On the mechanism of prostacyclin and thromboxane A2 biosynthesis

Selective inhibition of cyclooxygenase 2

Cyclooxygenase (COX), a key enzyme in the formation of prostanoids, is known to exist in two isoforms: an inducible enzyme (COX 2) and a constitutive from (COX 1). Both enzymes are inhibited by non-steroidal anti-inflammatory drugs (NSAID), but only marginal selectivity has thus far been reported. In this study, we report on a novel selective inhibitor of COX 2, CGP 28238 (6-(2,4-difluorophenoxy)-5-methyl-sulfonylamino-1-indanon e). Human washed platelets were used as a source of COX 1. For IL-1 stimulated rat mesangial cells we demonstrated the almost exclusive presence of COX 2 in western blot and mRNA analysis. Therefore these two model systems were chosen for selectivity testing. With an IC50 value of 15 nM, CGP 28238 blocked COX 2 activity in a similar concentration range to that of other potent NSAID such as indomethacin and diclofenac (IC50 = 1.17-8.9 nM). However, in contrast to these reference NSAIDs, CGP 28238 was at least 1000-fold less potent in inhibiting COX 1. Using other cell systems reported to express COX 1 or COX 2, we obtained a similar selectivity for COX 2. Thus, on the basis of our findings, CGP 28238 is a novel, highly potent and selective inhibitor of COX 2 and may be a lead compound for a new generation of potent anti-inflammatory drugs with an improved side-effect profile.

Characterization of monoclonal antibodies generated against bovine and porcine prostacyclin synthase and quantitation of bovine prostacyclin synthase

Monoclonal antibodies were raised against prostacyclin synthases purified from bovine and porcine aortae, respectively. Two monoclonal antibodies, RS1 and RS2, were purified and characterized. As shown by enzyme activity precipitation and Western blot analysis, in solubilized bovine and porcine aortae microsomes the monoclonal antibodies reacted only with prostacyclin synthase. The monoclonal antibody RS1 cross-reacts with partially purified prostacyclin synthase from human umbilical veins in an ELISA-based assay. None of the antibodies inhibited the enzyme activity. By combination of the monoclonal antibody RS2 with a polyclonal antibody we established an enzyme-linked immunosorbent assay (ELISA) for quantitation of bovine prostacyclin synthase. ELISA data were confirmed by Western blot analysis. Among different bovine tissues, aortae with 1665 +/- 200 ng/mg microsomal protein showed the highest content of PGIS. Significant lower concentrations were observed in tongue, lung, kidney and thymus ranging from 49 +/- 13.4 to 2.7 +/- 0.9 ng/mg protein. The monoclonal antibody RS1 binds to endothelial cells and vascular smooth muscle cells in human liver tissue.

Gender-related response to a tert-butyl hydroperoxide-induced oxidation in human neonatal tissue

Reports of gender-related differences in the activity of enzymes involved in the metabolism of intracellular antioxidants, led us to verify whether the prostaglandin response to tert-butyl hydroperoxide (TBH) differed according to the sex of infants. Segments of human umbilical veins were perfused in the presence or absence of TBH (0.25 mmol/l, and 1.0 mmol/l). Because TBH is quenched in the cell by glutathione peroxidase, total glutathione concentrations and production of glutathione-dependent prostaglandins (PGE2 and PGF2 alpha) as well as membrane-derived eicosanoids (PGI2 and thromboxane) were measured in the eluate. In veins from boys, TBH induced a sustained response for glutathione only, which was increased (p < 0.05). In female-derived tissue, the hydroperoxide induced a different response according to the dose of TBH. At 0.25 mmol/l, a drop (p < 0.005) in PGF2 alpha was associated with a rise (p < 0.001) in thromboxane. At 1.0 mmol/l, TBH had an opposite effect--there was a rise (p < 0.01) in PGE2 and PGI2. The prostaglandin concentration were not proportional to the oxidative stimulus, suggesting a critical level of TBH at which the oxidative state differs in tissues derived from boys or girls.

