O-acetylsalicylhydroxamic acid, a novel acetylating inhibitor of prostaglandin H2 synthase: structural and functional characterization of enzyme-inhibitor interactions

Structural and Chemical Biology of the Interaction of Cyclooxygenase with Substrates and Non-Steroidal Anti-Inflammatory Drugs

Cyclooxgenases are key enzymes of lipid signaling. They carry out the first step in the production of prostaglandins, important mediators of inflammation, pain, cardiovascular disease, and cancer, and they are the molecular targets for nonsteroidal anti-inflammatory drugs, which are among the oldest and most chemically diverse set of drugs known. Homodimeric proteins that behave as allosterically modulated, functional heterodimers, the cyclooxygenases exhibit complex kinetic behavior, requiring peroxide-dependent activation and undergoing suicide inactivation. Due to their important physiological and pathophysiological roles and keen interest on the part of the pharmaceutical industry, the cyclooxygenases have been the focus of a vast array of structural studies, leading to the publication of over 80 crystal structures of the enzymes in complex with substrates or inhibitors supported by a wealth of functional data generated by site-directed mutation experiments. In this review, we explore the chemical biology of the cyclooxygenases through the lens of this wealth of structural and functional information. We identify key structural features of the cyclooxygenases, break down their active site into regional binding pockets to facilitate comparisons between structures, and explore similarities and differences in the binding modes of the wide variety of ligands (both substrates and inhibitors) that have been characterized in complex with the enzymes. Throughout, we correlate structure with function whenever possible. Finally, we summarize what can and cannot be learned from the currently available structural data and discuss the critical intriguing questions that remain despite the wealth of information that has been amassed in this field.

Synthesis, characterization, and biological properties of oxidovanadium(IV) complexes of acetylsalicylhydroxamic acid (N-acetyloxy-2-hydroxybenzamide) as potential antimicrobials

New oxidovanadium(IV) complexes of composition [VO(AcSHA)2] 1 and [VO(acac)(AcSHA)] 2 are synthesized by reactions of VOSO4.5H2O and [VO(acac)2] with acetylsalicylhydroxamic acid AcSH2A (C6H4(OH)(CONHOCOCH3)) in a 1:2 molar ratio in absolute ethanol. The compounds are characterized by the Fourier-transform infrared spectroscopy, ultraviolet–visible spectroscopy, electron spin resonance, and mass spectrometry along with elemental analyses, molar conductivity, and magnetic moment measurements. The infrared spectra of the complexes suggest bonding through carbonyl and phenolic oxygen atoms (O,O coordination). The magnetic moment, electron spin resonance, and mass spectra of the complexes indicate that both exist as monomers, and a distorted square pyramidal geometry around vanadium is proposed. The thermal behavior of the complexes is studied by thermogravimetry and differential thermal analysis techniques under an N2 atmosphere, yielding VO2 as the decomposition product. The in vitro antimicrobial assays against pathogenic Gram-positive bacteria, Gram-negative bacteria, and fungi (minimum inhibitory concentration method) show the appreciable antimicrobial potential relative to the respective standard drugs, tetracycline hydrochloride, and fluconazole.

Identification of N-Hydroxycinnamamide analogues and their bio-evaluation against breast cancer cell lines

The present study demonstrates the identification of N-hydroxycinnamamide derivatives and their anticancer potential against human triple-negative breast cancer cell line MDA-MB‑231, MCF-7 and non-malignant origin cell line, HEK-293 (human embryonic kidney). MTT assay was studied with HEK-293 cell line. Anticancer potential of the N-hydroxycinnamamide derivatives were compared with marked drug Tamoxifen through in vitro study. The compound numbers 3b and 3h exhibit most potent activity against antagonistic breast cancer cells (MDA-MB-231) with IC₅₀13μM and 5μM respectively. Compound 3h promotes DNA fragmentation and induction of apoptosis. Furthermore, loss of mitochondrial membrane potential induced by compound 3h. The major mechanism of compound 3h for anti-breast cancer activity was probably initiation of reactive oxygen species (ROS) in cancer cells thereby persuading apoptotic cell deaths in cancer cells.

