Iodination of arachidonic acid mediated by eosinophil peroxidase, myeloperoxidase and lactoperoxidase. Identification and comparison of products

Non-union bone fractures

The human skeleton has remarkable regenerative properties, being one of the few structures in the body that can heal by recreating its normal cellular composition, orientation and mechanical strength. When the healing process of a fractured bone fails owing to inadequate immobilization, failed surgical intervention, insufficient biological response or infection, the outcome after a prolonged period of no healing is defined as non-union. Non-union represents a chronic medical condition not only affecting function but also potentially impacting the individual's psychosocial and economic well-being. This Primer provides the reader with an in-depth understanding of our contemporary knowledge regarding the important features to be considered when faced with non-union. The normal mechanisms involved in bone healing and the factors that disrupt the normal signalling mechanisms are addressed. Epidemiological considerations and advances in the diagnosis and surgical therapy of non-union are highlighted and the need for greater efforts in basic, translational and clinical research are identified.

Regulation of NADPH oxidase NOX4 by delta iodolactone (IL-δ) in thyroid cancer cells

INTRODUCTION

Iodine is not used only by the thyroid to synthesize thyroid hormones but also directly influences a number of thyroid parameters such as thyroid proliferation and function. Several iodinated lipids, biosynthesized by the thyroid, were postulated as intermediaries in the action of iodide. Among these, iodolactone (IL-δ) and 2-iodohexadecanal (2-IHDA) have shown to inhibit several thyroid parameters. The antiproliferative effect of IL-δ is not restricted to the thyroid gland. IL-δ exhibits anti-tumor properties in breast cancer, neuroblastoma, glioblastoma, melanoma and lung carcinoma cells suggesting that IL-δ could be used as a chemotherapeutic agent. Moreover in a colon cancer cell line (HT-29), IL-δ induced cell death, and this effect was mediated by reactive oxygen species (ROS) generation. The aim of the present study was to analyze the sources of reactive oxygen species induced by IL-δ and to explore the contribution of ROS induced by IL-δ on cell proliferation and apoptosis.

METHODOLOGY AND RESULTS

Cancer thyroid follicular (WRO) and papilar (TPC-1) cells lines were treated with IL-δ. Proliferation and apoptosis was analyzed. IL-δ caused a significant loss of cell viability on WRO and TPC-1 cells in a concentration dependent manner and induced apoptosis after 3 h of treatment. Furthermore, IL-δ (10 μM) increased ROS production (39% WRO and 20% TPC-1). The concomitant treatment of WRO and TPC-1 cells with Trolox or NAC plus IL-δ abrogated the augment of ROS induced by IL-δ exposure. Additionally Trolox and NAC reversed the effect of IL-δ on cell proliferation and apoptosis. Only in WRO cells IL-δ upregulates NADPH oxidase NOX4 expression, and siRNA targeted knock-down of NOX4 attenuates ROS production, apoptosis (p < 0.05) and the inhibitory effect of IL-δ on cell proliferation and PCNA expression (p < 0.05).

CONCLUSIONS

The antiproliferative and pro-apoptotic effect of IL-δ is mediated by different mechanisms and pathway involving different sources of ROS generation depending on the cellular context.

Broad-Spectrum Antimicrobial Effects of Photocatalysis Using Titanium Dioxide Nanoparticles Are Strongly Potentiated by Addition of Potassium Iodide

Photocatalysis describes the excitation of titanium dioxide nanoparticles (a wide-band gap semiconductor) by UVA light to produce reactive oxygen species (ROS) that can destroy many organic molecules. This photocatalysis process is used for environmental remediation, while antimicrobial photocatalysis can kill many classes of microorganisms and can be used to sterilize water and surfaces and possibly to treat infections. Here we show that addition of the nontoxic inorganic salt potassium iodide to TiO2 (P25) excited by UVA potentiated the killing of Gram-positive bacteria, Gram-negative bacteria, and fungi by up to 6 logs. The microbial killing depended on the concentration of TiO2, the fluence of UVA light, and the concentration of KI (the best effect was at 100 mM). There was formation of long-lived antimicrobial species (probably hypoiodite and iodine) in the reaction mixture (detected by adding bacteria after light), but short-lived antibacterial reactive species (bacteria present during light) produced more killing. Fluorescent probes for ROS (hydroxyl radical and singlet oxygen) were quenched by iodide. Tri-iodide (which has a peak at 350 nm and a blue product with starch) was produced by TiO2-UVA-KI but was much reduced when methicillin-resistant Staphylococcus aureus (MRSA) cells were also present. The model tyrosine substrate N-acetyl tyrosine ethyl ester was iodinated in a light dose-dependent manner. We conclude that UVA-excited TiO2 in the presence of iodide produces reactive iodine intermediates during illumination that kill microbial cells and long-lived oxidized iodine products that kill after light has ended.