Oxidative stress: free radical production in neural degeneration

It is not yet established whether oxidative stress is a major cause of cell death or simply a consequence of an unknown pathogenetic factor. Concerning chronic diseases, as Parkinson's and Alzheimer's disease are assumed to be, it is possible that a gradual impairment of cellular defense mechanisms leads to cell damage because of toxic substances being increasingly formed during normal cellular metabolism. This point of view brings into consideration the possibility that, besides exogenous factors, the pathogenetic process of neurodegeration is triggered by endogenous mechanisms, either by an endogenous toxin or by inherited metabolic disorders, which become progressively more evident with aging. In the following review, we focus on the oxidative stress theory of neurodegeneration, on excitotoxin-induced cell damage and on impairment of mitochondrial function as three major noxae being the most likely causes of cell death either independently or in connection with each other. First, having discussed clinical, pathophysiological, pathological and biochemical features of movement and cognitive disorders, we discuss the common features of these biochemical theories of neurodegeneration separately. Second, we attempt to evaluate possible biochemical links between them and third, we discuss experimental findings that confirm or rule out the involvement of any of these theories in neurodegeneration. Finally, we report some therapeutic strategies evolved from each of these theories.

Mass determination of 15-hydroxyprostaglandin dehydrogenase from human placenta and kinetic studies with (5Z, 8E, 10E, 12S)-12-hydroxy-5,8,10-heptadecatrienoic acid as substrate

NAD(+)-dependent 15-hydroxyprostaglandin dehydrogenase catalyzes the first step in the metabolism of prostaglandins which is usually associated with physiological inactivation. A highly purified homogenous enzyme preparation from human placenta was used to determine the molecular mass and lack of quaternary structure of the enzyme. Furthermore we have examined enzyme kinetics of the purified enzyme with (5Z,8E,10E,12S)-12-hydroxy-5,8,10-heptadecatrienoic acid (HHT) an equimolar coproduct of thromboxane biosynthesis. Using gel electrophoresis and gel filtration on FPLC, we could estimate a molecular mass of 28 +/- 1 kDa, indicating that the enzyme consists of one single protein chain. The exact molecular mass of the monomer was calculated by matrix-assisted laser desorption/ionization mass spectrometry to 28740 +/- 30 Da. (5Z,8E,10E)-12-oxo-5,8,10-heptadecatrienoic acid (oxo-HT) could be identified as the only product obtained from the enzymatic reaction with HHT. Quantification of this metabolite was achieved by gas chromatography/tandem mass spectrometry. The calculated enzyme kinetic constants for the formation of the metabolic product [Km (HHT) = 9.68 microM, Vi = 12.78 mU/micrograms] were in agreement with those determined for NADH formation (Km = 7.65 microM, Vi = 11.79 mU/micrograms). This demonstrates that HHT shows high affinity to the enzyme which is comparable to prostaglandin E2 (PGE2). As the product oxo-HT is a potent inhibitor of platelet aggregation, dehydrogenation of HHT might represent a biological activation step.

Increased 5-lipoxygenase activity in massive hepatic cell necrosis in the rat correlates with neutrophil infiltration

Rats were treated with heat-killed Propionibacterium acnes and subsequent injection of a small amount of lipopolysaccharide after 7 days. After 24 hr most of the rats died of massive liver cell necrosis. Nonparenchymal liver cells were isolated from this liver injury model and incubated with arachidonic acid. Reverse-phase high-pressure liquid chromatography detected the 5-lipoxygenase metabolites (leukotriene B4 and 5-hydroxy-arachidonic acid), whereas these compounds were produced in negligible amounts when the rats were treated with P. acnes only. Immunohistochemical studies with 5-lipoxygenase antiserum revealed that the injured livers contained a large number of positively stained round cells with segmented nuclei, which were rarely found in the livers treated with P. acnes only. These positively stained cells were histologically identified as neutrophils. The results suggested that the increased 5-lipoxygenase activity in the injured rat liver is attributable to the infiltrating neutrophils rather than to nonparenchymal hepatic cells.