Molecular basis for nonspecificity of nonsteroidal anti-inflammatory drugs (NSAIDs)

Inhibition of the production of inflammatory mediators by the action of nonsteroidal anti-inflammatory drugs (NSAIDs) is highly accredited to their recognition of cyclooxygenase enzymes. Along with inflammation relief, however, NSAIDs also cause adverse effects. Although NSAIDs strongly inhibit enzymes of the prostaglandin synthesis pathways, several other proteins also serve as fairly potent targets for these drugs. Based on their recognition pattern, these receptors are categorised as enzymes modifying NSAIDs, noncatalytic proteins binding to NSAIDs and enzymes with catalytic functions that are inhibited by NSAIDs. The extensive binding of NSAIDs is responsible for their limited in vivo efficacy as well as the large spectrum of their effects. The biochemical nature of drugs binding to multiple protein targets and its implications on physiology are discussed.

Physical evidence for substrate binding in preventing cyclooxygenase inactivation under nitrative stress

Prostaglandin biosynthesis is catalyzed by two spatially and functionally distinct active sites in cyclooxygenase (COX) enzymes. Despite the crucial role of COXs in biology, molecular details regarding the function and regulation of these enzymes are incompletely defined. Reactive nitrogen species, formed during oxidative stress, produce modifications that alter COX functionalities and prostaglandin biosynthesis. We previously established that COX-1 undergoes selective nitration on Tyr385 via a mechanism that requires the presence of bound heme cofactor. As this is a critical residue for COX-1 catalysis, nitration at this site results in enzyme inactivation. We now show that occupancy of the COX-1 active site with substrate protects against Tyr385 nitration and redirects nitration to alternative Tyr residues on COX-1, preserving catalytic activity. This study reveals a novel role for the substrate in protecting COX-1 from inactivation by nitration in pathophysiological settings.

Peroxide-induced radical formation at TYR385 and TYR504 in human PGHS-1

Prostaglandin H synthase isoforms 1 and -2 (PGHS-1 and -2) react with peroxide to form a radical on Tyr385 that initiates the cyclooxygenase catalysis. The tyrosyl radical EPR signals of PGHS-1 and -2 change over time and are altered by cyclooxygenase inhibitor binding. We characterized the tyrosyl radical dynamics using wild type human PGHS-1 (hPGHS-1) and its Y504F, Y385F, and Y385F/Y504F mutants to determine whether the radical EPR signal changes involve Tyr504 radical formation, Tyr385 radical phenyl ring rotation, or both. Reaction of hPGHS-1 with peroxide produced a wide singlet, whereas its Y504F mutant produced only a wide doublet signal, assigned to the Tyr385 radical. The cyclooxygenase specific activity and K(M) value for arachidonate of hPGHS-1 were not affected by the Y504F mutation, but the peroxidase specific activity and the K(M) value for peroxide were increased. The Y385F and Y385F/Y504F mutants retained only a small fraction of the peroxidase activity; the former had a much-reduced yield of peroxide-induced radical and the latter essentially none. After binding of indomethacin, a cyclooxygenase inhibitor, hPGHS-1 produced a narrow singlet but the Y504F mutant did not form a tyrosyl radical. These results indicate that peroxide-induced radicals form on Tyr385 and Tyr504 of hPGHS-1, with radical primarily on Tyr504 in the wild type protein; indomethacin binding prevented radical formation on Tyr385 but allowed radical formation on Tyr504. Thus, hPGHS-1 and -2 have different distributions of peroxide-derived radical between Tyr385 and Tyr504. Y504F mutants in both hPGHS-1 and -2 significantly decreased the cyclooxygenase activation efficiency, indicating that formation of the Tyr504 radical is functionally important for both isoforms.