6-iodolactone, key mediator of antitumoral properties of iodine

An iodinated derivative of arachidonic acid, 5-hydroxy-6-iodo-8,11,14-eicosatrienoic acid, δ-lactone (6-IL) has been implicated as a possible intermediate in the autoregulation of the thyroid gland by iodine. In addition to antiproliferative and apoptotic effects observed in thyrocytes, this iodolipid could also exert similar actions in cells derived from extrathyroidal tissues like mammary gland, prostate, colon, or the nervous system. In mammary cancer (solid tumors or tumor cell lines), 6-IL has been detected after molecular iodine (I2) supplement, and is a potent activator of peroxisome proliferator-activated receptor type gamma (PPARγ). These observations led us to propose I2 supplement as a novel coadjutant therapy which, by inducing differentiation mechanisms, decreases tumor progression and prevents chemoresistance. Some kinds of tumoral cells, in contrast to normal cells, contain high concentrations of arachidonic acid, making the I2 supplement a potential "magic bullet" that enables local, specific production of 6-IL, which then exerts antineoplastic actions with minimal deleterious effects on normal tissues.

Role of lipid peroxidation and oxidative stress in the association between thyroid diseases and breast cancer

Evidence is accumulating for a role of the thyroid in the natural history of breast cancer, although no plausible mechanism has been advanced to explain this association. We believe that the thyroid disease-breast cancer relationship provides a unique opportunity to find out the causes of breast cancer. Both diseases are female predominant, with specifically identified biological pathways and genetic and environmental determinants, and seeing them in concert provides an opportunity to identify the most relevant mechanistic pathways. In this communication, we advance a plausible mechanism to explain the thyroid disease-breast cancer relationship. We specifically propose that the reduction in risk associated with hyperthyroidism or increased levels of thyroid hormones, or iodine, may derive from the pro-oxidant properties of these compounds, i.e., from its ability to generate oxidative stress-induced apoptosis. Conversely, the increased risk from hypothyroidism may derive from its ability to inhibit this stress-mediated apoptotic process.

Differential action of iodine on mitochondria from human tumoral- and extra-tumoral tissue in inducing the release of apoptogenic proteins

Iodide is actively concentrated in the thyroid gland for thyroid hormone biosynthesis. Excess iodine has been observed to induce apoptosis in thyrocytes and mammary cells. The mechanism of iodine induced apoptosis is poorly understood. Among various cell organelles, mitochondria is known to provide conducive environment for the organification of iodine, i.e. iodination of different proteins. Mitochondria also play a central role in execution of apoptosis. To study the role of mitochondria in iodine induced apoptosis, we investigated the direct interaction of iodine and human breast mitochondria vis-a-vis its role in the initiation of apoptosis in vitro. We observed that mitochondria isolated from the tumor (TT) and extra-tumoral tissue (ET) of human breast display significant uptake of iodine. Mitochondrial proteins were observed to be predominantly iodinated in ET but not in TT mitochondria. Treatment with iodine showed an increase in mitochondrial permeability transition of TT and decrease in ET. Iodine induced released factor(s) other than cytochrome c from tumor mitochondria initiate(s) apoptosis in vitro, while those from ET mitochondria were non-apoptogenic in nature. To our knowledge, this is first report demonstrating that iodine acts differentially on mitochondria of tumor and extratumoral origin to release apoptogenic proteins from TT and has a protective effect on ET.

