On the haemoprotein character of prostaglandin endoperoxide synthetase

The Protective Role of Carbon Monoxide (CO) Produced by Heme Oxygenases and Derived from the CO-Releasing Molecule CORM-2 in the Pathogenesis of Stress-Induced Gastric Lesions: Evidence for Non-Involvement of Nitric Oxide (NO)

Carbon monoxide (CO) produced by heme oxygenase (HO)-1 and HO-2 or released from the CO-donor, tricarbonyldichlororuthenium (II) dimer (CORM-2) causes vasodilation, with unknown efficacy against stress-induced gastric lesions. We studied whether pretreatment with CORM-2 (0.1-10 mg/kg oral gavage (i.g.)), RuCl₃ (1 mg/kg i.g.), zinc protoporphyrin IX (ZnPP) (10 mg/kg intraperitoneally (i.p.)), hemin (1-10 mg/kg i.g.) and CORM-2 (1 mg/kg i.g.) combined with N(G)-nitro-l-arginine (l-NNA, 20 mg/kg i.p.), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 10 mg/kg i.p.), indomethacin (5 mg/kg i.p.), SC-560 (5 mg/kg i.g.), and celecoxib (10 mg/kg i.g.) affects gastric lesions following 3.5 h of water immersion and restraint stress (WRS). Gastric blood flow (GBF), the number of gastric lesions and gastric CO and nitric oxide (NO) contents, blood carboxyhemoglobin (COHb) level and the gastric expression of HO-1, HO-2, hypoxia inducible factor 1α (HIF-1α), tumor necrosis factor α (TNF-α), cyclooxygenase (COX)-2 and inducible NO synthase (iNOS) were determined. CORM-2 (1 mg/kg i.g.) and hemin (10 mg/kg i.g.) significantly decreased WRS lesions while increasing GBF, however, RuCl₃ was ineffective. The impact of CORM-2 was reversed by ZnPP, ODQ, indomethacin, SC-560 and celecoxib, but not by l-NNA. CORM-2 decreased NO and increased HO-1 expression and CO and COHb content, downregulated HIF-1α, as well as WRS-elevated COX-2 and iNOS mRNAs. Gastroprotection by CORM-2 and HO depends upon CO's hyperemic and anti-inflammatory properties, but is independent of NO.

Macrophages and Iron Metabolism

Iron is a transition metal that due to its inherent ability to exchange electrons with a variety of molecules is essential to support life. In mammals, iron exists mostly in the form of heme, enclosed within an organic protoporphyrin ring and functioning primarily as a prosthetic group in proteins. Paradoxically, free iron also has the potential to become cytotoxic when electron exchange with oxygen is unrestricted and catalyzes the production of reactive oxygen species. These biological properties demand that iron metabolism is tightly regulated such that iron is available for core biological functions while preventing its cytotoxic effects. Macrophages play a central role in establishing this delicate balance. Here, we review the impact of macrophages on heme-iron metabolism and, reciprocally, how heme-iron modulates macrophage function.

Substrate-dependent irreversible inactivation of cytochrome P-450: conversion of its haem moiety into modified porphyrins

2-Allyl-2-isopropylacetamide and other drugs containing either an allyl, a vinyl or an ethynyl unsaturated side chain are metabolized by liver cytochrome P-450 to reactive derivatives that irreversibly inhibit the haemoprotein by a suicidal type of inactivation. The main target is the haem moiety of cytochrome P-450 which is converted into abnormal porphyrins. These have been isolated from the liver of treated rats, extensively purified and compared with model porphyrins. The abnormal porphyrins incorporate metal ions in vitro much more readily than does their parent porphyrin, protoporphyrin. They are also much more basic than protoporphyrin, and on titration with a strong acid they readily give rise to a porphyrin monocation which then requires relatively large amounts of acid for conversion to the porphyrin dication. In all these respects and also in the intensity of their bathochromic shifts these abnormal porphyrins closely resemble N-alkylated porphyrins and they markedly differ from porphyrins that are substituted at one of their meso-carbon positions or which bear electron-withdrawing substituents at the beta-positions of the pyrrole rings. This suggests strongly that reactive derivatives of the unsaturated drugs act as electrophilic reagents and alkylate one of the pyrrole nitrogen atoms of cytochrome P-450. A model centred on the apoprotein of cytochrome P-450 is considered for the degradation of liver haem caused by unsaturated drugs. The apocytochrome may accept exchangeable pools of liver haem for degradation, leading to a state of haem depletion and to activation of delta-aminolaevulinate synthase.

Characterization of the heme environment in Arabidopsis thaliana fatty acid alpha-dioxygenase-1

Plant alpha-dioxygenases (PADOX) are hemoproteins in the myeloperoxidase family. We have used a variety of spectroscopic, mutagenic, and kinetic approaches to characterize the heme environment in Arabidopsis thaliana PADOX-1. Recombinant PADOX-1 purified to homogeneity contained 1 mol of heme bound tightly but noncovalently per protein monomer. Electronic absorbance, electron paramagnetic resonance, and magnetic circular dichroism spectra showed a high spin ferric heme that could be reduced to the ferrous state by dithionite. Cyanide bound relatively weakly in the ferric PADOX-1 heme vicinity (K(d) approximately 10 mm) but did not shift the heme to the low spin state. Cyanide was a very strong inhibitor of the fatty acid oxygenase activity (K(i) approximately 5 microm) and increased the K(m) value for oxygen but not that for fatty acid. Spectroscopic analyses indicated that carbon monoxide, azide, imidazole, and a variety of substituted imidazoles did not bind appreciably in the ferric PADOX-1 heme vicinity. Substitution of His-163 and His-389 with cysteine, glutamine, tyrosine, or methionine resulted in variable degrees of perturbation of the heme absorbance spectrum and oxygenase activity, consistent with His-389 serving as the proximal heme ligand and indicating that the heme has a functional role in catalysis. Overall, A. thaliana PADOX-1 resembles a b-type cytochrome, although with much more restricted access to the distal face of the heme than seen in most other myeloperoxidase family members, explaining the previously puzzling lack of peroxidase activity in the plant protein. PADOX-1 is unusual in that it has a high affinity, inhibitory cyanide-binding site distinct from the distal heme face and the fatty acid site.