Enhancement of prostaglandin E2 production by liver macrophages (Kupffer cells) after stimulation with biological response modifiers

PGE2 production by liver macrophages (Kupffer cells) activated by biological response modifiers was examined. Kupffer cells obtained from a normal rat liver possessed cyclooxygenase activity and produced TXB2, PGD2, and PGE2 from (1-14C)arachidonic acid. The major product was PGD2. When Kupffer cells were incubated in the presence of lipo-polysaccharide (LPS), OK-432, or heat-killed Propionibacterium acnes for 24 h, the amount of arachidonate cyclooxygenase products increased and the major product changed from PGD2 to PGE2. When liver macrophages including Kupffer cells were prepared from rats after an injection of LPS, OK-432, or heat-killed P. acnes, it was noticed that the number of cells obtained and PGE2 production increased compared with those of normal rat. These results suggested that PGE2 production by rat liver was induced when they were treated with biological response modifiers.

Ridogrel: a selective inhibitor of the cytochrome P450-dependent thromboxane synthesis

Ridogrel [(E)-5-[[[(3-pyridinyl)[3-(trifluoromethyl)phenyl] methylene]amino]oxy] pentanoic acid] is a potent inhibitor of the P450-dependent human platelet thromboxane A2 (TxA2) synthase. Fifty percent inhibition is already achieved at 5.0 +/- 0.37 nM. This IC50 value is close to half the P450 concentration used, i.e. 10.7 nM. Ridogrel binds to human platelet microsomal P450 as proven by the type II spectral changes induced by the addition of increasing concentrations of ridogrel to solubilized microsomes. The calculated half-maximal spectral change (SC50 value) is 3.78 +/- 1.79 nM. These results indicate that ridogrel binds stoichiometrically and suggest that inhibition of thromboxane synthesis may originate from liganding of its basic nitrogen to the haem-iron of P450 and from the attachment of the hydrophobic carboxylic side chain to or near the substrate binding place. Ridogrel is a selective inhibitor of the TxA2 synthase. At a high concentration (10 microM), ridogrel has a slight, if any, effect on the P450-mediated cholesterol synthesis in human liver and hepatoma cells and androgen synthesis from 17 alpha-hydroxy-20-dihydroprogesterone or pregnenolone in subcellular fractions from rat testes. These results indicate that ridogrel is a poor inhibitor of the P450-dependent 14 alpha-demethylase, 17 alpha-hydroxylase and 17,20-lyase. It has, up to 10 microM, no effect on the adrenal mitochondrial 11 beta-hydroxylase and cholesterol side-chain cleavage enzyme and does not inhibit aromatase activity in human placental microsomes. Ridogrel has no significant effect on the regio- and stereoselective P450-dependent oxidations of testosterone in liver microsomes from unpretreated or from 5-pregnen-3 beta-ol-20-one-16 alpha-carbonitrile-, phenobarbital- or 3-methylcholanthrene-pretreated male and female Sprague-Dawley rats. It does not interfere with the reduction of testosterone into 5 alpha-dihydrotestosterone and 5 alpha androstane 3 beta, 17 beta-diol.

Metabolic activation of carcinogens

Most chemical carcinogens are not active in themselves but require bioactivation to electrophiles that bind covalently to DNA and often act by producing mutations. In recent years it has been realized that mutations can be important at many stages of carcinogenesis. A variety of different enzymes are involved in bioactivation reactions, which include oxidation, reduction, thiol conjugation, acetyl transfer, sulfur transfer, methyl transfer, glucuronosyl transfer, and epoxide hydrolysis. These processes often occur in concert with a single carcinogen. Humans vary considerably in activities of these enzymes and this variation may contribute to differences in risk.

