Fluoride inhibits the antimicrobial peroxidase systems in human whole saliva

Fluoride (F-) ions at concentrations present in vivo at the plaque/enamel interface (0.05-10 mM) inhibited the activities of lactoperoxidase (LP), myeloperoxidase (MP) and total salivary peroxidase (TSP) in a pH- and dose-dependent way. The inhibition was observed only at pH < or = 6.5 and with F- concentrations > or = 0.1 mM. At pH 5.5 LP activity was inhibited by 85% and MP by 34% with 10 mM F-. TSP activity was also inhibited only at low pH (5.5) by approximately 25%. Furthermore, the generation of the actual antimicrobial agent in vivo, hypothiocyanite (HOSCN/OSCN-), of the oral peroxidase systems was inhibited by F-, again at low pH (5.0-5.5) both in buffer (by 45%) and in saliva (by 15%). This inhibition was observed only with the highest F- concentrations studied (5-10 mM). Fluoridated toothpaste (with 0.10 or 0.14% F) mixed with saliva did not inhibit TSP or HOSCN/OSCN- generation. This may have been due to the 'buffering' effect of toothpaste which did not allow salivary pH to drop below 5.9. We conclude that the F- ions in acidic fluoride products, e.g. in gels or varnishes (but not in toothpastes), may have the potential to locally inhibit the generation of a nonimmune host defense factor, HOSCN/OSCN/SCN-, produced by oral peroxidase systems. The possible clinical significance of this finding remains to be shown.

Chemical and enzymatic oxidation of benzimidazoline-2-thiones: a dichotomy in the mechanism of peroxidase inhibition

Derivatives of imidazole-2-thiones block reactions catalyzed by thyroid peroxidase (TPX) and the closely related lactoperoxidase (LPX), and this property is used therapeutically to treat hyperthyroidism. The reactions of a series of benzimidazoline-2-thiones with chemical and enzymatic oxidants were investigated to probe systematically the mechanism of inhibition. Oxidation of benzimidazoline-2-thione (I) and 1-methylbenzimidazoline-2-thione (II) with 3-chloroperbenzoic acid (PBA) yielded reaction products and stoichiometry consistent with benzimidazole-2-sulfenic acids as reactive intermediates. The N,N'-disubstituted nature of 1,3-dimethylbenzimidazoline-2-thione (III) precludes sulfenic acid formation by tautomerization, and the oxidation of III with PBA yielded products and stoichiometry that were consistent with a benzimidazole-2-sulfonyl ylide as the reactive intermediate. I and II are suicide inhibitors of LPX and TPX, but III was found to inhibit only peroxidase-catalyzed iodination reactions by an alternate substrate mechanism. These results provide support for the hypothesis that imidazole-2-sulfenic acids are important reactive intermediates in the suicide inactivation of TPX and LPX and relate the chemical reactivity of the inhibitor with both the potency and mechanism of inhibition. These results suggest that 1,3-disubstituted thiourea derivatives represent a new class of potential antihyperthyroid drugs that block TPX-catalyzed tyrosine iodination but do not cause irreversible enzyme inactivation.

Formation of molecular iodine during oxidation of iodide by the peroxidase/H2O2 system. Implications for antithyroid therapy

The first step in the biogenesis of thyroid hormones is the oxidation of iodides taken up by the thyroid gland. Oxidation of I- by the H2O2/peroxidase system leads to the formation of iodinium ions I+ which bond to thyroglobulin by electrophilic substitution. However, it is not clear whether I- is transformed directly to I+ or whether it passes through a molecular iodine intermediate. This latter possibility is indicated by the oxidation potentials of the reactions. I2 can be detected in vitro from the formation of I3- ions, although this has yet to be confirmed in vivo. The present study was designed to determine, albeit indirectly, whether this reaction occurs in vivo. If I2 is produced, it may form charge transfer complexes with numerous drugs. We also investigated the action of various drugs on lactoperoxidase and assessed their antithyroid activity in the rat by assay of plasma levels of T3, T4, and TSH. We found a good correlation between the value of Kc, the formation constant of the complex of the drug with molecular iodine, and the antithyroid activity in vivo. This correlation was observed in four different classes of compound. The possibility that molecular iodine is produced in the thyroid gland has implications for antithyroid therapy.