Expression of Sodium Iodide Symporter in the Lacrimal Drainage System: Implication for the Mechanism Underlying Nasolacrimal Duct Obstruction in I131-Treated Patients

PURPOSE

Nasolacrimal outflow obstruction has been associated with high-dose (>150 mCi) radioactive iodine (I(131)) treatment. Commonly used for thyroid cancer treatment, I(131) is effectively transported in the targeted tissue by the Na(+)/I symporter (NIS). We hypothesized that NIS is expressed in the lacrimal sac and nasolacrimal duct and that active accumulation of I(131) is responsible for the clinical observations seen in these patients.

METHODS

Reverse transcriptase-polymerase chain reaction and immunohistochemical analyses were used to evaluate NIS expression in both archived and fresh human tissues

RESULTS

Reverse transcriptase-polymerase chain reaction analysis showed that NIS mRNA is present in the lacrimal sac. Immunohistochemical analysis indicated that NIS protein is expressed in the stratified columnar epithelial cells of the lacrimal sac and nasolacrimal duct. NIS protein was undetectable in the lacrimal gland, Wolfring and Krause glands, conjunctiva, canaliculus, and nasal mucosa. NIS-expressing columnar epithelial cells were absent and fibrosis was evident in the lacrimal sacs from I(131)-treated patients undergoing dacryocystorhinostomy.

CONCLUSIONS

NIS is present in the lacrimal sac and nasolacrimal duct of humans, correlating to the anatomic areas of clinical obstruction that develop in patients treated with greater than 150 mCi of I(131). This suggests that NIS may be the vector of radiation-induced injury to the lacrimal system. To our knowledge, this is the first report of any ion transporter in the nasolacrimal outflow system and raises new questions as to the role the lacrimal sac plays in the modification of tears and in lacrimal outflow pathology.

Enhanced killing of Acanthamoeba cysts with a plant peroxidase-hydrogen peroxide-halide antimicrobial system

The activity of H(2)O(2) against the resistant cyst stage of the pathogenic free-living amoeba Acanthamoeba was enhanced by the addition of KI and either horseradish peroxidase or soybean peroxidase or, to a lesser degree, lactoperoxidase. This resulted in an increase in the cysticidal activity of 3% (wt/vol) H(2)O(2), and there was >3-log killing in 2 h, compared with the 6 h required for comparable results with the peroxide solution alone (P < 0.05). With 2% H(2)O(2), enhancement was observed at all time points (P < 0.05), and total killing of the cyst inoculum occurred at 4 h, compared with 6 h for the peroxide alone. The activity of sublethal 1% H(2)O(2) was enhanced to give 3-log killing after 8 h of exposure (P < 0.05). No enhancement was obtained when KCl or catalase was used as a substitute in the reaction mixtures. The H(2)O(2) was not neutralized in the enhanced system during the experiments. However, in the presence of a platinum disk used to neutralize H(2)O(2) in contact lens care systems, the enhanced 2% H(2)O(2) system gave 2.8-log killing after 6 h or total cyst killing by 8 h, and total neutralization of the H(2)O(2) occurred by 4 h. In contrast, 2% H(2)O(2) alone resulted in <0.8-log killing of cysts in the presence of the platinum disk due to rapid (<1 h) neutralization of the peroxide. Our observations could result in significant improvement in the efficacy of H(2)O(2) contact lens disinfection systems against Acanthamoeba cysts and prevention of acanthamoeba keratitis.

Functional expression of sodium iodide symporter (NIS) in human breast cancer tissue

Sodium iodide symporter (NIS) is a molecule involved in active accumulation of iodine in thyroid gland for the biosynthesis of thyroid hormone. Its expression has also been demonstrated in extra-thyroidal tissues including lactating mice mammary gland and also in human breast cancers. Iodide transport in thyroid cells through NIS is the basis for using radioiodine for diagnosis and treatment of differentiated thyroid carcinoma. The similar approach may prove beneficial for the diagnosis and treatment of breast cancer if iodine uptake, its retention and NIS expression can be shown unequivocally in malignant tumors. The aim of the present study was to investigate NIS expression, in vivo iodine transport ability and fate of iodine in human breast tumors. Women (age 33-58 years) with infiltrating duct carcinoma confirmed by FNAC and subsequent histopathology were the subject of this study. Expression of NIS RNA and protein was confirmed by RNAase protection assay, western blot and immunohistochemistry respectively in surgically excised breast tumor tissue. Iodine transport ability and its nature was assessed both in vivo and in vitro. We report high NIS expression at both transcriptional and translational level and its ability to transport iodine in human breast tumors. The in vivo iodine transport ability was confirmed by scintigraphy. Unlike thyroid, perchlorate and thiocyanate do not inhibit iodine transport in breast tumors. The presence of iodinated proteins suggests the longer retention time. The unequivocal demonstration of NIS expression, its functionality and retention of iodine by organification further provides supportive evidence for use of radioiodine as an additional treatment modality of human breast carcinoma.

