Inhibition of peroxidase activity by some non-steroidal anti-inflammatory drugs

Inhibitors of Eicosanoid Biosynthesis Reveal that Multiple Lipid Signaling Pathways Influence Malaria Parasite Survival in Anopheles gambiae

Eicosanoids are bioactive signaling lipids derived from the oxidation of fatty acids that act as important regulators of immune homeostasis and inflammation. As a result, effective anti-inflammatory drugs have been widely used to reduce pain and inflammation which target key eicosanoid biosynthesis enzymes. Conserved from vertebrates to insects, the use of these eicosanoid pathway inhibitors offer opportunities to evaluate the roles of eicosanoids in less-characterized insect systems. In this study, we examine the potential roles of eicosanoids on malaria parasite survival in the mosquito Anopheles gambiae. Using Plasmodium oocyst numbers to evaluate parasite infection, general or specific inhibitors of eicosanoid biosynthesis pathways were evaluated. Following the administration of dexamethasone and indomethacin, respective inhibitors of phospholipid A2 (PLA2) and cyclooxygenase (COX), oocyst numbers were unaffected. However, inhibition of lipoxygenase (LOX) activity through the use of esculetin significantly increased oocyst survival. In contrast, 12-[[(tricyclo[3.3.1.13,7]dec-1-ylamino)carbonyl]amino]-dodecanoic acid (AUDA), an inhibitor of epoxide hydroxylase (EH), decreased oocyst numbers. These experiments were further validated through RNAi experiments to silence candidate genes homologous to EH in An. gambiae to confirm their contributions to Plasmodium development. Similar to the results of AUDA treatment, the silencing of EH significantly reduced oocyst numbers. These results imply that specific eicosanoids in An. gambiae can have either agonist or antagonistic roles on malaria parasite survival in the mosquito host.

Mosquito Midgut Prostaglandin Release Establishes Systemic Immune Priming

Anopheles gambiae mosquitoes that have been infected with Plasmodium mount a more effective immune response to a subsequent infection. Priming is established when Plasmodium invasion of the mosquito midgut allows contact of the gut microbiota with epithelial cells. This event is followed by a systemic release of a hemocyte differentiation factor (HDF) consisting of Lipoxin A4 bound to Evokin, a lipocalin carrier, which increases the proportion of circulating hemocytes. We show that mosquito midgut cells produce and release prostaglandin E2 (PGE2), which attracts hemocytes to the midgut surface and enhances their patrolling activity. Systemic injection of prostaglandins (PGs) recapitulates the priming response and enhances antiplasmodial immunity by triggering HDF production. Although insects lack cyclooxygenases, two heme peroxidases, HPX7 and HPX8, catalyze essential steps in PG biosynthesis in mosquitoes. Mosquito midgut PGE2 release attracts hemocytes and establishes a long-lasting enhanced systemic cellular immune response to Plasmodium infection.

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Plasmodium invasion or bacterial exposure triggers midgut prostaglandin synthesis

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Prostaglandins attract mosquito hemocytes and increase their patrolling activity

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Two midgut peroxidases, HPX7 and HPX8, catalyze midgut prostaglandin synthesis

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Systemic release of midgut prostaglandins is essential to establish immune priming

Lactoperoxidase-catalysed oxidation of indomethacin, a nonsteroidal antiinflammatory drug, through the formation of a free radical