Role of human heme oxygenase-1 in attenuating TNF-alpha-mediated inflammation injury in endothelial cells

Heme oxygenase (HO) is the rate-limiting enzyme in the formation of bilirubin, an antioxidant, and carbon monoxide (CO), a cell cycle modulator and a vasodilator. Cyclooxygenase (COX) is a hemeprotein that catalyzes the conversion of arachidonic acid (AA) to various prostanoids, which play an important role in the regulation of vascular endothelial function in normal and disease states. The influence of suppression or overexpression of HO isoforms on COX expression and synthesis of prostanoids is of considerable physiological importance. Consequently, the goal of the present study was to determine whether the heme-HO system regulates COX enzyme expression and activity in vascular endothelial cells in the absence and presence of TNF-alpha (100 ng/ml). Endothelial cells stably transfected with the retrovirus containing the human HO-1 gene exhibited a several-fold increase in HO-1 protein levels, which was accompanied by an increase in HO activity and a marked decrease in PGE(2) and 6-keto PGF(1alpha) levels. We also assessed the effect of retrovirus-mediated HO-1 gene transfer in the sense and antisense orientation on HO-1 expression and cell cycle progression in human endothelial cells. The levels of CO and HO activity were increased in cells transduced with the HO-1 sense and were greatly suppressed in cells transduced with HO-1 antisense as compared to control sham-transduced cells (P < 0.05). The percentage of the G(1)-phase in cells transduced with HO-1 significantly increased (41.4% +/- 9.1) compared with control endothelial cells (34.8% +/- 4.9). We measured COX activity by determining the levels of PGI(2) and PGE(2). The levels of PGI(2) decreased in cells transduced with HO-1 sense and increased in cells transduced with HO-1 in antisense orientation. The expression of p27 was also studied and showed a marked decrease in cells transduced with HO-1 sense and a marked increase in the HO-1 antisense transduced cells. Cell cycle analysis of endothelial cell DNA distributions indicated that the TNF-alpha-induced decrease in the proportion of G(1)-phase cells and increase in apoptotic cells in control cultures could be abrogated by transfection with HO-1 in the sense orientation. Tin mesoporphyrin (SnMP) reversed the protective effect of HO-1. These results demonstrate that overexpressing HO-1 mitigated the TNF-alpha-mediated changes in cell cycle progression and apoptosis, perhaps by a decrease in the levels of COX activity.

Cyclooxygenase-2 expression in canine mammary tumors

Mammary tumors are the most common neoplasms in female dogs. Induction of cyclooxygenase-2 (COX-2) has been implicated in various cancers in humans. However, expression of COX-2 has not been investigated in canine mammary tumors. Normal mammary gland (n = 4), simple or complex adenomas (n = 63), and simple or complex adenocarcinomas (n = 84) were studied by immunohistochemistry. Results showed that COX-2 was not expressed in the normal gland but was detected in 24% of adenomas and in 56% of adenocarcinomas (P < 0.001). The incidence of COX-2 expression and the intensity of the COX-2 signal were higher in adenocarcinomas than in adenomas (P < 0.001). These results demonstrate for the first time that COX-2 is induced in a proportion of canine mammary tumors and that COX-2 expression is more frequent and more intense in malignant than in benign tumors, suggesting a potential role for COX-2 in canine mammary tumorigenesis.

Induction of cyclo-oxygenase-2 expression in naturally occurring gastric ulcers

Cyclo-oxygenase-2 (COX-2) is believed to participate in the repair of gastric ulcer. Like humans, pigs frequently develop gastric ulcers and thus represent an attractive animal model in which to study the repair process of naturally occurring gastric ulcers. However, expression of COX in the pig stomach has not been reported. The objectives of this study were to determine whether COX isoenzymes are expressed in porcine gastric ulcers and to characterize the porcine COX-2 cDNA. Normal stomachs (n=5) and those with gastric ulcers (n=35) were studied by immunohistochemistry and immunoblotting analysis. Reverse transcription-polymerase chain reaction (RT-PCR) was used to isolate the complete porcine COX-2 cDNA. COX-1 staining was present in normal stomach and in ulcerated areas. No COX-2 was detected in normal stomach, but COX-2 was strongly expressed in the ulcerated area in 28/35 (80%) gastric ulcers (p<0.01). Immunoblotting analysis confirmed the restricted expression of COX-2 in the ulcerated areas. The porcine COX-2 cDNA was shown to code for a 604 amino acid protein that is 89% identical to human COX-2. These results provide the complete primary structure of porcine COX-2 and demonstrate for the first time that the enzyme is induced in naturally occurring porcine gastric ulcers.

Catalytic properties of rice alpha-oxygenase. A comparison with mammalian prostaglandin H synthases

Long-chain fatty acids can be metabolized to C(n)(-1) aldehydes by alpha-oxidation in plants. The reaction mechanism of the enzyme has not been elucidated. In this study, a complete nucleotide sequence of fatty acid alpha-oxygenase gene in rice plants (Oryza sativa) was isolated. The deduced amino acid sequence showed some similarity with those of mammalian prostaglandin H synthases (PGHSs). The gene was expressed in Escherichia coli and purified to apparently homogeneous state. It showed the highest activity with linoleic acid and predominantly formed 2-hydroperoxide of the fatty acid (C(n)), which is then spontaneously decarboxylated to form corresponding C(n)(-1) aldehyde. With linoleic or linoleic acids as a substrate, rice alpha-oxygenase formed no product having a lambda(max) at approximately 234 nm, which indicated that the enzyme could not oxygenize the pentadiene system in the substrate. The spectroscopic feature of the purified enzyme in its ferrous state is similar to that of mammalian PGHS, whereas that of dithionite-reduced state showed significant difference. Site-directed mutagenesis revealed that His-158, Tyr-380, and Ser-558 were essential for the alpha-oxygenase activity. These residues are conserved in PGHS and known as a heme ligand, a source of a radical species to initiate oxygenation reaction and a residue involved in substrate binding, respectively. This finding suggested that the initial step of the oxygenation reaction in alpha-oxygenase has a high similarity with that of PGHS. The rice alpha-oxygenase activity was inhibited by imidazole but hardly inhibited by nonsteroidal anti-inflammatory drugs, such as aspirin, ibuprofen, and flurbiprofen, which are known as typical PGHS inhibitors. In addition, peroxidase activity could not be detected with alpha-oxygenase when palmitic acid 2-hydroperoxide was used as a substrate. From these findings, the catalytic resemblance between alpha-oxygenase and PGHS seems to be evident, although there still are differences in their substrate recognitions and peroxidation activities.