Salicylhydroxamic acid inhibits myeloperoxidase activity

Salicylhydroxamic and benzohydroxamic acids were found to bind to the resting state of myeloperoxidase and inhibit ligand binding to the heme iron. An ionizable group on the enzyme with pKa = 4 affects salicylhydroxamic acid binding; binding occurs when this group is not protonated. The binding of the heme iron ligands (e.g. cyanide, nitrite, and chloride) is probably controlled by the same ionizable group. The equilibrium dissociation constant of the salicylhydroxamic acid-myeloperoxidase complex is about 2 x 10(-6) M, and the association rate constant is 7.4 x 10(6) M-1.s-1. Salicylhydroxamic acid serves as a donor to the higher oxidation state of myeloperoxidase and thereby inhibits guaiacol oxidation. Salicylhydroxamic acid was also found to bind to intestinal peroxidase and lactoperoxidase. Salicylhydroxamic acid binding to all three mammalian peroxidases was about 3 orders of magnitude stronger than benzohydroxamic acid binding. We conclude that the salicylhydroxamic and benzohydroxamic acids bind in the distal heme cavity of these peroxidases and interact with the heme ligand binding site.

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.

Superoxide peroxidase activity of ovoperoxidase, the cross-linking enzyme of fertilization

Ovoperoxidase, an enzyme secreted by the eggs of the sea urchin Stronglycocentrotus purpuratus upon activation, catalyzes the formation of dityrosine residues in the fertilization envelope. This cross-linking reaction requires extracellular H2O2, which is produced by the egg during the cyanide-insensitive "respiratory burst" of fertilization. While investigating the possibility that the sea urchin oxidase might generate O2- as a precursor to H2O2, we discovered that ovoperoxidase possessed O2- degrading activity. Ovoperoxidase catalyzed the breakdown of O2- in a reaction that was sensitive to inhibition by catalase, indicating a requirement for H2O2. High concentrations of either O2- or H2O2 inhibited the O2- degrading activity of ovoperoxidase, as did the peroxidase inhibitors aminotriazole, azide, and phenylhydrazine. When ovoperoxidase was heated at 56 degrees C, it lost O2- degrading activity in parallel with peroxidase activity. In contrast, the copper-chelating agent diethyldithiocarbamate, which completely inactivated CuZn superoxide dismutase, failed to affect ovoperoxidase. The requirement for H2O2 and the inhibition by aminotriazole, azide, and phenylhydrazine support the hypothesis that ovoperoxidase catalyzes the breakdown of O2- by a peroxidative mechanism. Ovoperoxidase may play a role in protecting the developing embryo from oxidants derived from O2-.

Peroxidase-catalyzed displacement of tritium from regiospecifically labeled estradiol and 2-hydroxyestradiol

Estradiol and 2-hydroxyestradiol with 3H at different positions in rings A, B or D were incubated with lactoperoxidase without added H2O2 and their oxidative transformation was followed by transfer of 3H into 3H2O. With estradiol, 3H loss from different positions in the aromatic ring was almost equal and also occurred to a lesser extent from the alicyclic portion of the molecule. Glutathione had less effect on the formation of 3H2O for the aromatic ring of estradiol than from that of the catechol estrogen where it increased the yield 6-fold. The rate of 3H loss was also very much greater from tritiated 2-hydroxyestradiol than from estradiol and NADPH was inhibitory with both steroids. Conditions for the release of 3H from estradiol and 2-hydroxyestradiol by peroxidase as well as the effect of some biochemical inhibitors were also investigated. The possible contribution of peroxidative formation of 3H2O during the radiometric assay for catechol estrogen biosynthesis by tissue monooxygenases is discussed.

