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

Extracellular vesicles of Norway spruce contain precursors and enzymes for lignin formation and salicylic acid

Lignin is a phenolic polymer in plants that rigidifies the cell walls of water-conducting tracheary elements and support-providing fibers and stone cells. Different mechanisms have been suggested for the transport of lignin precursors to the site of lignification in the cell wall. Extracellular vesicle (EV)-enriched samples isolated from a lignin-forming cell suspension culture of Norway spruce (Picea abies L. Karst.) contained both phenolic metabolites and enzymes related to lignin biosynthesis. Metabolomic analysis revealed mono-, di-, and oligolignols in the EV isolates, as well as carbohydrates and amino acids. In addition, salicylic acid (SA) and some proteins involved in SA signaling were detected in the EV-enriched samples. A proteomic analysis detected several laccases, peroxidases, β-glucosidases, putative dirigent proteins, and cell wall-modifying enzymes, such as glycosyl hydrolases, transglucosylase/hydrolases, and expansins in EVs. Our findings suggest that EVs are involved in transporting enzymes required for lignin polymerization in Norway spruce, and radical coupling of monolignols can occur in these vesicles.

Structural evidence of the oxidation of iodide ion into hyper-reactive hypoiodite ion by mammalian heme lactoperoxidase

Lactoperoxidase (1.11.1.7, LPO) is a mammalian heme peroxidase found in the extracellular fluids of mammals including plasma, saliva, airway epithelial lining fluids, nasal lining fluid, milk, tears, gastric juices, and intestinal mucosa. To perform its innate immune action against invading microbes, LPO utilizes hydrogen peroxide (H₂ O₂ ) to convert thiocyanate (SCN^- ) and iodide (I^- ) ions into the oxidizing compounds hypothiocyanite (OSCN^- ) and hypoiodite (IO^- ). Previously determined structures of the complexes of LPO with SCN^- , OSCN^- , and I^- show that SCN^- and I^- occupy appropriate positions in the distal heme cavity as substrates while OSCN^- binds in the distal heme cavity as a product inhibitor. We report here the structure of the complex of LPO with IO^- as the first structural evidence of the conversion of iodide into hypoiodite by LPO. To obtain this complex, a solution of LPO was first incubated with H₂ O₂ , then mixed with ammonium iodide solution and the complex crystallized by the addition of PEG-3350, 20% (wt/vol). These crystals were used for X-ray intensity data collection and structure analysis. The structure determination revealed the presence of four hypoiodite ions in the substrate binding channel of LPO. In addition to these, six other hypoiodite ions were observed at different exterior sites. We surmise that the presence of hypoiodite ions in the distal heme cavity blocks the substrate binding site and inhibits catalysis. This was confirmed by activity experiments with the colorimetric substrate, ABTS (2,2'-azino-bis(3-ethylbenzthiazoline-sulfonic acid)), in the presence of hypoiodite and iodide ions.

Selective C-H Iodination of (Hetero)arenes

Iodoarenes are versatile intermediates and common synthetic targets in organic synthesis. Here, we present a strategy for selective C-H iodination of (hetero)arenes with a broad functional group tolerance. We demonstrate the utility and differentiation to other iodination methods of supposed sulfonyl hypoiodites for a set of carboarenes and heteroarenes.

Reaction Product Variability and Biological Activity of the Lactoperoxidase System Depending on Medium Ionic Strength and pH, and on Substrate Relative Concentration

The potential of ions produced in water by the lactoperoxidase system against plant pests has shown promising results. We tested the bioactivity of ions produced by the lactoperoxidase oxidation of I^- and SCN^- in several buffers or in tap water and characterized the ions produced. In vitro biological activity was tested against Penicillium expansum, the causal agent of mold in fruits, and the major cause of patulin contamination of fruit juices and compotes. In buffers, the ionic concentration was increased 3-fold, and pathogen inhibition was obtained down to the 1:15 dilution. In tap water, the ionic concentration was weaker, and pathogen inhibition was obtained only down to the 1:3 dilution. Acidic buffer increased ion concentrations as compared to less acidic (pH 5.6 or 6.2) or neutral buffers, as do increased ionic strength. ¹³ C-labelled SCN^- and MS showed that different ions were produced in water and in buffers. In specific conditions the ion solution turned yellow and a product was formed, probably diiodothiocyanate (I₂ SCN^- ), giving an intense signal at 49.7 ppm in ¹³ C-NMR. The formation of the signal was unambiguously favored in acidic media and disadvantaged or inhibited in neutral or basic conditions. It was enhanced at a specific SCN^- : I^- ratio of 1:4.5, but decreased when the ratio was 1:2, and was inhibited at ratio SCN^- >I^- . We demonstrated that the formation of the signal required the interaction between I₂ and SCN^- , and MS showed the presence of I₂ SCN^- .