Regulation of cyclooxygenase by the heme-heme oxygenase system in microvessel endothelial cells

Heme oxygenase (HO) is a microsomal enzyme that oxidatively cleaves heme to form biliverdin, with the release of iron and carbon monoxide (CO). HO not only controls the availability of heme for the synthesis of heme proteins but also is responsible for the generation of CO, which binds to the heme moiety of heme proteins thus affecting their enzymatic activity. Cyclooxygenase (COX) is a heme protein that catalyzes the conversion of arachidonic acid to prostaglandin H(2), the precursor of prostanoids that participate in the regulation of vascular function. The goal of the present study was to determine whether the heme-HO system regulates COX enzyme expression and activity in vascular endothelial cells. Endothelial cells stably transfected with the human HO-1 gene exhibited a severalfold increase in human HO-1 mRNA levels, which was accompanied by an increase in HO activity and a marked decrease in prostaglandin (PG) E(2) and 6-keto-PGF(1alpha) levels. Exposure of cells to CoCl(2), an inducer of HO-1 gene expression, resulted in increases in HO-1 protein levels and HO activity. The increase in HO activity was associated with a subsequent decrease in COX activity, which returned to normal levels following normalization of HO activity. The addition of heme resulted in an increase in COX activity with an increase in PGE(2) and 6-keto-PGF(1alpha) levels. The degree of HO-1 expression and, consequently, the level of cellular heme, were directly related to COX activity. These results demonstrate that the heme-HO system can function as a cellular regulator of the expression of vascular COX, thus influencing the generation of prostanoids, PGE(2) and PGI(2), known to play a role in vascular homeostasis.

Effects of heme oxygenase system on the cyclooxygenase in the primary cultured hypothalamic cells

Endogenous carbon monoxide (CO) shares with nitric oxide (NO) a role as a putative neural messenger in the brain. Both gases are believed to modulate CNS function via an increase in cytoplasmic cGMP concentrations secondary to the activation of soluble guanylate cyclase (sGC). Recently CO and NO were proposed as a possible mediator of febrile response in hypothalamus. NO has been reported to activate both the constitutive and inducible isoform of the cyclooxygenase (COX). Thus, we investigated whether CO arising from heme catabolism by heme oxygenase (HO) is involved in the febrile response via the activation of COX in the hypothalamus. PGE2 which is a final mediator of febrile response released from primary cultured hypothalamic cells was taken as a marker of COX activity. PGE2 concentration was measured with EIA kits. Exogenous CO (CO-saturated medium) and hemin (a substrate and potent inducer of HO) evoked an increase in PGE2 release from hypothalamic cells, and these effects were blocked by methylene blue (an inhibitor of sGC). And membrane permeable cGMP analogue, dibutyryl-cGMP elicited significant increases in PGE2 release. These results suggest that there may be a functional link between HO and COX enzymatic activities. The gaseous product of hemin through the HO pathway, CO, might play a role through the modulation of the COX activity in the hypothalamus.

Expression of cyclo-oxygenase-2 in naturally occurring squamous cell carcinomas in dogs

Squamous cell carcinoma is one of the most common cancers in humans and is also a frequently diagnosed neoplasm in dogs. Induction of cyclo-oxygenase-2 (COX-2), a key rate-limiting enzyme in prostaglandin biosynthesis, has been implicated in the oncogenesis of various cancers in humans, including squamous cell carcinomas. However, expression of COX-2 has not been reported in spontaneous squamous cell carcinomas of non-human species. Canine squamous cell carcinomas share several similarities with the human disease. Therefore, the objective of this study was to determine whether COX isoenzymes were expressed in naturally occurring cases of squamous cell carcinomas in dogs. Canine normal skin (n=4) and squamous cell carcinomas (n=40) were studied by immunohistochemistry and immunoblotting analysis using polyclonal antibodies selective for COX-1 or COX-2. COX-2 was strongly expressed by neoplastic keratinocytes in all cases of squamous cell carcinomas, whereas no COX-2 was detected in normal skin and in the non-neoplastic skin and oral mucosa included in the tumor tissue samples (p<0.01). Immunoblotting analysis confirmed the restricted expression of COX-2 (72,000--74,000 molecular weight doublet) in squamous cell carcinomas only. In contrast, faint COX-1 staining was found in normal skin and in squamous cell carcinomas. This study demonstrates for the first time that COX-2 is induced in canine squamous cell carcinomas, and provides a new model to investigate the role and regulation of COX-2 gene expression in naturally occurring squamous cell carcinomas. (J Histochem Cytochem 49:867-875, 2001)

Molecular characterization of bovine prostaglandin G/H synthase-2 and regulation in uterine stromal cells