Inactivation of urate oxidase by a system composed of lactoperoxidase, hydrogen peroxide and bromide

Urate oxidase from Candida utilis, an enzyme containing an essential thiol, was examined for its sensitivity to lactoperoxidase, an oxidant present in breast milk. Upon exposure to a system composed of lactoperoxidase, hydrogen peroxide and bromide at moderately alkaline pH, the urate oxidase exhibited comparable activity to the untreated enzyme; but upon exposure at moderately acidic pH, it lost its activity completely. Thus the lactoperoxidase-H2O2-bromide system significantly inactivated urate oxidase only at moderately acidic pH. This inactivation was prevented by the presence of N-acetylmethionine, a methionine analogue, or glutathione, which is a thiol compound analogous to an amino acid, indicating that it was probably due to the oxidation and damage of the methionine residue and/or the thiol group in the urate oxidase by the lactoperoxidase system, that loss of catalytic activity of the urate oxidase occurred.

Molecular chlorine generated by the myeloperoxidase-hydrogen peroxide-chloride system of phagocytes converts low density lipoprotein cholesterol into a family of chlorinated sterols

Oxidation of low density lipoprotein (LDL) may be of critical importance in triggering the pathological events of atherosclerosis. Myeloperoxidase, a heme protein secreted by phagocytes, is a potent catalyst for LDL oxidation in vitro, and active enzyme is present in human atherosclerotic lesions. We have explored the possibility that reactive intermediates generated by myeloperoxidase target LDL cholesterol for oxidation. LDL exposed to the myeloperoxidase-H2O2-Cl- system at acidic pH yielded a family of chlorinated sterols. The products were identified by mass spectrometry as a novel dichlorinated sterol, cholesterol alpha-chlorohydrin (6beta-chlorocholestane-(3beta,5alpha)-diol), cholesterol beta-chlorohydrin (5alpha-chlorocholestane-(3beta, 6beta)-diol), and a structurally related cholesterol chlorohydrin. Oxidation of LDL cholesterol by myeloperoxidase required H2O2 and Cl-, suggesting that hypochlorous acid (HOCl) was an intermediate in the reaction. However, HOCl failed to generate chlorinated sterols under chloride-free conditions. Since HOCl is in equilibrium with molecular chlorine (Cl2) through a reaction which requires Cl- and H+, this raised the possibility that Cl2 was the actual chlorinating intermediate. Consonant with this hypothesis, HOCl oxidized LDL cholesterol in the presence of Cl- and at acidic pH. Moreover, in the absence of Cl- and at neutral pH, Cl2 generated the same family of chlorinated sterols as the myeloperoxidase-H2O2-Cl- system. Finally, direct addition of Cl2 to the double bond of cholesterol accounts for dichlorinated sterol formation by myeloperoxidase. Collectively, these results indicate that Cl2 derived from HOCl is the chlorinating intermediate in the oxidation of cholesterol by myeloperoxidase. Our observations suggest that Cl2 generation in acidic compartments may constitute one pathway for oxidation of LDL cholesterol in the artery wall.

Cholesterol chlorohydrin synthesis by the myeloperoxidase-hydrogen peroxide-chloride system: potential markers for lipoproteins oxidatively damaged by phagocytes