Molecular cloning of human platelet thromboxane A synthase

Complementary DNA coding for thromboxane A synthase was amplified by polymerase chain reaction using primers synthesized according to the partial amino acid sequences of human platelet thromboxane A synthase (Nüsing, R., Schneider-Voss, S., and Ullrich, V. (1990) Arch. Biochem. Biophys. 280, 325-330) and cloned into pBluescript SK II(-). The primary structure of human platelet enzyme was deduced from the nucleotide sequence of the cDNA. The enzyme is composed of 533 amino acids with a molecular weight of 60,487. The primary structure of the enzyme exhibited a 34-36% homology to the amino acid sequences of cytochrome P450s classified in the P450 III gene family. The highly conserved cysteine-containing sequence involved in the heme-binding site of P450 was found near the carboxyl terminus (residues 472-492). The size of the major thromboxane A synthase mRNA from human platelets and human erythroleukemia cells was estimated to be approximately 2.2 kilobases by RNA blot analysis.

Isolation of partial complementary DNA encoding human thromboxane synthase

Thromboxane synthase catalyzes the biosynthesis of thromboxane A2 which plays a key role in the proaggregatory and vasoconstrictive processes. In this communication, we reported the successful cloning of thromboxane synthase cDNA from a human lung cDNA library. Oligonucleotides were synthesized according to the direct amino acid sequence of 2 peptides derived from purified human thromboxane synthase. Polymerase chain reaction was carried out using these oligonucleotides as primers to isolate a complementary DNA from human lung cDNA library. The longest cDNA thus obtained was 687 base pairs in length. Amino acid sequences deduced from the cDNA contained all three peptide sequences reported, confirming the authenticity of the cDNA clone.

Hydroperoxide-induced broncho- and vasoconstriction in the isolated rat lung

The effects of different hydroperoxides on lung mechanics and perfusate flow rate and their mechanisms of action were studied in isolated perfused rat lungs. The administration of hydrogen peroxide, t-butyl hydroperoxide, cumene hydroperoxide, linoleic acid hydroperoxide, and linoleic acid ethylester hydroperoxide (0.1-2 mM) to the perfusate caused a marked decrease in lung compliance, conductance, and perfusate flow rate, with constriction strength of t-butyl hydroperoxide greater than hydrogen peroxide greater than cumene hydroperoxide greater than linoleic acid ethylester hydroperoxide greater than linoleic acid hydroperoxide. Although the hydroperoxides probably had to enter lung cells to exert their effects, no relationship was found between constriction strength and amount of hydroperoxide taken up by the lung. Reduced sensitivity was apparent after repeated dosing, depending on the length of time between dosing. The addition of the iron chelator Desferal (1 mM) had no effect on the hydroperoxide-induced broncho- and vasoconstriction, although free iron was reduced by 50% in the lungs. The administration of the antioxidants diphenyl-p-phenylenediamine (50 microM) or butylated hydroxyanisole (200 microM) to the perfusate 20 min prior to the hydroperoxide attenuated the hydroperoxide-induced effects as well as arachidonic acid-induced broncho- and vasoconstriction. Our findings have shown that hydroperoxides that can enter the lung cells will also induce both vaso- and bronchoconstriction in the isolated perfused rat lung.

Mechanisms of hydroperoxide-induced broncho- and vasoconstriction in isolated and perfused rat lung