Prostaglandin G/H synthase (PGHS) is a key rate-limiting enzyme in the prostaglandin biosynthetic pathway, and prostaglandins play a central role in the control of the reproductive cycle. The objectives of this study were to clone and characterize the primary structure of bovine PGHS-2 and to study its regulation in uterine stromal cells in vitro. The bovine PGHS-2 cDNA was cloned by a combination of reverse transcription-polymerase chain reaction and cDNA library screening. Results showed that the complete bovine PGHS-2 cDNA is composed of a 5'-untranslated region of 128 bp, an open reading frame of 1815 bp, and a 3'-untranslated region of 1565 bp containing multiple repeats (n = 11) of the Shaw-Kamen sequence 5'-ATTTA-3'. The open reading frame encodes a 604-amino acid protein that is 86-97% identical to other mammalian PGHS-2 homologs. The regulation of PGHS-2 mRNA and protein was studied in primary cultures of bovine uterine stromal cells stimulated with phorbol 12-myristate 13-acetate (PMA; 100 nM). Northern and Western blot analyses reveal a marked induction in PGHS-2 transcript (4.0 kilobases) and protein (M(r) = 72 000) after 3-12 h of PMA stimulation (P < 0.05). However, this induction was transient in nature as levels of PGHS-2 mRNA and protein returned to basal levels after 24 h of PMA stimulation. In contrast, PMA had no effect on levels of PGHS-1 (P > 0.05). The PMA-dependent induction of PGHS-2 was associated with a significant increase in prostaglandin E2 secretion in the culture media (P < 0.05). To study promoter activity of the 5'-flanking DNA region of the bovine PGHS-2 gene, the genomic fragment -1574/-2 (+1 = transcription start site), as well as a series of 5'-deletion mutants, were fused upstream of the firefly luciferase gene and transiently transfected into primary cultures of bovine uterine stromal cells. Results showed that a first promoter region located between -1574 and -492 and a second region between -88 and -39 appear to play important roles in PMA-dependent regulation of PGHS-2 promoter activity in bovine uterine cells. Thus, this study characterizes for the first time the structure of the bovine PGHS-2 transcript and the deduced amino acid sequence of its encoded protein and establishes an in vitro model to study the regulation of PGHS-2 gene expression in bovine uterine tissue.

Inhibition of gastric mucosal prostaglandin synthetase activity by mercaptomethylimidazole, an inducer of gastric acid secretion--plausible involvement of endogenous H2O2

We have reported earlier that mercaptomethylimidazole (MMI), an antithyroid drug of thionamide group, induces gastric acid secretion at least partially through the liberation of histamine, sensitive to cimetidine. Now, we show that the drug has a significant inhibitory effect on the cyclooxygenase and peroxidase activity of the prostaglandin (PG) synthetase of the gastric mucosal microsomal preparation. The effect can also be mimicked by low concentrations of H2O2. While studying the possible intracellular effect of MMI on acid secretion, a cell fraction (F3) enriched in parietal cell was isolated by controlled digestion of the mucosa with protease. This cell fraction is activated by MMI as measured by increased O2 consumption. The activation is sensitive to omeprazole, a proton-pump inhibitor, indicating that the activation is due to increased acid secretion by MMI. MMI was also found to directly inhibit the peroxidase activity of the F3 cell fraction and may thus increase the intracellular level of H2O2. The cyclooxygenase activity of the PG synthetase of the F3 cell fraction is also inhibited by MMI and the effect can be reproduced by low concentrations of H2O2. Both MMI and H2O2 can also inhibit the peroxidase activity of the PG synthetase. We suggest that in addition to the activation of the parietal cell by MMI possibly through endogenous H2O2, MMI induces acid secretion in vivo by inactivating the PG synthetase thereby inhibiting the biosynthesis of PG and removing its inhibitory influence on acid secretion so that the histamine released by MMI can stimulate acid secretion with maximum efficiency.

Interactions between inducible isoforms of nitric oxide synthase and cyclo-oxygenase in vivo: investigations using the selective inhibitors, 1400W and celecoxib

1. Exposure of tissues to endotoxin (LPS) and/or cytokines leads to the induction of both inducible nitric oxide synthase (iNOS) and cyclo-oxygenase-2 (COX-2). It has previously been reported that there is 'cross-talk' between these two systems. However, such previous studies have been limited by the availability of highly selective inhibitors. Here we have investigated the interactions between iNOS and COX-2 in vivo using 1400W, an iNOS-selective inhibitor, and celecoxib, a COX-2-selective inhibitor. 2. Infusion of LPS to rats for 6 h caused a time-dependent increase in the plasma concentrations of 6 keto-prostaglandin F1alpha (6 keto-PGF1alpha) and nitrite/nitrate (NO2/NO3), consistent with the induction of iNOS and COX-2. Bolus injection of arachidonic acid (AA) at t=6 h resulted in a further increase of circulating levels of 6 keto-PGF1alpha in LPS-treated animals. 3. Treatment of rats with 1400W or the non-selective NOS inhibitor N(G)-monomethyl-L-arginine (L-NMMA) inhibited the increase in plasma NO2/NO3 but were both without effect on the plasma concentration of 6 keto-PGF1alpha before or after AA. 4. Treatment with the non-steroidal anti-inflammatory drugs (NSAIDs), A771726 or diclofenac, or with celecoxib significantly reduced the increase in circulating 6 keto-PGF1alpha caused by LPS, and the large increase in 6 keto-PGF1alpha following injection of AA. None of the COX inhibitors affected the increase in plasma NO2/NO3. Dexamethasone, however, significantly inhibited both the increase in 6 keto-PGF1alpha and the increase in NO2/NO3. 5. In conclusion, the use of selective inhibitors does not support the concept of cross talk in vivo between iNOS and COX-2.

Evidence that carbon monoxide stimulates prostaglandin endoperoxide synthase activity in rat hypothalamic explants and in primary cultures of rat hypothalamic astrocytes

Carbon monoxide (CO) shares with nitric oxide (NO) the ability to modulate the release of hypophysiotropic peptides from rat hypothalamic explants. While both gases are believed to act as neural messengers in the brain via the activation of soluble guanylyl cyclase, the latter is almost undetectable in the rat hypothalamus. NO has been shown to exert some of its biological actions through the modulation of prostaglandin endoperoxide synthase (PGHS) activity. We have, therefore, investigated whether CO also can use PGHS as a signaling pathway in the hypothalamus. Endogenous CO is produced in equimolar amounts with biliverdin (BV) by the catabolism of hemin through heme oxygenase (HO). Hemin, two inhibitors of HO, zinc-protoporphyrin-9 (ZnPP9) and tin-mesoporphyrin-9 (SnMP9), ferrous hemoglobin (Hb), indomethacin and dexamethasone (DEX) were used as pharmacological tools. Prostaglandin E2 (PGE2) released from rat hypothalamic explants or primary cultures of hypothalamic astrocytes was taken as a marker of PGHS activity. It was found that: (1) hemin evokes an increase in PGE2 release from hypothalamic explants; (2) this effect is counteracted by ZnPP9, SnMP9, Hb and indomethacin; (3) the metallo-porphyrins and indomethacin, but not Hb, are also able to inhibit basal PGE2 release from hypothalamic explants; and (4) dexamethasone does not inhibit, and even potentiates, the stimulatory effect of hemin on PGE2 release from hypothalamic astrocytes. The evidence presented here suggests that the catabolism of endogenous or exogenously added hemin is associated with an increase in PGE2 production in the rat hypothalamus. This effect can be attributed to the formation of CO, since the other end-product of HO, BV, does not enhance PGE2 release. Thus, at least some of the biological effects of CO at the hypothalamic level might be mediated by the activation of the PGHS pathway.