Interaction of lactoperoxidase with enzymes and immunoglobulins in bovine milk

The interaction of lactoperoxidase with lysozyme and ribonuclease as well as immunoglobulins from cow milk has been investigated. As gel filtration and enzyme kinetics experiments have shown, the lactoperoxidase was slightly activated by complexing to lysozyme, while IgA and IgM were inhibitory for the peroxidase. Oh the other hand, IgG and ribonuclease had no effect on the enzyme activity although the latter did form a complex with the lactoperoxidase. The interaction between the lysozyme and lactoperoxidase appears to be rather specific since the alteration of the lactoperoxidase sugar moiety by periodate oxidation, prevented the formation of the lactoperoxidase-lysozyme complex.

EDTA inhibits peroxidase-catalyzed iodide oxidation through interaction at the iodide binding site

EDTA inhibits the formation of I3- from iodide catalysed by various pure peroxidases. The inhibition is concentration-dependent and chloroperoxidase (CPO) is more sensitive than horseradish peroxidase (HRP) and lactoperoxidase (LPO). EDTA is more active than EGTA or other biological chelators tested. Zn2+, Mn2+ and Co2+ are equally active in reversing the effect of EDTA on both CPO and HRP almost completely, but ineffective in the case of LPO. The effect of EDTA on HRP can be reversed by a higher concentration of iodide but not by H2O2. EDTA causes a hypsochromic change in the absorption of the Soret band of HRP at 402 nm, and iodide can reverse this effect. EDTA can effectively displace radioiodide specifically bound to HRP. It is suggested that EDTA inhibits iodide oxidation by interacting at the iodide binding site of the HRP.

Suicide inactivation of lactoperoxidase by 3-amino-1,2,4-triazole

Amitrole (3-amino-1,2,4-triazole) meets the criteria for a suicide (mechanism-based) inhibitor of lactoperoxidase. Amitrole causes rapid inactivation of lactoperoxidase only in the presence of hydrogen peroxide, and the kinetics are consistent with a suicide mechanism. Approximately 7 mol of radiolabeled amitrole binds covalently per equivalent of lactoperoxidase activity lost. The visible spectrum of lactoperoxidase inactivated by amitrole is unchanged, suggesting that covalent modification of the heme prosthetic group does not occur. The 13C NMR spectrum of lactoperoxidase inactivated by [13C]amitrole shows unique resonances which support the hypothesis that covalent binding occurs on the protein moiety. The similarities between lactoperoxidase and thyroid peroxidase suggest a similar mechanism for inhibition of thyroid hormone synthesis by amitrole.

Vanadate as an inhibitor of plant and mammalian peroxidases

Vanadate ions are shown to inhibit horseradish, squash, and rat intestinal peroxidases by following the reaction spectrophotometrically in a wide range of vanadate concentrations. I50 in phosphate buffer were 43, 9.4, and 535 microM, respectively. No inhibitory effect was found on cow milk lactoperoxidase and beef liver catalase. Gel filtration of peroxidases in the presence of vanadate, as carried out by radioactive 48V for horseradish peroxidases (either in aerobic or anoxic conditions) and neutron activation analysis (NAA) for squash peroxidase, demonstrated a binding of vanadium to these enzymes in stoichiometric amounts. Electron paramagnetic resonance spectra of the eluted peaks for the former peroxidase indicated that vanadium is in the +5 oxidation state, but an equilibrium between V (V) and V (IV) in the assay conditions cannot be discarded. Although the inhibitory mechanism remains obscure, some hypotheses are considered. The potential implications that the inhibitory effect of vanadium might have on plant and animal metabolism are also discussed.