Myeloperoxidase, a heme protein secreted by activated phagocytes, uses hydrogen peroxide to produce potent cytotoxins. One important substrate is chloride, which is converted to hypochlorous acid (HOCl). This diffusible oxidant plays a critical role in the destruction of invading pathogens. Under pathological conditions, HOCl may also injure normal tissue. Recent studies have shown that myeloperoxidase is a component of human atherosclerotic lesions. Because oxidized lipoproteins may play a central role in atherogenesis, we have explored the possibility that cholesterol is a target for damage by myeloperoxidase. Three major classes of sterol oxidation products were apparent when cholesterol-phosphatidylcholine multilamellar vesicles which had been exposed to a myeloperoxidase-hydrogen peroxide-chloride system were subsequently analyzed by normal-phase thin layer chromatography. The products were identified by gas chromatography-mass spectrometry as cholesterol alpha- and beta-chlorohydrins (6 beta-chlorocholestane-3 beta,5 alpha-diol and 5 alpha-chlorocholestane-3 beta,6 beta-diol), cholesterol alpha- and beta-epoxides (cholesterol 5 alpha,6 alpha-epoxide and cholesterol 5 beta,6 beta-epoxide), and a novel cholesterol chlorohydrin. Conversion of cholesterol to the oxidation products required active myeloperoxidase, hydrogen peroxide, and halide and could be blocked by catalase or by scavengers of HOCl. Moreover, in the absence of the enzymatic system, reagent HOCl generated the same distribution of products. These results indicate that myeloperoxidase can convert cholesterol to chlorohydrins and epoxides by a reaction involving HOCl. Other oxygenated sterols are cytotoxic and mutagenic and are potent regulators of cholesterol homeostasis in cultured mammalian cells. Cholesterol chlorohydrins might similarly mediate powerful biological effects in the artery wall. Because chlorohydrins are stable under our experimental conditions, chlorinated sterols may prove useful as markers for lipoproteins oxidatively damaged by activated phagocytes.

Tritium isotope effect in high-performance liquid chromatography of eicosanoids

A significant difference in retention time between unlabeled and the corresponding multi-tritium-labeled eicosanoid has been observed in the high performance liquid chromatography (HPLC) analysis of 11 eicosanoids. Variations in retention time range from 3-7%, depending on the separation conditions as well as the number and position of the tritium substitution. Multi-tritium-labeled eicosanoids were eluted earlier than the corresponding unlabeled eicosanoid in reversed phase HPLC, whereas no isotope effect was seen with 14C- and 3H2-eicosanoids. Considerations must be given to this tritium isotope effect whenever both multi-tritium-labeled and unlabeled eicosanoids are used for HPLC cochromatography or recovery studies.

Chlorohydrin formation from unsaturated fatty acids reacted with hypochlorous acid

Stimulated neutrophils produce hypochlorous acid (HOCl) via the myeloperoxidase-catalyzed reaction of hydrogen peroxide with chloride. The reactions of HOCl with oleic, linoleic, and arachidonic acids both as free fatty acids or bound in phosphatidylcholine have been studied. The products were identified by gas chromatography-mass spectrometry of the methylated and trimethylsilylated derivatives. Oleic acid was converted to the two 9,10-chlorohydrin isomers in near stoichiometric yield. Linoleic acid, at low HOCl:fatty acid ratios, yielded predominantly a mixture of the four possible monochlorohydrin isomers. Bischlorohydrins were also formed, in increasing amounts at higher HOCl concentrations. Arachidonic acid gave a complex mixture of mono- and bischlorohydrins, the relative proportions depending on the amount of HOCl added. Linoleic acid appears to be slightly more reactive than oleic acid with HOCl. Reactions of oleic and linoleic acids with myeloperoxidase, hydrogen peroxide, and chloride gave chlorohydrin products identical to those with HOCl. Lipid chlorohydrins have received little attention as products of reactions of neutrophil oxidants. They are more polar than the parent fatty acids, and if formed in cell membranes could cause disruption to membrane structure. Since cellular targets for HOCl appear to be membrane constituents, chlorohydrin formation from unsaturated lipids could be significant in neutrophil-mediated cytotoxicity.