Study of the coordination chemistry of prostaglandin G/H synthase by resonance Raman spectroscopy

Resonance Raman spectra of prostaglandin G/H synthase (PGHS) in its ferric and ferrous states have been obtained by Soret excitation. In native PGHS, which contained only 0.25 heme/monomeric apoprotein, the ferric heme was in a high-spin hexacoordinated state. The presence of a vibration at 289 cm-1 that was responsive to H(2)16O -> H(2)18O replacement was taken as evidence for the presence of a H-bonded H2O molecule as the sixth ligand of the Fe. A study, by CD and resonance Raman spectroscopy, of heme incorporation into the apoprotein showed that, for heme/protein ratios lower than 0.5, the heme was in the same ferric high-spin hexacoordinated state as in the native enzyme. For heme/protein ratios higher than 0.5, the concomitant formation of two minor species was observed: a low-spin hexacoordinated species which could be due to the axial coordination of a distal histidine to the Fe trans to its proximal histidine ligand; and a high-spin pentacoordinated species that corresponded to non-specific binding of the heme to the apoprotein. In the reduced state, the heme of PGHS contained a high-spin pentacoordinated Fe(II) with a histidine as the proximal ligand. However, this species shifted spontaneously towards a low-spin hexacoordinated Fe(II) species in which the iron was probably coordinated by a distal histidine as the sixth axial ligand. The PGHS Fe(II).CO derivative displayed an Fe-CO stretching mode at 529 cm-1, which is in the range observed for peroxidases. Such a high frequency could be due to H-bonding between the oxygen atom of the CO ligand and the distal histidine, His207. Since this histidine plays an important role, by coordination of Fe(II) or Fe(III) of PGHS and stabilization of the ligands of the Fe, H2O or CO by H-bonding, it is suggested that this histidine could also play a key role in the cleavage of the O-O bond of peroxides by peroxidases.

Prostaglandin metabolism during growth and differentiation of the regenerating vertebrate appendage

House lizards are able to regenerate their tails. This is an ideal model to study the growth and differentiation of an organ. Prostaglandins (PGs) are local hormones having diverse and potent biological activities. In an effort to understand PG metabolism during the growth and differentiation of the regenerating lizard tail, we analysed the fatty acid (FA) composition of phospholipids are free FAs by GC, the activity of two rate-limiting enzymes (phospholipases A and C), the activity of the enzyme responsible for the oxygenation of polyunsaturated fatty acids to PGs (cyclooxygenase) and characterized the endogenous PGs by HPLC. It was observed that on the 20th day, i.e. the tissue differentiation period, there was an increase in phospholipase A activity, together with a sudden fall in the free arachidonic acid (AA) level, an increase in cyclooxygenase activity and the appearance of endogenous PGE2. PGE2 can stimulate cyclic adenosine monophosphate (cAMP) production and it may stimulate a cascade of events associated with tissue differentiation.

Reactions of prostaglandin H synthase with monosubstituted hydrazines and diazenes. Formation of iron(II)-diazene and iron(III)-sigma-alkyl or iron(III)-sigma-aryl complexes

The reaction of p-chlorophenylhydrazine with prostaglandin H synthase (PGHS) Fe(III) under aerobic conditions leads to a partial destruction of the heme and to a new complex absorbing at 436 nm. This complex is also obtained by reaction of p-chlorophenyldiazene (pClPhN = NH) with PGHS Fe(III) under anaerobic conditions and by oxidation of the PGHS Fe(II)(pClPhN = NH) diazene complex by Fe(CN)6K3. The similarity between those reactions and those of arylhydrazines and aryldiazenes with other hemoproteins such as cytochrome P450 and hemoglobin and myoglobin, as well as the similarities between the spectroscopic and chemical properties of this complex and those of the sigma-aryl complexes of other hemoproteins such as hemoglobin and myoglobin, strongly suggested a PGHS Fe(III)-pClPh structure for this complex. It was completely established after the extraction of its heme, by butan-2-one at 0 degree C under neutral or acidic conditions, which led to the sigma-aryl PGHS-Fe(III)-pClPh complex and to N-phenylprotoporphyrin IX, respectively. A mechanism is proposed for the formation of the PGHS Fe(III) pClPh complex; it includes the reduction of PGHS Fe(III) into PGHS Fe(II) with formation of the diazene pClPhN = NH. This diazene can bind to PGHS Fe(II) or be oxidized with formation of pClPh free radicals. These radicals can react with PGHS Fe(II) to form the PGHS Fe(III)-pClPh complex or with the protein, or may initiate free radical oxidations which could lead to destruction of the heme or of the protein. Other alkylhydrazines or arylhydrazines also react with PGHS Fe(III) under aerobic conditions with the formation of PGHS Fe(III)-R or aryl (Ar) complexes and heme destruction. Alkylhydrazines such as methylhydrazine, which lead to very reactive alkyl radicals, lead to very low amounts of PGHS Fe(III)-R complex and high amounts of heme destruction, whereas arylhydrazines bearing electron-withdrawing substituents such as 3,4-dichlorophenylhydrazine, which lead to stabilized aryl radicals, lead to a high amounts of PGHS Fe(III)-Ar complex and low amounts of heme destruction.(ABSTRACT TRUNCATED AT 400 WORDS)

Selectivity of nonsteroidal antiinflammatory drugs as inhibitors of constitutive and inducible cyclooxygenase