N-(4-Hydroxyphenyl)acetamide against diiodine towards polyiodide dianion

N-(4-Hydroxyphenyl)acetamide decreases the total amount of diiodine which is available for the iodination of tyrosil residues of thyroglobulin, while it inhibits the activity of thyroid peroxidase.

Coniferyl alcohol hinders the growth of tobacco BY-2 cells and Nicotiana benthamiana seedlings

MAIN CONCLUSION

Externally added coniferyl alcohol at high concentrations reduces the growth of Nicotiana cells and seedlings. Coniferyl alcohol is metabolized by BY-2 cells to several compounds. Coniferyl alcohol (CA) is a common monolignol and a building block of lignin. The toxicity of monolignol alcohols has been stated in the literature, but there are only few studies suggesting that this is true. We investigated the physiological effects of CA on living plant cells in more detail. Tobacco (Nicotiana tabacum) Bright yellow-2 cells (BY-2) and Nicotiana benthamiana seedlings both showed concentration-dependent growth retardation in response to 0.5-5 mM CA treatment. In some cases, CA addition caused cell death in BY-2 cultures, but this response was dependent on the growth stage of the cells. Based on LC-MS/MS analysis, BY-2 cells did not accumulate the externally supplemented CA, but metabolized it to ferulic acid, ferulic acid glycoside, coniferin, and to some other phenolic compounds. In addition to growth inhibition, CA caused the formation of a lignin-like compound detected by phloroglucinol staining in N. benthamiana roots and occasionally in BY-2 cells. To prevent this, we added potassium iodide (KI, at 5 mM) to overcome the peroxidase-mediated CA polymerization to lignin. KI had, however, toxic effects on its own: in N. benthamiana seedlings, it caused reduction in growth; in BY-2 cells, reduction in growth and cell viability. Surprisingly, CA restored the growth of KI-treated BY-2 cells and N. benthamiana seedlings. Our results suggest that CA at high concentrations is toxic to plant cells.

Mode of action of lactoperoxidase as related to its antimicrobial activity: a review

Lactoperoxidase is a member of the family of the mammalian heme peroxidases which have a broad spectrum of activity. Their best known effect is their antimicrobial activity that arouses much interest in in vivo and in vitro applications. In this context, the proper use of lactoperoxidase needs a good understanding of its mode of action, of the factors that favor or limit its activity, and of the features and properties of the active molecules. The first part of this review describes briefly the classification of mammalian peroxidases and their role in the human immune system and in host cell damage. The second part summarizes present knowledge on the mode of action of lactoperoxidase, with special focus on the characteristics to be taken into account for in vitro or in vivo antimicrobial use. The last part looks upon the characteristics of the active molecule produced by lactoperoxidase in the presence of thiocyanate and/or iodide with implication(s) on its antimicrobial activity.

Peroxidasin forms sulfilimine chemical bonds using hypohalous acids in tissue genesis

Collagen IV comprises the predominant protein network of basement membranes, a specialized extracellular matrix, which underlie epithelia and endothelia. These networks assemble through oligomerization and covalent crosslinking to endow mechanical strength and shape cell behavior through interactions with cell-surface receptors. A recently discovered sulfilimine (S=N) bond between a methionine sulfur and hydroxylysine nitrogen reinforces the collagen IV network. We demonstrate that peroxidasin, an enzyme found in basement membranes, catalyzes formation of the sulfilimine bond. Drosophila peroxidasin mutants have disorganized collagen IV networks and torn visceral muscle basement membranes, pointing to a critical role for the enzyme in tissue biogenesis. Peroxidasin generates hypohalous acids as reaction intermediates, suggesting a paradoxically anabolic role for these usually destructive oxidants. This work highlights sulfilimine bond formation as what is to our knowledge the first known physiologic function for peroxidasin, a role for hypohalous oxidants in tissue biogenesis, and a possible role for peroxidasin in inflammatory diseases.

New protein footprinting: fast photochemical iodination combined with top-down and bottom-up mass spectrometry

We report a new approach for the fast photochemical oxidation of proteins (FPOP) whereby iodine species are used as the modifying reagent. We generate the radicals by photolysis of iodobenzoic acid at 248 nm; the putative iodine radical then rapidly modifies the target protein. This iodine-radical labeling is sensitive, tunable, and site-specific, modifying only histidine and tyrosine residues in contrast to OH radicals that modify 14 amino-acid side chains. We iodinated myoglobin (Mb) and apomyoglobin (aMb) in their native states and analyzed the outcome by both top-down and bottom-up proteomic strategies. Top-down sequencing selects a certain level (addition of one I, two I's) of modification and determines the major components produced in the modification reaction, whereas bottom-up reveals details for each modification site. Tyr146 is found to be modified for aMb but less so for Mb. His82, His93, and His97 are at least 10 times more modified for aMb than for Mb, in agreement with NMR studies. For carbonic anhydrase and its apo form, there are no significant differences of the modification extents, indicating their similarity in conformation and providing a control for this approach. For lispro insulin, insulin-EDTA, and insulin complexed with zinc, iodination yields are sensitive to differences in insulin oligomerization state. The iodine radical labeling is a promising addition to protein footprinting methods, offering higher specificity and lower reactivity than ∙OH and SO(4)(-∙), two other radicals already employed in FPOP.