Myeloperoxidase-catalyzed iodination and coupling

Myeloperoxidase (MPO), which displays considerable amino acid sequence homology with thyroid peroxidase (TPO) and lactoperoxidase (LPO), was tested for its ability to catalyze iodination of thyroglobulin and coupling of two diiodotyrosyl residues within thyroglobulin to form thyroxine. After 1 min of incubation in a system containing goiter thyroglobulin, I-, and H2O2, the pH optimum of MPO-catalyzed iodination was markedly acidic (approximately 4.0), compared to LPO (approximately 5.4) and TPO (approximately 6.6). The presence of 0.1 N Cl- or Br- shifted the pH optimum for MPO to about 5.4 but had little or no effect on TPO- or LPO-catalyzed iodination. At pH 5.4, 0.1 N Cl- and 0.1 N Br- had a marked stimulatory effect on MPO-catalyzed iodination. At pH 4.0, however, iodinating activity of MPO was almost completely inhibited by 0.1 N Cl- or Br-. Inhibition of chlorinating activity of MPO by Cl- at pH 4.0 has been previously described. When iodination of goiter thyroglobulin was performed with MPO plus the H2O2 generating system, glucose-glucose oxidase, at pH 7.0, the iodinating activity was markedly increased by 0.1 N Cl-. Under these conditions iodination and thyroxine formation were comparable to values observed with TPO. MPO and TPO were also compared for coupling activity in a system that measures coupling of diiodotyrosyl residues in thyroglobulin in the absence of iodination. MPO displayed very significant coupling activity, and, like TPO, this activity was stimulated by a low concentration of free diiodotyrosine (1 microM). The thioureylene drugs, propylthiouracil and methimazole, inhibited MPO-catalyzed iodination both reversibly and irreversibly, in a manner similar to that previously described for TPO-catalyzed iodination.

Ferritin-dependent lipid peroxidation by stimulated neutrophils: inhibition by myeloperoxidase-derived hypochlorous acid but not by endogenous lactoferrin

Human neutrophils stimulated with phorbol myristate acetate or formylmethionylleucylphenylalanine caused superoxide-dependent release of iron from feritin, measured as the formation of a ferrous-ferrozine complex. The stimulated cells also caused ferritin-dependent peroxidation of phospholipid liposomes. Peroxidation was inhibited by lactoferrin, but only at concentrations considerably in excess of what could be achieved by release of endogenous lactoferrin. Peroxidation was enhanced by catalase and methionine, especially when stimulants that release myeloperoxidase were used. Peroxidation was inhibited by added myeloperoxidase. These results are explained by myeloperoxidase catalysing the formation of hypochlorous acid (HOCl) and the HOCl reacting with the lipid to inhibit peroxidation. Thus, neutrophils are able to use ferritin to promote lipid peroxidation. This may be limited under some conditions by iron binding to lactoferrin or transferrin, and more generally by reactions of the lipid with myeloperoxidase-derived HOCl. However, the latter reactions themselves may be harmful.

Activation and release of enzymes and major basic protein from guinea pig eosinophil granulocytes induced by different inflammatory stimuli and other substances. A histochemical, biochemical, and electron microscopic study

In order to investigate the availability and release of enzymes from eosinophilic granulocytes in response to a variety of stimuli, guinea pig peritoneal eosinophils were obtained after repeated intraperitoneal injections of freeze-dried Trichinella spiralis larvae. The activities of the enzymes peroxidase, arylsulfatase B, beta-glucuronidase, aminopeptidase, histaminase, cytochrome c oxidase, acid phosphatase, adenosine triphosphatase and glucose 6-phosphatase, and the major basic protein (MBP) were studied histochemically and, in part, also biochemically. Eosinophils were incubated with the following substances: histamine, platelet activating factor, calcium ionophore, compound 48/80, leukotriene B4, prostaglandins E1, and E2, heparin, and eosinophil-chemotactic factors from neutrophils and lymphocytes. Eosinophils displayed a selective and stimulus-dependent enzyme and MBP reaction. Calcium ionophore and compound 48/80 provoked a release of cytotoxic major basic protein, partly associated with peroxidase release, while leukotriene B4 and eosinophil chemotactic factors caused histaminase and peroxidase release and activated leucinaminopeptidase. Heparin and calcium ionophore induced release of both MBP and histaminase. These data support the concept that eosinophils exhibit either inflammatory or cytotoxic, or antiinflammatory properties upon stimulation by various agents.