Constitutive cyclooxygenase (COX-1; prostaglandin-endoperoxide synthase, EC 1.14.99.1) is present in cells under physiological conditions, whereas COX-2 is induced by some cytokines, mitogens, and endotoxin presumably in pathological conditions, such as inflammation. Therefore, we have assessed the relative inhibitory effects of some nonsteroidal antiinflammatory drugs on the activities of COX-1 (in bovine aortic endothelial cells) and COX-2 (in endotoxin-activated J774.2 macrophages) in intact cells, broken cells, and purified enzyme preparations (COX-1 in sheep seminal vesicles; COX-2 in sheep placenta). Similar potencies of aspirin, indomethacin, and ibuprofen against the broken cell and purified enzyme preparations indicated no influence of species. Aspirin, indomethacin, and ibuprofen were more potent inhibitors of COX-1 than COX-2 in all models used. The relative potencies of aspirin and indomethacin varied only slightly between models, although the IC50 values were different. Ibuprofen was more potent as an inhibitor of COX-2 in intact cells than in either broken cells or purified enzymes. Sodium salicylate was a weak inhibitor of both COX isoforms in intact cells and was inactive against COX in either broken cells or purified enzyme preparations. Diclofenac, BW 755C, acetaminophen, and naproxen were approximately equipotent inhibitors of COX-1 and COX-2 in intact cells. BF 389, an experimental drug currently being tested in humans, was the most potent and most selective inhibitor of COX-2 in intact cells. Thus, there are clear pharmacological differences between the two enzymes. The use of such models of COX-1 and COX-2 activity will lead to the identification of selective inhibitors of COX-2 with presumably less side effects than present therapies. Some inhibitors had higher activity in intact cells than against purified enzymes, suggesting that pure enzyme preparations may not be predictive of therapeutic action.

Phenylhydrazones as new good substrates for the dioxygenase and peroxidase reactions of prostaglandin synthase: formation of iron(III)-sigma-phenyl complexes

Phenylhydrazones of various aromatic and aliphatic aldehydes or ketones act as good substrates of the dioxygenase reaction of prostaglandin synthase (PGHS). Corresponding alpha-azo hydroperoxides are formed as intermediates with maximum initial rates of O2 consumption between 8 and 230 mol (mol of PGHS)-1 s-1 for benzophenone and hexanal phenylhydrazone, respectively. The Km values for these reactions vary from 100 to 300 microM. These alpha-azo hydroperoxides are then converted to the corresponding alpha-azo alcohols by the peroxidase reaction of PGHS. During such oxidations of phenylhydrazones by PGHS, a new complex of this hemeprotein characterized by peaks at 438 and 556 nm is formed. This complex was obtained both by direct reaction of PGHS Fe(III) with phenyldiazene and by reaction of PGHS Fe(III) with phenylhydrazine in the presence of O2. By analogy to results previously reported for hemoglobin, myoglobin, catalase, and cytochrome P450, this species should be a sigma-phenyl PGHS FeIII-Ph complex. The PGHS FeIII-Ph complex should derive from an oxidation of the intermediate alpha-azo alcohol by PGHS Fe(III), cleavage of the resulting alkoxy radical with formation of a ketone (or aldehyde) and Ph*, and combination of PGHS Fe(II) with Ph*. Such an oxidation of alpha-azo alcohols by lipoxygenase-FeIII with formation of Ph* was reported previously. The formation of Ph* and of PGHS FeIII-Ph is likely the cause of the inhibitory effects previously reported for arylhydrazones toward PGHS.

Target size analysis of prostaglandin endoperoxide synthase. Radiation inactivation of both cyclooxygenase and peroxidase correlated with the monomer of 72 kDa

To determine the size of the functional catalytic unit of prostaglandin endoperoxide (prostaglandin H) synthase, radiation inactivation experiments were performed. Both microsomes from ovine seminal vesicles and purified enzyme were irradiated with 10 MeV electrons. The enzymic activities of prostaglandin H synthase, cyclooxygenase and peroxidase, showed mono-exponential inactivation curves dependent on radiation dose, indicating molecular masses of approximately 72 kDa. The enzyme in microsomes, in its native environment, as well as in its purified state after solubilisation with nonionic detergent showed identical molecular masses. The results clearly demonstrate that the monomer of the enzyme with an apparent molecular mass of 72 kDa (SDS/PAGE) is the functional unit for catalysis of both activities. Hence the two active sites of cyclooxygenase and peroxidase reside on the same polypeptide chain.

Hemin and hemeprotein bleaching during linoleic acid oxidation by lipoxygenases

Hemin and hemoglobin are bleached by lipoxygenases, type 1 (from soybean) or type 2 (from platelets), during linoleic acid oxidation. This process has been found to be related to the inhibition of the lipoxygenase activity, measured as hydroperoxide generation and to produce oxodienes as well. All these parameters have been determined simultaneously from measurements of the absorbance at 234, 285, 375 and 410 nm to detect hydroperoxides, oxodienes, hemin and hemoglobin, respectively, using a diode array spectrophotometer. The inhibition of lipoxygenase activity by these pigments has been found to be competitive with linoleic acid, showing an increase of 4-7-fold of the Km value of linoleic acid in the presence of concentrations of hemin and hemoglobin as low as 0.2 and 0.02 microM, respectively, for the case of platelet lipoxygenase activity. The concentrations of hemin and of hemoglobin producing the inhibition of 50% of lipoxygenase activity are: 0.25 and 0.02 microM for the platelet isoenzyme, and 1.4 and 0.18 microM for the soybean isoenzyme, respectively. From the quenching of the intrinsic fluorescence of soybean lipoxygenase activity by hemin, we have obtained a dissociation constant of hemin-soybean lipoxygenase of 0.5 microM. The results obtained in this paper for the cooxidation process of hemin and hemoglobin by lipoxygenase can be rationalized in terms of hemin binding at or near to the catalytic center, resulting in a lesser binding of linoleic acid and an enhanced release of radicals, and pigment bleaching by radicals and lipid hydroperoxides.