Lactoperoxidase (LPO, EC 1.11.1.7; donor-H2O2 oxidoreductase) catalyses the oxidation of indomethacin, a nonsteroidal antiinflammatory drug by H2O2 as measured by time-dependent decay of indo-methacin extinction at 280 nm and concurrent appearance of stable oxidation product(s) at 412 nm. From a plot of log Vmax against varying pH of indomethacin oxidation, involvement of an ionizable group of the enzyme having pka = 5.7 could be ascertained for controlling the oxidation process. Spectral studies revealed that LPO-compound II oxidises indomethacin through one-electron transfer and is reduced to the native ferric state as shown by its spectral shift from 430 nm to 412 nm through an isosbestic point at 421 nm. The one-electron oxidation product is a nitrogen-centered free radical detected as a 5,5-dimethyl-l-pyrroline N-oxide (DMPO) adduct (alpha N = 15 G, alpha H beta = 16 G) in electron spin resonance spectroscopy. The free radical is scavenged by reaction with O2 as shown by O2 consumption sensitive to the free-radical trap, DMPO. Binding studies by optical difference spectroscopy indicate that indomethacin binds to LPO with an apparent KD value of 24.5 microM. The free energy change, delta G', for the binding is -26.3 KJ mol-1, suggesting that the interaction is favourable for oxidation. Indomethacin binding remains unaltered by a change of pH from 5.25 to 7.5, presumably because of hydrophobic interaction. The binding is competitive with resorcinol, an aromatic electron donor, showing the KD value to be as high as 100 microM. We suggest that indomethacin interacts at the aromatic donor binding site and is oxidised by one-electron transfer by LPO catalytic intermediates to stable oxidation product(s) through the formation of a free radical.

Effects of indole-3-acetic acid on croton oil- and arachidonic acid-induced mouse ear edema

The indole-3-acetic acid (IAA) is a plant growth hormone (auxin) being considered as a tryptophan metabolite in animals. The main purpose of this work was to verify IAA's topical anti-inflammatory action using croton oil- or arachidonic acid-induced mouse ear edema, in comparison to known anti-inflammatory agents. IAA antioxidant activity was also verified by measuring the inhibition of brain homogenate lipid peroxidation with the thiobarbituric acid reactive substances (TBARS) test. IAA inhibited the action of both croton oil-and arachidonic acid-induced edema in a dose-dependent manner (4.0 mumoles IAA inhibited 75.8% in croton oil and 82.5% in arachidonic acid induced ear edema). Both IAA (5.3 mM) and indomethacin (8.0 mM) inhibited TBARS formation. Data suggest that IAA exhibits antiinflammatory effect possibly by its anti-oxidant activity.

Inhibition of intestinal peroxidase activity by nonsteroidal antiinflammatory drugs

The peroxidase activity of the mitochondrial fraction of rat intestine is inhibited in vitro by non-steroidal antiinflammatory drugs (NSAIDs), such as indomethacin (IMN) and acetylsalicylic acid (ASA), the former being more potent than the latter. The peroxidase was solubilised by cetab-NH4Cl extraction and purified to apparent homogeneity by Sephadex G-150 gel filtration and affinity chromatography on Con-A Sepharose. The purified enzyme activity was 80% inhibited by 150 microM IMN and 50% by 2.67 mM ASA. IMN could also inhibit lactoperoxidase activity to the same extent but not the horseradish peroxidase activity. The inhibition of peroxidase-catalysed iodide oxidation by IMN and ASA was optimal at pH 5.5 and 4.5, respectively. Kinetic studies revealed that the inhibition by IMN was competitive with respect to iodide or guaiacol, while the inhibition by ASA was noncompetitive and reversible in nature. Studies of some structural analogues showed that indole-3-acetic acid was as effective as IMN, while salicylic acid was more potent than ASA. Spectral studies showed a small bathochromic shift of the Soret band of the enzyme by IMN, suggesting its possible interaction at or near the heme moiety. The competitive nature of IMN may be explained as due to its oxidation by the peroxidase to a product absorbing at 412 nm, the formation of which is inhibited by iodide. We suggest that IMN inhibits intestinal peroxidase activity by acting as a competitive substrate for the enzyme. As intestinal peroxidase is mainly contributed by the invading eosinophils, NSAIDs may affect the host defence mechanism by inhibiting the activity of the enzyme.