Hydroperoxide-dependent metabolism of goitrogenic thiocarbamides by lactoperoxidase

1. Inhibition of lactoperoxidase by thiocarbamides is consistent with a suicide mechanism whereby enzyme-catalysed S-oxygenation produces reactive intermediates which covalently modify the active site haem. 2. The reaction of thiocarbamide goitrogens with lactoperoxidase in the presence of hydroperoxides results in time-dependent and irreversible enzyme inactivation and an altered visible spectrum of the haem prosthetic group of the inactivated enzyme. 3. A mechanism of S-oxygenation for the inactivation is suggested by lactoperoxidase-catalysed formation of stable S-oxides from thioamide and organosulphur functional groups, and by a common dependence of substrate and inhibitor binding constants on their electrochemical oxidation potentials. 4. Hydroperoxide-dependent inactivation of lactoperoxidase by benzimidazoline-2-thiones occurs concomitantly to the covalent binding of stoichiometric amounts of 14C- or 35S-labelled inhibitors per mole of enzyme, and the formation of turnover products derived from the hydroperoxide cosubstrate and inhibitor.

Mechanism-based inhibition of lactoperoxidase by thiocarbamide goitrogens. Identification of turnover and inactivation pathways

Direct evidence is presented in support of mechanism-based (suicide) inactivation of lactoperoxidase by thiocarbamide thyroid inhibitors. The turnover of 1-methylbenzimidazolidine-2-thione was demonstrated by identifying the inhibitor-derived products 1-methylbenzimidazole and bisulfite ion that are formed concurrent to enzyme inactivation. The turnover of a hydroperoxide cosubstrate, 5-phenyl-4-pentenyl hydroperoxide, was quantitated from formation of the corresponding alcohol during enzyme inactivation. A specific inactivation pathway is suggested by the covalent binding of 1 mol of 14C- and 35S-labeled benzimidazolidine-2-thione and 1-methylbenzimidazolidine-2-thione per mole of inactivated lactoperoxidase. These results are explained by partitioning of inhibitor-derived S-oxygenated intermediates between turnover and inactivation pathways. The properties of the inactivation process are unique among thiono-sulfur compounds and suggest that benzimidazolinesulfenic acids are the reactive intermediates.

The role of hydroxyl radicals in irreversible inactivation of lactoperoxidase by excess H2O2. A spin-trapping/ESR and absorption spectroscopy study

H2O2 is catalytically metabolized by ferric lactoperoxidase (LPO)----compound (cpd) I----cpd II----ferric LPO cycles. An excess of the substrate, however, is degraded by a ferric LPO----cpd I----cpd II----cpd III----ferrous LPO----ferric LPO cycle. This latter pathway leads to the partial or total irreversible inactivation of the enzyme depending on the excess of H2O2 (H. Jenzer, W. Jones, and H. Kohler (1986) J. Biol. Chem. 261, 15550-15556). Spin-trapping/ESR data indicate that in the course of the reaction superoxide (HO2./O2-) and hydroxyl radicals (OH.) are formed. Since many substances known to scavenge radicals, such as a spin trap (e.g., 5,5-dimethyl-1-pyrroline-N-oxide) desferrioxamine, albumin, or mannitol, do not prevent enzyme inactivation, we conclude that OH. generation is a site-specific reaction at or near the active center of LPO where bulky scavenger molecules may not be able to penetrate. We suggest the formation of OH. by a Fenton-like reaction between H2O2 and the intermediate ferrous state of the enzyme, which substitutes for Fe2+ in the Fenton reaction. OH. is a powerful oxidant which in turn may attack rapidly the nearest partner available, either H2O2 to produce HO2. and H2O, or the prosthetic group to give rise to oxidative cleavage of the porphyrin ring structure of the heme moiety of LPO and thus to the liberation of iron.

Irreversible inactivation of lactoperoxidase in the course of iodide oxidation

In the course of lactoperoxidase-catalysed I- oxidation, which is a model for the initial step of thyroid hormone biosynthesis, irreversible enzyme inactivation can occur if free molecular iodine (I2) or other oxidized iodine species accumulate. Evidence is presented that the breakdown of the catalytic activity is the result of the iodination of the peroxidase-apoprotein. This kind of enzyme inactivation, which can be prevented by iodine acceptors' such as thyroglobulin or high concentrations of I-, may well play a role in the regulation of the synthesis of thyroid hormones in vivo.