Localization of prostaglandin endoperoxide synthase in neurons and glia in monkey brain

The localization of prostaglandin endoperoxide synthase in monkey brain was investigated by the immunoperoxidase method using the monoclonal antibody (PES-7) raised against the enzyme purified from bovine seminal vesicle. The frozen sections with 30-microns thickness were employed after the brain was fixed with perfusion of 2% paraformaldehyde in phosphate-buffered saline. The immunoreactivity was most intense in the neurons of cerebral cortex and hippocampus, and was moderate in the neurons of caudate nucleus, putamen, globus pallidus and amygdala, while it was relatively weak in glial cells in the whole brain regions including the white matter. The majority of neurons showed the immunoreactivity in the somata and proximal dendrites, but exceptionally in the pyramidal cells of the hippocampus, positive staining was also observed in the apical dendrites. In the cerebellum, the immunoreactivity in both neurons and glia was rather faint as compared with that in other regions. Positive staining was not significantly observed in the vasculatures and arachnoid membranes. These findings indicate that most of neuronal and glial cells in monkey brain contain the enzyme of the rate-limiting and initial step of the biosynthesis of prostaglandins which regulate a variety of neural functions.

Use of a voltage-responsive timer to quantitate hydroperoxides by the cyclooxygenase activation assay

The importance of lipid hydroperoxides in the interpretations of physiological and pathological processes requires a valid and reliable microanalytical procedure. However, quantitation of the small amounts of lipid hydroperoxides in biological samples is often attempted with methods that have low sensitivity, low validity, and/or low reliability. The highly sensitive and specific enzymatic assay based upon cyclooxygenase activation has many complex kinetic features that have caused it to be cumbersome and unreliable. Analysis of the cyclooxygenase kinetics showed that a fixed percentage (20%) of full reaction accurately substituted for the measurements of the time required to reach optimal velocity. Consequently, a voltage-responsive timer can report endpoints that are dependent upon picomole amounts of lipid hydroperoxide. Additional features of the timer are described which make the hydroperoxide assay simpler, faster, more reliable, and easier to perform than the previous method while still retaining its sensitivity and specificity.

Prostaglandin and thromboxane biosynthesis

We describe the enzymological regulation of the formation of prostaglandin (PG) D2, PGE2, PGF2 alpha, 9 alpha, 11 beta-PGF2, PGI2 (prostacyclin), and thromboxane (Tx) A2 from arachidonic acid. We discuss the three major steps in prostanoid formation: (a) arachidonate mobilization from monophosphatidylinositol involving phospholipase C, diglyceride lipase, and monoglyceride lipase and from phosphatidylcholine involving phospholipase A2; (b) formation of prostaglandin endoperoxides (PGG2 and PGH2) catalyzed by the cyclooxygenase and peroxidase activities of PGH synthase; and (c) synthesis of PGD2, PGE2, PGF2 alpha, 9 alpha, 11 beta-PGF2, PGI2, and TxA2 from PGH2. We also include information on the roles of aspirin and other nonsteroidal anti-inflammatory drugs, dexamethasone and other anti-inflammatory steroids, platelet-derived growth factor (PDGF), and interleukin-1 in prostaglandin metabolism.

"Enzymatic" lipid peroxidation: reactions of mammalian lipoxygenases

Lipoxygenase is a dioxygenase which incorporates one molecule of oxygen at a certain position of unsaturated fatty acids such as arachidonic and linolenic acids. The enzymatic oxygenation of unsaturated fatty acids is stereospecific concomitant with a stereoselective abstraction of hydrogen atom. Fatty acid cyclooxygenase is an atypical lipoxygenase incorporating two molecules of oxygen, and initiates the biosynthesis of prostaglandins and thromboxanes. Arachidonate 5-lipoxygenase is responsible for the leukotriene synthesis. No such bioactive compound has been found as a metabolite of the 12- and 15-lipoxygenase pathways, and their physiological roles are still unclarified. These enzymes have been purified, and their molecular and catalytic properties have been investigated. Their cDNA clones have been isolated, and their nucleotide sequences have been determined deducing the primary structures of the enzymes.

Topographic studies of microsomal and pure prostaglandin H synthase

Prostaglandin H synthase catalyzes the first step in the conversion of polyunsaturated fatty acids to prostaglandins, thromboxanes, and prostacyclins. The enzyme is normally bound to the endoplasmic reticulum membrane, but can be purified to homogeneity after solubilization with detergent. The topologies of the microsomal and the pure detergent-solubilized forms of the synthase were compared by an examination of their sensitivity to degradation by proteases, of the effect of heme on this protease sensitivity, and of the sizes of proteolytic fragments produced. For the microsomal synthase, the localization of proteolytic fragments was also determined. Analysis of the microsomal proteins after proteolytic digests involved separation by polyacrylamide gel electrophoresis and selective detection of the synthase-derived polypeptides with a polyclonal antibody against the pure synthase. With both the microsomal and the pure synthase, incubation with trypsin led to a progressive loss of cyclooxygenase activity and cleavage of the synthase subunit (70K Da) into two fragments of 38K and 33K Da. Incubation of the detergent-solubilized form of the synthase with proteinase K and chymotrypsin also produced a very similar pair of fragments (38K and 33K Da). After incubation of the microsomes with trypsin both the 38K and 33K Da fragments from the synthase remained bound to the membrane; no cyclooxygenase activity was released in soluble form from the microsomes by trypsin. Further, neither trypsin nor proteinase K released soluble radiolabeled peptides from microsomes whose synthase had been labeled with [acetyl-14C]-aspirin. With the microsomal synthase the sensitivity to protease (66% of the cyclooxygenase activity was lost after 90 min incubation with proteinase K) was enhanced by depletion of heme (84% of activity lost) and was decreased by addition of heme (only 20% of activity lost), just as had been previously demonstrated for the detergent-solubilized synthase. At each of several intervals during an incubation of the pure synthase with trypsin the extent of cleavage of the synthase polypeptide correlated reasonably well with the extent of loss of cyclooxygenase activity; a similar relation between proteolytic cleavage and loss of activity was observed in digests of the pure synthase supplemented with differing amounts of heme.(ABSTRACT TRUNCATED AT 400 WORDS)

Membrane peroxidases

It is well known that the partial reduction of oxygen can result in the formation of highly reactive oxygen products. Hydrogen peroxide is one of these metabolites of oxygen. Peroxidases utilize this metabolite for a variety of functions. It is the purpose of this treatise to review the nature and function of various membrane peroxidases in the body.