Chemical and structural diversity in cyclooxygenase protein active sites

A major pharmaceutical problem is designing diverse and selective lead compounds. The human genome sequence provides opportunities to discover compounds that are protein selective if we can develop methods to identify specificity determinants from sequence alone. We have analyzed sequence and structural diversity of sheep COX-1 and mouse COX-2 proteins by Active Site Profiling (ASP). Eleven residues that should serve as specificity determinants between COX-1 and COX-2 were identified; however, the literature suggests that only one has been utilized in structure-based discovery. ASP was used to create a position-specific scoring matrix, which was used to identify possible cross-reacting proteins from the human sequences. This method proved selective for cyclooxygenases, comparing well with results using BLAST. The methods identify a probable misannotation of a cyclooxygenase in which there is high sequence similarity scores using BLAST, but ASP shows it does not contain the residues necessary for cyclooxygenase function. ASP Analysis of human COX proteins suggests that some specificity determinants that distinguish COX-1 and COX-2 proteins are similar between sheep COX-1/mouse COX-2 and human COX-1/COX2; however, residue identities at those positions are not necessarily conserved. Our results lay groundwork for development of family-specific pattern recognition methods to selectively match compounds with proteins.

Role of Tyr348 in Tyr385 radical dynamics and cyclooxygenase inhibitor interactions in prostaglandin H synthase-2

Both prostaglandin H synthase (PGHS) isoforms utilize a radical at Tyr385 to abstract a hydrogen atom from arachidonic acid, initializing prostaglandin synthesis. A Tyr348-Tyr385 hydrogen bond appears to be conserved in both isoforms; this hydrogen bonding has the potential to modulate the positioning and reactivity of the Tyr385 side chain. The EPR signal from the Tyr385 radical undergoes a time-dependent transition from a wide doublet to a wide singlet species in both isoforms. In PGHS-2, this transition results from radical migration from Tyr385 to Tyr504. Localization of the radical to Tyr385 in the recombinant human PGHS-2 Y504F mutant was exploited in examining the effects of blocking Tyr385 hydrogen bonding by introduction of a further Y348F mutation. Cyclooxygenase and peroxidase activities were found to be maintained in the Y348F/Y504F mutant, but the Tyr385 radical was formed more slowly and had greater rotational freedom, as evidenced by observation of a transition from an initial wide doublet species to a narrow singlet species, a transition not seen in the parent Y504F mutant. The effect of disrupting Tyr385 hydrogen bonding on the cyclooxygenase active site structure was probed by examination of cyclooxygenase inhibitor kinetics. Aspirin treatment eliminated all oxygenase activity in the Y348F/Y504F double mutant, with no indication of the lipoxygenase activity observed in aspirin-treated wild-type PGHS-2. Introduction of the Y348F mutation also strengthened the time-dependent inhibitory action of nimesulide. These results suggest that removal of Tyr348-Tyr385 hydrogen bonding in PGHS-2 allows greater conformational flexibility in the cyclooxygenase active site, resulting in altered interactions with inhibitors and altered Tyr385 radical behavior.

Heme catalyzes tyrosine 385 nitration and inactivation of prostaglandin H2 synthase-1 by peroxynitrite

The mechanism by which the inflammatory enzyme prostaglandin H(2) synthase-1 (PGHS-1) deactivates remains undefined. This study aimed to determine the stabilizing parameters of PGHS-1 and identify factors leading to deactivation by nitric oxide species (NO(x)). Purified PGHS-1 was stabilized when solubilized in beta-octylglucoside (rather than Tween-20 or CHAPS) and when reconstituted with hemin chloride (rather than hematin). Peroxynitrite (ONOO(-)) activated the peroxidase site of PGHS-1 independently of the cyclooxygenase site. After ONOO(-) exposure, holoPGHS-1 could not metabolize arachidonic acid and was structurally compromised, whereas apoPGHS-1 retained full activity once reconstituted with heme. After incubation of holoPGHS-1 with ONOO(-), heme absorbance was diminished but to a lesser extent than the loss in enzymatic function, suggesting the contribution of more than one process to enzyme inactivation. Hydroperoxide scavengers improved enzyme activity, whereas hydroxyl radical scavengers provided no protection from the effects of ONOO(-). Mass spectral analyses revealed that tyrosine 385 (Tyr 385) is a target for nitration by ONOO(-) only when heme is present. Multimer formation was also observed and required heme but could be attenuated by arachidonic acid substrate. We conclude that the heme plays a role in catalyzing Tyr 385 nitration by ONOO(-) and the demise of PGHS-1.