Thyroid autoregulation. Inhibitory effects of iodinated derivatives of arachidonic acid on iodine metabolism

Thyroid autoregulation has been linked to an organified iodocompound. Since several iodolipids are produced by the gland their possible role in thyroid autoregulation was examined. The following pure synthetic compounds were prepared: 1) 14-iodo-15-hydroxy-5,8,11-eicosatrienoic acid (I-OH-A); 2) its omega lactone (IL-omega); 3) 5-hydroxy-6-iodo-8,11,14-eicosatrienoic acid delta lactone (IL-delta). Their action on iodine metabolism was studied. Iodine uptake was measured in calf thyroid slices. At 10(-4)M I-OH-A caused a 64% decrease in the T/M ratio, while IL-omega inhibited it by 36% and IL-delta was without effect. At 10(-5)M the inhibition was 44% for I-OH-A and 19% for IL-omega, while T3 was without action. A possible isotopic dilution effect was excluded, and no change in iodine efflux was observed. The inhibition by I-OH-A of iodide uptake was observed after only 15 min preincubation. This compound also decreased 125I accumulation in rats. In calf thyroid slices, I-OH-A at 10(-4)M, inhibited PB125I formation by 80%, IL-omega by 62% and IL-delta by 37%. T3 and arachidonic acid were without action. I-OH-A also caused a dose-dependent inhibition of TSH-stimulated iodide organification. The present results demonstrate, for the first time, that iodinated derivatives of arachidonic acid inhibit thyroid function and mimic the effect of iodide on thyroid autoregulation.

Arachidonic acid metabolism in isolated pancreatic islets. II. The effects of glucose and of inhibitors of arachidonate metabolism on insulin secretion and metabolite synthesis

Isolated pancreatic islets from the rat incubated with 28 mM glucose have been found to secrete more insulin and to synthesize greater amounts of arachidonate lipoxygenase and cyclooxygenase products than islets incubated with 3 mM glucose. This effect was not apparent in studies examining metabolism of radiolabeled arachidonate and was revealed only when the metabolites were quantitated with mass spectrometric measurements. That the glucose-induced synthesis of arachidonate metabolites may participate in insulin secretion was suggested by studies with inhibitors of arachidonate metabolism. Eicosa 5,8,11,14 tetrynoic acid (ETYA) suppressed glucose-induced insulin secretion by 63-74% at a concentration (20 microM) which inhibited the synthesis of arachidonate lipoxygenase and cyclooxygenase products by 90%. Indomethacin (10 microM) completely prevented islet synthesis of cyclooxygenase products but did not influence glucose-induced insulin secretion. Although indomethacin did not inhibit the conversion of exogenous, 3H-labeled arachidonate to [3H]12-HETE, it did significantly inhibit (41-72%) the synthesis of 12-HETE from endogenous precursor. This is presumed to reflect indirect effects of indomethacin on hydrolysis of arachidonate from phospholipids, as recently reported in platelets. These studies constitute the first demonstration that glucose stimulates the synthesis of a lipoxygenase product (12-HETE) from endogenous arachidonate by isolated islets, and that suppression of 12-HETE synthesis with ETYA reduces glucose-induced insulin secretion from isolated islets.

Arachidonic acid metabolism in isolated pancreatic islets. I. Identification and quantitation of lipoxygenase and cyclooxygenase products

The metabolism of arachidonic acid by pancreatic islets has been studied with purified populations of large numbers of islets isolated from the rat. Sequential high-performance liquid chromatographic analyses of islet-derived metabolites of 3H-labeled arachidonate in both reversed and normal phases with 14C-labeled internal standards have demonstrated synthesis by the islets of the cyclooxygenase products prostaglandin E2, prostaglandin F2 alpha, thromboxane B2 and 12- hydroxyheptadecatrienoic acid as well as the lipoxygenase product 12-hydroxyeicosatetraenoic acid (12-HETE). Islet synthesis of these compounds was suppressed with appropriate inhibitors of arachidonate metabolism. Synthesis of the identified metabolites from endogenous arachidonate has also been quantitated with the use of deuterated internal standards, capillary column gas chromatographic analyses, and negative ion-chemical ionization mass spectrometric measurements. The relative abundances of metabolites derived from exogenous, radiolabeled arachidonate versus endogenous precursor differed considerably, and 12-HETE was by far the most abundant of these metabolites synthesized from endogenous arachidonate. Platelets contaminating the isolated islet preparations have been excluded as the source of the identified arachidonate metabolites. These studies establish that cells intrinsic to pancreatic islets synthesize a clearly characterized profile of arachidonate lipoxygenase and cyclooxygenase products. The sensitive and specific mass spectrometric methods for quantitation of these compounds permit detailed evaluation of their possible participation in insulin secretion from isolated islets.