The mechanisms of hydroperoxide-induced broncho- and vasoconstriction were investigated in the perfused and ventilated rat lung. Hydrogen peroxide (500 microM), tertiary butylhydroperoxide (500 microM) and arachidonic acid (100 microM) induced similar profiles of broncho- and vasoconstriction which could be prevented by the inhibitor of cyclooxygenase, diclofenac (100 microM) but not by nordihydroguaiaretic acid (5 and 25 microM), an inhibitor of lipoxygenase. The hydroperoxides also caused a time-dependent increase in the levels of thromboxane and prostacycline, products of cyclooxygenase. Furthermore, the thromboxane agonist, U44069 (100 pmoles), caused a very rapid broncho- and vasoconstriction that was preventable by the thromboxane antagonist L655.240 (1 microM). L655.240 also inhibited hydrogen peroxide-induced broncho- and vasoconstriction. The phospholipase A2 inhibitors, quinacrine (100 microM) and dibucaine (100 microM), did not prevent hydroperoxide-induced broncho- and vasoconstriction. The Ca2+ chelator, EGTA, prevented hydroperoxide and arachidonic acid-induced lung constriction, although it did not inhibit the release of thromboxane. The infusion of arachidonic acid and hydroperoxides resulted in edema in the lung which was prevented by prior administration of diclofenac, indomethacin or L655.240. These results indicate that hydroperoxide-induced broncho- and vasoconstriction and lung edema are mediated by thromboxane, a product of cyclooxygenase. The mechanism of hydroperoxide-induced release of arachidonic acid is not clear but does not seem to involve Ca2+ nor the activation of phospholipase A2.

Chemistry and biochemistry of 4-hydroxynonenal, malonaldehyde and related aldehydes

Lipid peroxidation often occurs in response to oxidative stress, and a great diversity of aldehydes are formed when lipid hydroperoxides break down in biological systems. Some of these aldehydes are highly reactive and may be considered as second toxic messengers which disseminate and augment initial free radical events. The aldehydes most intensively studied so far are 4-hydroxynonenal, 4-hydroxyhexenal, and malonaldehyde. The purpose of this review is to provide a comprehensive summary on the chemical properties of these aldehydes, the mechanisms of their formation and their occurrence in biological systems and methods for their determination. We will also review the reactions of 4-hydroxyalkenals and malonaldehyde with biomolecules (amino acids, proteins, nucleic acid bases), their metabolism in isolated cells and excretion in whole animals, as well as the many types of biological activities described so far, including cytotoxicity, genotoxicity, chemotactic activity, and effects on cell proliferation and gene expression. Structurally related compounds, such as acrolein, crotonaldehyde, and other 2-alkenals are also briefly discussed, since they have some properties in common with 4-hydroxyalkenals.

Substrate inactivation of lung thromboxane synthase preferentially decreases thromboxane A2 production

Bovine lung thromboxane synthase was immobilized on phenyl-Sepharose beads by adsorption. The immobilized enzyme was catalytically active and synthesized both TXA2 and HHT. The production of both products was inhibited by 1-benzylimidazole and furegrelate. Multiple additions of PGH2 dramatically reduced the ability of the enzyme to synthesize TXA2, but did not effect the synthesis of HHT. In addition, 1-benzylimidazole did not protect thromboxane synthase from inactivation with multiple additions of PGH2. When the enzyme was incubated with PGH2 in the presence of 1-benzylimidazole, the synthesis of TXA2 was inhibited. When the inhibitor was removed the enzyme had still been inactivated by PGH2 in the presence of 1-benzylimidazole. Thus the substrate inactivation of the enzyme does not require the production of TXA2. Our data suggests that the synthesis of TXA2 and HHT can be differentially inactivated and may occur at different sites on the enzyme.

Immunoaffinity purification of human thromboxane synthase

A recently produced monoclonal antibody against human thromboxane synthase was used to purify the enzyme from platelets in a one-step procedure with good yields. The isolated protein exhibited a single band of about 58 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and contained one heme/mol. Although the visible spectrum of the oxidized enzyme displayed a peak at 418 nm like the previously isolated enzyme after dithionite reduction and CO addition, it shifted to 419 nm but not to 450 nm where only a small shoulder could be detected. Its catalytic activity was only 1-5% of the previous preparations, but with the same Km of about 10 microM and a ratio of thromboxane B2: 12-hydroxyheptadecatrienoic acid of 1:1. Studies with EPR spectrometry and inhibitors confirmed that only a minor part of the enzyme was in its native heme-thiolate conformation, whereas the major part had been converted to the inactive P420 form by the elution procedure. The amino acid analysis revealed 46% hydrophobic residues. According to the sequence of 26 amino acids from the N-terminus and two tryptic peptides no homology to one of the cytochrome P450 monooxygenases, or to cyclooxygenase, or to prostacyclin synthase was detected.

