was found to be only a poor substrate for the oxene transferase activity of thromboxane synthase (1% yield) which indicates a preference for the 9-methylene group of U46619 which is orientated close to the heme iron of thromboxane synthase as evidenced by spectroscopic studies. Low-level chemiluminescence detected following incubation of iodosylbenzene with partly purified thromboxane synthase is in agreement with the formation of an activated (FeO)3+ oxygen species. In summary, these results point to a common role of the thiolate ligand in the oxygen activation mechanism by thromboxane and prostacyclin synthase and liver cytochrome P-450 monooxygenases.
Blood Platelets↗
Identification of novel arachidonic acid metabolites formed by prostaglandin H synthase.
The metabolism of [1-14C]arachidonic acid by microsomal and purified prostaglandin (PG) H synthase was investigated. HPLC analysis confirmed that arachidonic acid (20:4) was extensively converted into prostaglandin G2 (PGG2) and/or prostaglandin H2 (PGH2) but several minor labelled products were formed in addition. Their formation, mediated by PGH synthase was established by inhibition with aspirin and indomethacin [Hecker, M., Hatzelmann, A. & Ullrich, V. (1987) Biochem. Pharmacol. 36, 851-855]. Upon comparison with authentic reference material these unknown PGH synthase metabolites were identified with respect to chromatographic properties, ultraviolet spectroscopy and mass spectrometry as 11 (R)-hydroperoxy-5Z,8Z,12E,14Z-eicosatetraenoic acid (11-OOH-20:4), 12(S)-hydroperoxy-5Z,8E,10E-heptadecatrienoic acid (OOH-17:3), 12(S)-hydroxy-5Z,8E,10E-heptadecatrienoic acid (OH-17:3), 15(RS)-hydroperoxy-5Z,8Z,11Z,13E-eicosatetraenoi c acid (15-OOH-20:4), 15(RS)-hydroxy-5Z,8Z,11Z,13E-eicosatetraenoic acid (15-OH-20:4), 13-hydroxy-5Z,14Z-prostaglandin H2, 15(S)-hydroxy-8-iso-5Z,13E-prostaglandin H2 and 15-oxo-prostaglandin H2. Unlike PGG2 and PGH2, 8-iso-PGH2, 13-hydroxy-PGH2 and 15-oxo-PGH2 failed to induce aggregation of washed human platelets and to form thromboxane upon incubation with homogeneous human platelet thromboxane synthase. In contrast to the formation of OOH-17:3, 15-oxo-PGH2 and OH-17:3 which can be attributed to the heme-catalyzed decomposition of PGG2 and PGH2, 11-OOH-20:4,15-(O)OH-20:4-,8 iso-PGH2 and 13-hydroxy-PGH2 represent potential side products of arachidonic acid conversion into PG endoperoxides. Their formation allows to conclude on PGH synthase mechanism and its intermediates for which an extended reaction scheme is proposed.
Animals↗
Regulation of 5-lipoxygenase activity by the glutathione status in human polymorphonuclear leukocytes.
The influence of the glutathione status of human polymorphonuclear leukocytes (PMN) on 5-lipoxygenase activity was studied by treating cells with increasing concentrations of 1-chloro-2,4-dinitrobenzene (Dnp-Cl) or azodicarboxylic acid bis(dimethylamide) (Diamide). Subsequent incubation with arachidonate resulted in an up to tenfold-stimulated formation of 5-hydroxyeicosatetraenoic acid, leukotriene B4, leukotriene B4 isomers and omega-hydroxyleukotriene B4. Higher concentrations of the GSH reagents were inhibitory. At maximal stimulation by Dnp-Cl, 5-hydroperoxyeicosatetraenoic acid started to be built up at the expense of 5-HETE at glutathione levels which were diminished by about 50% compared to resting cells. No increase in cytosolic Ca2+ could be measured under these conditions by the fura-2 method. In PMN homogenates Dnp-Cl and Diamide were without effect and even caused inhibition when 5-lipoxygenase was stimulated by Ca2+ and ATP. 15-Lipoxygenase was either unchanged in the case of Diamide, or even increased after pretreatment with Dnp-Cl. The results allow us to conclude that 5-lipoxygenase activity in intact PMN is regulated not only by Ca2+ but in a complex manner also by the glutathione redox status. Conditions of oxidative stress increase the activity which may reflect the in vivo situation under phagocytosis and oxidative burst.
Adult↗
Preparative HPLC purification of prostaglandin endoperoxides and isolation of novel cyclooxygenase-derived arachidonic acid metabolites.
A preparative HPLC purification scheme for the isolation of prostaglandin endoperoxides prepared by short-time incubation of [1-14C]-labelled arachidonic acid (AA) with sheep seminal vesicle microsomes was developed. Milligram quantities of prostaglandin G2 (PGG2) and prostaglandin H2 (PGH2) were obtained in greater than or equal to 95% purity within shortest time. Furthermore, careful application of this HPLC technique led to the isolation of two minor [1-14C]-labelled fractions which according to their spectral and chromatographic characteristics, were identical with 15(S)-hydroperoxy-5,8,11,13-eicosatetraenoic acid (15-HPETE) and 15(S)-hydroxy-5,8,11,13-eicosatetraenoic acid (15-HETE). Another HETE substituted at either C11 or C12 was also present. The formation of these products was mediated by cyclooxygenase as evidenced by aspirin (100 microM) and indomethacin (10 microM) inhibition. Sulfhydryl-blocking agents such as p-hydroxymercuribenzoate (1 mM) and/or the 12-lipoxygenase inhibitor esculetin (100 microM) were without effect. In addition to these AA metabolites four other fractions contained arachidonate-derived endoperoxides with antiaggregatory properties, all of which released malondialdehyde upon incubation with thromboxane A2 synthase. No thromboxane formation was observed although turnover numbers were comparable to those of PGG2 and PGH2. The formation of these endoperoxides did not occur via enzymatic or non-enzymatic degradation of PGG2 or PGH2. The exact chemical nature of these endoperoxides remains to be established.