[Biochemical characteristics of iodperoxidase activity of human saliva]

Peroxidase-catalyzed oxidation of iodide in human saliva leads to the formation of a brown product with lambda max 287 nm and 353 nm (I3-) identified by the method of UV-spectrophotometry. I3- directly reacts with starch producing the characteristic blue complex. Salivary iodide peroxidase activity was found to be from 1.2 to 2.3 times higher then the activity of salivary peroxidases with natural substrates (SCN- and Cl-). Optimum for the iodide peroxidase activity in human saliva was found to be near pH 5.8. Salivary iodide peroxidase activity progressively lowers with the rise of pH value of the reaction mixture until total loss at the pH>7.4 was observed. Iodide peroxidase activity in human saliva at pH>7.4 is masked due to decomposition of I3- with the increase of pH along with the inhibition of peroxidases and I3- reduction by low molecular weight dialyzable salivary components possibly by Cl- and NCS-. Salivary iodide peroxidase activity was completely inhibited by peroxidase inhibitors (NaN3, 2-mercaptoethanol, thiourea), while addition of the peroxidase alternative substrates (ascorbate, quercetin, thiocyanate) resulted in partial inhibition of iodide peroxidase activity. The results of the study confirm the idea, that high activity of human saliva peroxidase with iodide as a substrate may play a crucial role in the bioavailability and metabolism of biologically active iodide.

Kinetics and mechanism of the peroxidase-catalyzed iodination of tyrosine

The kinetics of iodination of tyrosine by hydrogen peroxide and iodide, catalyzed by both horseradish peroxidase (HRP) and lactoperoxidase (LPO), were studied. The initial rates of formation of both molecular I2 and monoiodotyrosine (MIT) were measured with stopped flow techniques. The following reactions occur in both systems. Enzymatic: FeIII + H2O2-->Fev = O + H2O; Fev = O + I(-)-->FeIII-O-I-; FeIII-O-I- + H(+)-->FeIII + HOI; FeIII-O-I- + I- + H(+)-->FeIII + I2 + HO-. Iodine equilibria: I2 + I-<-->I3-; I2 + H2O<-->HOI + I- + H+. Nonenzymatic iodination, one or both of the following: Tyr + HOI-->MIT + H2O; Tyr + I2-->MIT + I- + H+, where FeIII is native peroxidase, Fev = O is compound I and Tyr is tyrosine. The big difference in the two systems is that the following reaction also occurs with LPO: FeIII-O-I- + Tyr-->MIT + FeIII + HO-, which is the dominant mechanism of iodination for the mammalian enzyme. The overall rate of formation of MIT is about 10 times faster for LPO compared to HRP under comparable conditions. A small decrease in rate occurs when D-tyrosine is substituted for L-tyrosine in the LPO reaction. Thus LPO has a tyrosine binding site near the heme. A kinetically controlled maximum is observed in I3- concentration. Once equilibrium is established, I2 is the dominant form of inorganic iodine in solution. However, hypoiodous acid may be the inorganic iodination reagent.

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.

Oxidative radioiodination damage to human lactoferrin

Oxidative iodination of human lactoferrin (Lf) as commonly performed by using the chloramine-T, the Iodogen or the lactoperoxidase method produces an unreliable tracer protein because of excessive and heterogeneous polymer formation. Before iodination a minor tetramer fraction may be demonstrable in iron-saturated Lf only. Iodination-induced polymerization of iron-poor as well as iron-saturated Lf occurs independently of the presence or absence of 10 mM-EDTA and the 125I-/Lf molar ratio used for iodination. 125I-Lf polymers are mainly covalently linked, as suggested by the lack of substantial dissociation in SDS/polyacrylamide-gel electrophoresis. Damage to the 125I-Lf monomer may be another consequence of oxidative iodination. This is demonstrated in SDS/polyacrylamide-gel electrophoresis where 50% of the radioactivity of apparently normal monomer (Mr 75,000) is displaced to a lower-Mr region (30,000-67,000) after reduction with dithiothreitol. Non-oxidative iodination by the Bolton-Hunter technique produces an antigenetically stable tracer that is not being subjected to polymerization and monomer degradation as judged by high-performance gel chromatography and SDS/polyacrylamide-gel electrophoresis with and without dithiothreitol treatment. It is concluded that oxidation in itself leads to covalent non-disulphide cross-linking between human Lf molecules and, possibly, to intramolecular peptide-bond breaking becoming unmasked under reducing conditions. In biological experiments with human 125I-Lf this problem should be carefully considered.

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.