On the molecular mechanism of lactoperoxidase-catalyzed H2O2 metabolism and irreversible enzyme inactivation

Lactoperoxidase-catalyzed H2O2 metabolism proceeds through one of three different pathways, depending on the nature and the concentration of the second substrate as an e- donor and/or on pH conditions. In the lactoperoxidase (LPO)-H2O2 system, at low H2O2 concentrations and/or alkaline conditions the peroxidatic cycle involves ferric LPO----compound I----compound II----ferric LPO conversion, whereas high H2O2 concentrations and/or acidic conditions favor the ferric LPO----compound I----compound II----compound III----ferrous LPO----ferric LPO pathway. The compound III/ferroperoxidase states are associated with irreversible enzyme inactivation by cleavage of the heme moiety and liberation of iron. It is likely that either singlet oxygen or superoxide and hydroxyl radicals are involved in the attack on heme iron, because inactivation correlates with oxygen production and can be decreased to a certain degree by scavengers such as ethanol, 1-propanol, 2-propanol, or mannitol. In the LPO-H2O2-I- system, the enzyme may also be inactivated by I2 generated in the course of enzymatic I- oxidation (i.e. during ferric LPO----compound I----ferric LPO cycles).

The role of superoxide radicals in lactoperoxidase-catalysed H2O2-metabolism and in irreversible enzyme inactivation

Irreversible inactivation of lactoperoxidase in the presence of excess H2O2 has been investigated. Serial overlay absorption spectra of the Soret region show that the rate and total amount of enzyme inactivation depend on the proton concentration. Perhydroxyl or superoxide radicals (HO.2 or O-2) cannot be established as the inactivating species in this mechanism, but they influence the rate of reconversion of the intermediate lactoperoxidase-compound III back to the resting ferric form of the enzyme.

Mechanism-based inhibition of lactoperoxidase by thiocarbamide goitrogens

The irreversible inactivation of bovine lactoperoxidase by thiocarbamide goitrogens was measured, and the kinetics were consistent with a mechanism-based (suicide) mode. Sulfide ion inactivated, 2-mercaptobenzimidazole-inactivated, and 1-methyl-2-mercaptoimidazole-inactivated lactoperoxidases have different visible spectra, suggesting different products were formed. The results support a mechanism in which reactive intermediates are formed by S-oxygenation reactions catalyzed by lactoperoxidase compound II. It is proposed that the reaction of electron-deficient intermediates with the heme prosthetic group is responsible for the observed spectral changes and inactivation by thiocarbamides.

The role of compound III in reversible and irreversible inactivation of lactoperoxidase

In the presence of iodide (I-, 10 mM) and hydrogen peroxide in a large excess (H2O2, 0.1-10 mM) catalytic amounts of lactoperoxidase (2 nM) are very rapidly irreversibly inactivated without forming compound III (cpd III). In contrast, in the absence of I- cpd III is formed and inactivation proceeds very slowly. Increasing the enzyme concentration up to the micromolar range significantly accelerates the rate of inactivation. The present data reveal that irreversible inactivation of the enzyme involves cleavage of the prosthetic group and liberation of heme iron. The rate of enzyme destruction is well correlated with the production of molecular oxygen (O2), which originates from the oxidation of excess H2O2. Since H2O2 and O2 per se do not affect the heme moiety of the peroxidase, we suggest that the damaging species may be a primary intermediate of the H2O2 oxidation, such as oxygen in its excited singlet state (1 delta gO2), superoxide radicals (O-.2), or consequently formed hydroxyl radicals (OH.).