Parallel synthesis and in vitro activity of novel anthranilic hydroxamate-based inhibitors of the prostaglandin H2 synthase peroxidase activity

Currently available non-steroidal anti-inflammatory drugs (NSAIDs) such as aspirin are directed at the cyclooxygenase (COX) site, but not the peroxidase (POX) activity of prostaglandin H2 synthase (PGHS). They are thus unable to inhibit the free-radical induced tissue injury associated with PGHS peroxidase activity, which can occur independently of the COX site. A lead compound, anthranilic hydroxamic acid (AHA) was found to have significant PGHS-POX inhibitory activity (IC50= 72 microM). To define the critical parameters for PGHS-POX inhibition, we investigated 29 AHA derivatives, synthesised from their acid precursors, using solid phase synthesis. In vitro analysis demonstrated a ten-fold improvement in inhibition with 3,5-diiodoanthranilic hydroxamic acid (IC50= 7 microM).

Aspirin induces nitric oxide release from vascular endothelium: a novel mechanism of action

1. The study was designed to test the hypothesis that aspirin may stimulate nitric oxide (NO) release from vascular endothelium, a pivotal factor for maintenance of vascular homeostasis. 2. Clinical evidence suggests that low-dose aspirin may improve vascular endothelial function. Since other cyclooxygenase (COX) inhibitors showed no beneficial vascular effects, aspirin may exhibit a vasculoprotective, COX-independent mechanism. 3. Luminal NO release was monitored in real time on dissected porcine coronary arteries (PCA) by an amperometric, NO-selective sensor. Additionally, endothelial NO synthase (eNOS) activity was measured in EA.hy 926 cell homogenates by an l-[(3)H]citrulline/l-[(3)H]arginine conversion assay. Superoxide scavenging capacity was assessed by lucigenin-enhanced luminescence. 4. Aspirin induced an immediate concentration-dependent NO release from PCA with an EC(50) of 50 nm and potentiated the NO stimulation by the receptor-dependent agonist substance P. These effects were independent of an increase in intracellular calcium and could be mimicked by stimulation with acetylating aspirin derivatives. The aspirin metabolite salicylic acid or the reversible cyclooxygenase inhibitor indomethacin failed to modulate NO release. Incubation of soluble eNOS for 15 min with 100 microm aspirin or acetylating aspirin analogues increased the l-[(3)H]citrulline yield by 40-80%, while salicylic acid had no effect. Aspirin and salicylic acid showed a similar, but only modest, magnitude and velocity of superoxide scavenging. 5. Our findings demonstrate that therapeutically relevant concentrations of aspirin elicit NO release from vascular endothelium. This effect appears to be due to a direct acetylation of the eNOS protein, but is independent of COX inhibition or inhibition of superoxide-mediated NO degradation.

Histidine 386 and its role in cyclooxygenase and peroxidase catalysis by prostaglandin-endoperoxide H synthases