Nonsteroidal anti-inflammatory drugs inhibit gastric peroxidase activity

The peroxidase activity of the mitochondrial fraction of rat gastric mucosa was inhibited with various nonsteroidal anti-inflammatory drugs (NSAIDs) in vitro. Indomethacin was found to be more effective than phenylbutazone (PB) or acetylsalicylic acid (ASA). Mouse gastric peroxidase was also very sensitive to indomethacin inhibition. Indomethacin has no significant effect on submaxillary gland peroxidase activity of either of the species studied. Purified rat gastric peroxidase activity was inhibited 75% with 0.15 mM indomethacin showing half-maximal inhibition at 0.04 mM. The inhibition could be withdrawn by increasing the concentration of iodide but not by H2O2. NSAIDs inhibit gastric peroxidase activity more effectively at acid pH (pH 5.2) than at neutral pH. Spectral studies showed a bathochromic shift of the Soret band of the enzyme with indomethacin indicating its interaction at or near the heme part of the enzyme.

Peroxidase catalysed formation of prostaglandins from arachidonic acid

Horseradish peroxidase and bovine lactoperoxidase (EC 1.11.1.7), when incubated aerobically with arachidonate, gave rise to the formation of substances identified by bioassay as prostaglandin F2 alpha (PGF2 alpha)- and prostaglandin E2 (PGE2)-like compounds. Boiling of enzymes, which suppressed their capacity to peroxidize guaiacol, also destroyed their capacity to convert arachidonate into PG-like compounds. The rates of formation of PG-like compounds rapidly declined with time, approaching zero after 10 and 20 min for PGE2 alpha- and PGE2-like compounds, respectively. Addition of more enzyme further promoted the reaction. Horseradish and lacto-peroxidases showed optimum pH values of 9.0 and 10.0, respectively. Both enzymes exhibited apparent Km values of about 5 x 10(-5) M for arachidonate. Some reducing agents such as ascorbic acid, NADH and adrenaline dose-dependently inhibited this reaction. The haem poison, phenylhydrazine, also inhibited, with an IC50 of 1 x 10(-7) M. Indomethacin inhibited only the formation of PGE2-like compounds with an IC50 of about 3 x 10(-6) M. As compared to a standard commercial preparation of horseradish peroxidase, the purified horseradish basic and acidic isoenzymes exhibited a higher activity, towards arachidonate whereas other haemoproteins, possessing peroxidase activity, were less active. TLC and GC-MS analyses performed on the reaction products led to the identification of PGF2 alpha, PGE2 and PG6K1 alpha and other unidentified arachidonate derivatives. At 25 degrees, pH 9.5, horseradish peroxidase, acting on saturating concentration of arachidonate, catalysed the formation of 60 mumol/min/mmole enzyme of PGE2 + PGF2 alpha. This appears to be the first report of the synthesis of prostaglandins catalysed by peroxidases.

The amplified chemiluminescence test to characterize antirheumatic drugs as oxygen radical scavengers

High levels of reactive oxygen species (ROS) are generated by phagocytes involved in host defence and inflammation. Thus, it appears highly desirable to learn more about the potential of antirheumatic drugs to scavenge ROS or to inhibit their enzymatic generation. Amplified chemiluminescence (CL) allows detection of O-2 using lucigenin (LgCL) or H2O2 using luminol (LuCL). A total of 43 compounds have been tested quantitatively in vitro (10(-6) to 10(-4) mol/l) with respect to three test parameters; varying cell-activity, and incubation-time employing two different phagocyte populations (neutrophils/macrophages). The most active compounds with LgCL were the known radical scavengers nordihydroguaiaretic acid (NDGA), N-propyl gallate, superoxide dismutase and chloroquine, the non-steroidal anti-inflammatory drugs (NSAID) benzydamine, timegadine, carprofen, enolicam, the known lipoxygenase inhibitors (e.g. CBS 1108/1114, BW 755C) and glucosaminoglucan polysulfate. Inactive in this system were corticosteroids (prednisolone, dexamethasone) most of the tested NSAID (N = 16/20), most disease modifying drugs (D-penicillamine, levamisole, gold-TM) and the anti-gout drugs (sulfinpyrazone, allopurinol, colchicine). Therefore amplified CL with lucigenin appears to be a rapid, kinetic, reproducible means of pharmacological profiling in vitro new anti-inflammatory drugs for radical scavenger activity.