EPR study of ferric native prostaglandin H synthase and its ferrous NO derivative

Purified prostaglandin H synthase (EC 1.14.99.1) apoprotein, a polypeptide of 72 kDA, was titrated with hemin and EPR spectra of high-spin ferric heme were observed at liquid-helium temperature. With up to one hemin per polypeptide, a signal at g = 6.6 and 5.4, rhombicity 7.5%, evolved owing to specifically bound, catalytic active heme. At higher heme/polypeptide ratios signals at g = 6.3 and 5.9 were observed which were assigned to non-specific heme with no catalytic function. In microsomes from ram seminal vesicles the native enzyme showed the signal at g = 6.7 and 5.2 which could not be increased by the addition of hemin. Cyanide, an inhibitor of the enzyme, reacted at lower concentrations with the specific heme abolishing its signal at g = 6.6 and 5.4. Higher concentrations of cyanide were needed for the disappearance of the signal of non-specific heme. The reduced enzyme reacted with NO and formed two types of NO complexes. A transient complex, with a rhombic signal at gx = 2.07, gz = 2.01 and gy = 1.97, was assigned to a six-coordinate complex. The final, stable complex showed an axial signal at g = 2.12 and g = 2.001 and was assigned to a five-coordinate complex, where the protein ligand was no longer bound to the heme iron. Neither type of signal showed a hyperfine splitting from nitrogen of histidine indicating the absence of a histidine-iron bond in the enzyme. From these results and the similarity of the EPR signal at g = 6.6 and 5.4 to the signal of native catalase (EC 1.11.1.6) we speculated that tyrosinate might be the endogenous ligand of the heme in prostaglandin H synthase.

Purification and characterization of sheep platelet cyclo-oxygenase. Acetylation by aspirin prevents haemin binding to the enzyme

Arachidonate cyclo-oxygenase (prostaglandin synthetase; prostaglandin endoperoxide synthetase; EC 1.14.99.1) was purified from sheep platelets. The purification procedure involved hydrophobic column chromatography using either Ibuprofen-Sepharose, phenyl-Sepharose or arachidic acid-Sepharose as the first step followed by metal-chelate Sepharose and haemin-Sepharose affinity chromatography. The purified enzyme (Mr approximately 65,000) was homogeneous as observed by SDS/polyacrylamide-gel electrophoresis and silver staining. The enzyme was a glycoprotein with mannose as the neutral sugar. Haemin or haemoglobin was essential for activity. The purified enzyme could bind haemin exhibiting a characteristic absorption maximum at 410 nm. The enzyme after metal-chelate column chromatography could undergo acetylation by [acetyl-3H]aspirin. The labelled acetylated enzyme could not bind to haemin-Sepharose, presumably due to acetylation of a serine residue involved in the binding to haemin. The acetylated enzyme also failed to show its characteristic absorption maximum at 410 nm when allowed to bind haemin.

Regional distribution of prostaglandin endoperoxide synthase studied by enzyme-linked immunoassay using monoclonal antibodies

Prostaglandin endoperoxide synthase transforms arachidonic acid to prostaglandin H2 via prostaglandin G2. The enzyme purified from bovine vesicular gland was given to mice as antigen, and monoclonal antibodies were raised by the hybridoma technique. Two species of the monoclonal antibody recognizing different sites of the enzyme were utilized to establish a peroxidase-linked immunoassay of prostaglandin endoperoxide synthase. Fab' fragment of one of the antibodies was prepared and conjugated to horseradish peroxidase. The conjugate was then bound to prostaglandin endoperoxide synthase, and the labeled enzyme was precipitated by the addition of the other antibody. The peroxidase activity of the immunoprecipitate correlated linearly with the amount of prostaglandin endoperoxide synthase. This sensitive and convenient method to determine the enzyme amount rather than the enzyme activity was utilized to extensively screen the amount of prostaglandin endoperoxide synthase in various bovine tissues. In addition to vesicular gland, platelets and kidney medulla previously known as rich enzyme sources, the immunoenzymometric assay demonstrated a high content of the enzyme in various parts of alimentary tract and a low but significant amount of enzyme in some parts of brain.

The combined effects of 2,3-DPG and Na-arachidonate on platelet aggregation and on TXA2 formation

We have investigated the effects of 2,3-DPG on platelet aggregation in the presence of suboptimal concentrations of Na-Arachidonate by using the two cuvette transfer experiments of Hamberg, Svensson and Samuelsson (3). The results show that 2,3-DPG enhanced or induced platelet aggregation in the presence of suboptimal concentrations of Na-Arachidonate. Imidazole, a TXA2 synthetase inhibitor, and Lasix, when added inhibited 2,3-DPG effects on platelet aggregation, suggesting that 2,3-DPG may act either on cyclooxygenase or on TXA2 synthetase of prostaglandin synthesis. A specific RIA assay showed that 2,3-DPG when added to suboptimal concentrations of Na-Arachidonate enhanced the formation of TXB2, a stable metabolite of TXA2. We have concluded that during intravascular hemolysis 2,3-DPG release may be a key component in preventing and/or inducing thrombosis.

Purification of prostaglandin H synthetase and a fluorometric assay for its activity

Prostaglandin H synthetase (PGH synthetase) has been purified to homogeneity from sheep vesicular glands. The pure enzyme has a specific activity of about 40 microM of arachidonic acid consumed per minute per milligram of protein, which corresponds to a turnover number of 2800 min-1 per subunit. The purified enzyme was obtained by one-stage chromatography on DEAE-Toyopearl 650 from Tween 20-solubilized microsomes. A sensitive fluorometric assay for PGH synthetase activity using homovanillic acid (HVA) as electron donor has been proposed. It has been shown that homovanillic acid may be used as the electron donor and that in the presence of HVA the enzyme has an activity of approximately 40 microM/min/mg.