Enzymatic oxidation of xenobiotic chemicals

Studies with biomimetic models can yield considerable insight into mechanisms of enzymatic catalysis. The discussion above indicates how such information has been important in the cases of flavoproteins, hemoproteins, and, to a lesser extent, the copper protein dopamine beta-hydroxylase. Some of the moieties that we generally accept as intermediates (i.e., high-valent iron oxygen complex in cytochrome P-450 reactions) would be extremely hard to characterize were it not for biomimetic models and more stable analogs such as peroxidase Compound I complexes. Although biomimetic models can be useful, we do need to keep them in perspective. It is possible to alter ligands and aspects of the environment in a way that may not reflect the active site of the protein. Eventually, the model work needs to be carried back to the proteins. We have seen that diagnostic substrates can be of considerable use in understanding enzymes and examples of elucidation of mechanisms through the use of rearrangements, mechanism-based inactivation, isotope labeling, kinetic isotope effects, and free energy relationships have been given. The point should be made that a myriad of approaches need to be applied to the study of each enzyme, for there is potential for misleading information if total reliance is placed on a single approach. The point also needs to be made that in the future we need information concerning the structures of the active sites of enzymes in order to fully understand them. Of the enzymes considered here, only a bacterial form of cytochrome P-450 (P-450cam) has been crystallized. The challenge to determine the three-dimensional structures of these enzymes, particularly the intrinsic membrane proteins, is formidable, yet our further understanding of the mechanisms of enzyme catalysis will remain elusive as long as we have to speak of putative specific residues, domains, and distances in anecdotal terms. The point should be made that there is actually some commonality among many of the catalytic mechanisms of oxidation, even among proteins with different structures and prosthetic groups. Thus, we see that cytochrome P-450 has some elements of a peroxidase and vice versa; indeed, the chemistry at the prosthetic group is probably very similar and the overall chemistry seems to be induced by the protein structure. The copper protein dopamine beta-hydroxylase appears to proceed with chemistry similar to that of the hemoprotein cytochrome P-450 and, although not so thoroughly studied, the non-heme iron protein P. oleovarans omega-hydroxylase.(ABSTRACT TRUNCATED AT 400 WORDS)

Substrate specificities and functions of the P450 cytochromes

Currently, the major recognized biochemical functions of members of the large superfamily of P450 hemoproteins (referred to commonly as the cytochromes P450) include catalyses of the monooxygenations of a wide variety of endogenous and exogenous lipophilic chemicals. Substrates that have attracted the greatest attention thus far are steroids, fatty acids, eicosanoids, retinoids, other endogenous lipids, therapeutic agents, pesticides/herbicides, chemical carcinogens, industrial chemicals and other environmental contaminants and toxic xenobiotic organics of low molecular weight. Commonly, monooxygenation of such substrates results in the generation of metabolites capable of producing biological effects that are profoundly different (qualitatively as well as quantitatively) from those elicitable by the parent chemical per se. P45OXIX-dependent conversion of testosterone to estradiol-17 beta provides a dramatic example. Thus, these hemoproteins serve as extremely important but, as yet, largely unpredictable regulators of the biological effects producible by endobiotics as well as by xenobiotics. Current focus is on the identification and acquisition of sequence information on hereto unidentified and/or uncharacterized P450 isoforms and ascertainment of the specific functions of specific, individual isoforms. The regulation of quantities and activities of such isoforms in specific species/tissues, understandably, is also of great current interest. This interest has been further intensified by recent results indicating that substrate specificity associated with one P450 may not be the same as the corresponding isoform derived from a different animal species. Recent technological advances promise to greatly hasten the acquisition of knowledge concerning the functions of these important hemoproteins.