Inactivation of peroxidase and glucose oxidase by H2O2 and iodide during in vitro thyroglobulin iodination

Thyroglobulin iodination and thyroxine synthesis in vitro require the presence of peroxidase, H2O2 and iodide. H2O2 is usually continuously generated by glucose oxidase (GO) and glucose. The aim of this study was to investigate whether the two enzymes could possibly be inactivated by a particular concentration of H2O2 or iodide present during incubation. The results revealed that both enzymes were indeed inactivated under two distinct conditions: Lactoperoxidase and thyroid peroxidase were inactivated by modest concentrations of H2O2 accumulating during incubation. Glucose oxidase was inactivated by an oxidized species of iodine or singlet oxygen produced in the catalytic cycle. The results may explain some hitherto unsolved discrepancies between different iodination procedures. Moreover they may have an impact on the regulation of in vivo thyroglobulin iodination and hormone synthesis.

pH-dependent fluoride inhibition of peroxidase activity

Fluoride was found to be a potent inhibitor of bovine lactoperoxidase and of salivary peroxidase at acid pH values. Inhibition was reversible at neutral pH, and appeared to involve HF binding by the enzyme. Fluoride inhibition of lactoperoxidase occurred with all reductants tested, including thiocyanate, iodide, and guaiacol. Fluoride concentrations for 50% inhibition of enzymatic activity with iodide as reductant were: less than 0.05 mM at a pH value of 4.0, 0.3 mM at 5.0, 4.0 mM at pH 6.0, and more than 10.0 mM at pH 7.0. Salivary peroxidases were found to have lower pH optima but to be approximately as sensitive to acid-dependent fluoride inhibition as was purified bovine lactoperoxidase. The findings suggest that the fluoride in dental plaque may be inhibitory to the antimicrobial peroxidase system.

Mechanism of enzymatic and non-enzymatic tyrosine iodination. Inhibition by excess hydrogen peroxide and/or iodide

Non-enzymatic (I2-mediated) and lactoperoxidase-catalyzed iodination of tyrosine are inhibited by excess iodide (I-) and/or hydrogen peroxide (H2O2). This phenomenon is a consequence of the concentration-dependent dual role of I- and H2O2 in the iodinating system. I- and H2O2, in addition to their function as primary substrates of peroxidase, may act as alternative 'iodine acceptors' and therefore compete with tyrosine for the active iodinating agent, irrespective of whether this compound is an enzyme-associated iodinium cation (E X I delta +) or an equivalent oxidized iodine species (IOH, IC1, I2). The competitive reaction pathways resulting from excess I- and/or H2O2 in the iodination system are I2/I-3 generation and/or pseudo-catalatic degradation of H2O2, respectively. Our results also demonstrate that I2 (and alternative medium-dependent oxidized iodine species such as IOH and IC1) generated in the iodination system may play an important role as iodinating agent(s). They serve as a substitute for the enzyme-bound iodinium species (E X I delta +), if the prevailing I- concentration favours this pathway. The proposed mechanism of the various antagonistic and interactive reaction pathways is summarized in a scheme.

Cystine antagonism of the antibacterial action of lactoperoxidase-thiocyanate-hydrogen peroxide on Streptococcus agalactiae

Cystine reduction in Streptococcus agalactiae, resulting in sulfhydryl formation, may account for antagonism of the antibacterial effect of lactoperoxidase-thiocyanate-hydrogen peroxide when cystine is present in excess of the amount needed for maximum growth. Accumulation of cystine by S. agalactiae and its reduction to form sulfhydryl compounds were demonstrated. The reduction of cystine appeared to occur by a couple reaction between glutathione reductase and glutathione-disulfide transhydrogenase activity, both of which were found in the supernatant fraction from cell homogenates. NADPH-specific glutathione reductase activity was found in the pellet and supernatant fractions from cell homogenates. Two sulfhydryls were formed for each mole of NADPH used during cystine reduction. The information presented offers a plausible explanation of how cystine, when present in excess of growth needs, may be reduced to generate sulfhydryl compounds which neutralize the antibacterial effect of lactoperoxidase-thiocyanate-hydrogen peroxide on S. agalactiae.