Animals↗
Products, kinetics, and substrate specificity of homogeneous thromboxane synthase from human platelets: development of a novel enzyme assay.
Homogeneous thromboxane synthase from human platelets converted prostaglandin H2 (PGH2) to thromboxane A2 (measured as thromboxane B2, TxB2), 12(L)-hydroxy-5,8,10-heptadecatrienoic acid (HHT), and malondialdehyde (MDA) in equimolar amounts under a variety of experimental conditions. PGG2 was transformed to MDA and corresponding 15- and 12-hydroperoxy products. PGH1 was enzymatically transformed into 12(L)-hydroxy-8,10-heptadecadienoic acid (HHD) and PGH3 into TxB3 and 12(L)-hydroxy-5,8,10,14-heptadecatetraenoic acid (delta 14-HHT) as earlier reported for solubilized and partially purified thromboxane synthase preparations. The ratio of thromboxane to C17 hydroxy fatty acid formation was 1:1 with PGG2, PGH2, and PGH3 as substrates. These results confirm and extend earlier observations with partially purified enzyme that the three products are formed in a common enzymatic pathway (Diczfalusy, U., Falardeau, P., and Hammarström, S. (1977) FEBS Lett. 84, 271-274). A convenient spectrophotometric assay for thromboxane synthase activity measuring the ultraviolet light absorption of the C17 hydroxy acid formed (e.g., HHT) was developed. The validity of the assay was determined employing specific inhibitors for thromboxane synthase. The substrate specificity of thromboxane synthase was determined using this assay. PGG2 and PGH3 showed Vmax and KM values similar to those of PGH2. The KM value of PGH1 was also identical to that of PGH2 but the Vmax value PGH1 was more than twice as high as that of PGH2.
Blood Platelets↗
Inhibition of platelet aggregation by carbon monoxide is mediated by activation of guanylate cyclase.
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.
15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5↗
Spectral studies on structure-activity relationships of thromboxane synthase inhibitors.
Thromboxane A2 synthase is a cytochrome P450-type enzyme and its interaction with imidazole or pyridine-based inhibitors could be studied by absolute and difference spectroscopy with the solubilized as well as the purified enzyme. Nitrogenous bases shift the 418-nm Soret absorption by 4-6 nm to the red and among them the best inhibitors of enzyme activity showed a stoichiometric binding to the enzyme. The structural and energetic prerequisites for such high binding affinities were primarily the liganding of the basic nitrogen to the hemin but also the attachment of a hydrophobic carboxylic side chain to the active site at an about 1 nm distance from the nitrogen. In addition, the side chain seemed to be oriented almost parallel to the plane of the heme. If this geometry was changed, a decrease in affinity was observed and if the ligand binding was sterically hindered, a spectral shift to a five-coordinated complex absorbing at 390 nm occurred. This is best explained by the displacement of an endogenous oxygen ligand, presumably water, from the sixth coordination position of the heme. From these results it can be concluded that the inhibitors mimic the binding of prostaglandin H2 (PGH2) with its carboxylic group at the carboxyl side chain and the endoperoxide oxygen atom at C9 as previously reported. The methyl side chain of PGH2 does not seem to play a role in the formation of the enzyme-substrate complex.
Blood Platelets↗
Isolation and characterization of thromboxane synthase from human platelets as a cytochrome P-450 enzyme.
Thromboxane synthase from human platelets was purified to apparent homogeneity by conventional chromatographic techniques. A 423-fold enrichment over the specific content in the 100,000 X g sediment from platelet homogenates was obtained. The enzyme gave a single band on sodium dodecyl sulfate-gel electrophoresis corresponding to a monomeric molecular weight of 58,800. One heme per polypeptide chain was present, and by optical and EPR spectroscopy a close analogy to the group of cytochrome P-450 proteins was established. From its substrate prostaglandin H2, the stable end product thromboxane B2 is formed with a specific activity of 24.1 mumol min-1 mg of protein-1 which corresponds to a molecular activity of 1628 min-1. The enzyme formed 12L-hydroxy-5,8,10-heptadecatrienoic acid together with thromboxane B2 in a 1:1 ratio. Both products were identified by gas chromatography-mass spectrometry analysis. As reported previously for platelet microsomes (Ullrich, V., and Haurand, M. (1983) Adv. Prostaglandin Thromboxane Leukotriene Res. 11, 105-110), the pure hemoprotein spectrally interacts with pyridine- or imidazole-based inhibitors and for the potent inhibitor imidazo-(1,5-a)pyridine-5-hexanoic acid a stoichiometric binding to the heme was shown. Substrate analogs with a methylene group replacing the oxygen in either the 9- or 11-position caused difference spectra showing spectral shifts towards 387 and 407 nm, respectively. The identification of thromboxane synthase as a P-450 protein suggests that the heme-thiolate group of the enzyme is required to split and activate the endoperoxide bond of prostaglandin H2.
Blood Platelets↗