Prostaglandin-endoperoxide H synthases (PGHSs) have a cyclooxygenase that forms prostaglandin (PG) G2 from arachidonic acid (AA) plus oxygen and a peroxidase that reduces the PGG2 to PGH2. The peroxidase activates the cyclooxygenase. This involves an initial oxidation of the peroxidase heme group by hydroperoxide, followed by oxidation of Tyr385 to a tyrosyl radical within the cyclooxygenase site. His386 of PGHS-1 is not formally part of either active site, but lies in an extended helix between Tyr385, which protrudes into the cyclooxygenase site, and His388, the proximal ligand of the peroxidase heme. When His386 was substituted with alanine in PGHS-1, the mutant retained <2.5% of the native peroxidase activity, but >20% of the native cyclooxygenase activity. However, peroxidase activity could be restored (10-30%) by treating H386A PGHS-1 with cyclooxygenase inhibitors or AA, but not with linoleic acid; in contrast, mere occupancy of the cyclooxygenase site of native PGHS-1 had no effect on peroxidase activity. Heme titrations indicated that H386A PGHS-1 binds heme less tightly than does native PGHS-1. The low peroxidase activity and decreased affinity for heme of H386A PGHS-1 imply that His386 helps optimize heme binding. Molecular dynamic simulations suggest that this is accomplished in part by a hydrogen bond between the heme D-ring propionate and the N-delta of Asn382 of the extended helix. The structure of the extended helix is, in turn, strongly supported by stable hydrogen bonding between the N-delta of His386 and the backbone carbonyl oxygens of Asn382 and Gln383. We speculate that the binding of cyclooxygenase inhibitors or AA to the cyclooxygenase site of ovine H386A PGHS-1 reopens the constriction in the cyclooxygenase site between the extended helix and a helix containing Gly526 and Ser530 and restores native-like structure to the extended helix. Being less bulky than AA, linoleic acid is apparently unable to reopen this constriction.

The structure of mammalian cyclooxygenases

Cyclooxygenases-1 and -2 (COX-1 and COX-2, also known as prostaglandin H2 synthases-1 and -2) catalyze the committed step in prostaglandin synthesis. COX-1 and -2 are of particular interest because they are the major targets of nonsteroidal antiinflammatory drugs (NSAIDs) including aspirin, ibuprofen, and the new COX-2-selective inhibitors. Inhibition of the COXs with NSAIDs acutely reduces inflammation, pain, and fever, and long-term use of these drugs reduces the incidence of fatal thrombotic events, as well as the development of colon cancer and Alzheimer's disease. In this review, we examine how the structures of COXs relate mechanistically to cyclooxygenase and peroxidase catalysis and how alternative fatty acid substrates bind within the COX active site. We further examine how NSAIDs interact with COXs and how differences in the structure of COX-2 result in enhanced selectivity toward COX-2 inhibitors.

A novel family of hydroxamate-based acylating inhibitors of cyclooxygenase

Aspirin irreversibly inhibits cyclooxygenase (COX) by acetylating a serine residue in the active site. We synthesized a series of novel acylating agents based on our previously reported acetylating compound, O-acetylsalicylhydroxamic acid. One of these, triacetylsalicylhydroxamic acid (TriAcSHA) was more effective than aspirin and O-acetylsalicylhydroxamic acid in inactivating both COX-1 and COX-2. Preincubation of COX-1 with inhibitor for 5 min yielded IC(50) values of 18 microM for TriAcSHA and 60 microM for acetylsalicylic acid. Inhibition was time-dependent, with complete inhibition within 10 min at a concentration of 50 microM. As with aspirin, mutation of the serine 530 of COX-1 to alanine abolished the activity of the TriAcSHA. Mutation of the alanine 119 to a glutamine markedly reduced the sensitivity to TriAcSHA, suggesting that this residue was necessary for the interaction with the enzyme. TriAcSHA was also more effective than aspirin as an inhibitor of platelet aggregation induced by arachidonic acid. The diacetylated phenylhydroxamates N-methyl-O,O-diacetylsalicylhydroxamic acid, N,O-diacetylbenzohydroxamic acid, and 2-methyl-O,N-diacetylbenzohydroxamic acid showed reduced or absent activity against COX-1. In addition, we synthesized a series of triacylsalicylhydroxamic acids with progressively longer acyl groups (three to six carbons). All of the compounds inhibited COX-1 and demonstrated progressively greater COX-1 selectivity with increasing number of carbons. Hence, salicylhydroxamic acid provides a versatile backbone for the generation of a family of acylating inhibitors of cyclooxygenase.