Investigations into lactoperoxidase-catalysed bromination of tyrosine and thyroglobulin

EFSA Scientific Committee, Bennekou SH, Allende A, Bearth A, Casacuberta J, Castle L, Coja T, Crépet A, Halldorsson T, Hoogenboom L(R, Knutsen H, Koutsoumanis K, Lambré C, Nielsen S, Turck D, Civera AV, Villa R, Zorn H, Bampidis V, Castenmiller J, Chagnon M, Cottrill B, Darney K, Gropp J, Puente SL, Rose M, Vinceti M, Bastaki M, Gergelová P, Greco L, Innocenti ML, Janossy J, Lanzoni A, Terron A, Benford D. Risks to human and animal health from the presence of bromide in food and feed

The European Commission mandated EFSA to assess the toxicity of bromide, the existing maximum residue levels (MRLs), and possible transfer from feed into food of animal origin. The critical effects of bromide in experimental animals are on the thyroid and central nervous system. Changes in thyroid hormone homeostasis could result in neurodevelopmental toxicity, among other adverse effects. Changes in thyroid hormone concentrations and neurophysiological parameters have also been observed in experimental human studies, but the evidence was limited. Dose-response modelling of decreased blood thyroxine concentrations in rats resulted in a reference point of 40 mg/kg body weight (bw) per day. The Scientific Committee established a tolerable daily intake (TDI) of 0.4 mg/kg bw per day and an acute reference dose (ARfD) of 0.4 mg/kg bw per day to protect against adverse neurodevelopmental effects. The TDI value is supported by the results of experimental human studies with a NOAEL of 4 mg/kg bw per day and 10-fold interindividual variability. The TDI and ARfD are considered as conservative with 90% certainty. Insufficient evidence related to the toxicological effects of bromide was available for animals, with the exception of dogs. Therefore, the reference point of 40 mg/kg bw per day was extrapolated to maximum safe concentrations of bromide in complete feed for other animal species. Bromide can transfer from feed to food of animal origin, but, from the limited data, it was not possible to quantify the transfer rate. Monitoring data exceeded the current MRLs for some food commodities, generally with a low frequency. A conservative safety screening of the MRLs indicated that the TDI and ARfD are exceeded for some EU diets. Dietary exposure assessment for animals was not feasible due to insufficient data. The Scientific Committee recommends data be generated to allow robust dietary exposure assessments in the future, and data that support the risk assessment.

The role of reactive N-bromo species and radical intermediates in hypobromous acid-induced protein oxidation

Activated eosinophils, and hypobromous acid (HOBr) generated by these cells, have been implicated in the tissue injury in asthma, allergic reactions, and some infections. Proteins are major targets for this oxidant, but limited information is available on the mechanisms of damage and intermediates formed. Reaction of HOBr with proteins is shown to result in the formation of bromamines and bromamides, from side-chain and backbone amines and amides, and 3-bromo- and 3,5-dibromo-Tyr, from Tyr residues; these materials account for ca. 70% of the oxidant consumed. Protein carbonyls, dityrosine, and 3,4-dihydroxyphenylalanine are also formed, though these are minor products (<5% of HOBr added). With BSA, extensive (selective and nonspecific) protein fragmentation and limited aggregation are also observed. The bromamines/bromamides are unstable and induce further oxidation and free radical formation as detected by EPR spin trapping. Evidence was obtained for the generation of nitrogen-centered radicals on side-chain and backbone amide groups of amino acids, peptides, and proteins. These radicals readily undergo rearrangement reactions to give carbon-centered radicals. With proteins, alpha-carbon (backbone) radicals are detected, which may play a role in protein fragmentation. A novel damage transfer pathway from Gln side-chain amide groups to backbone sites was also observed.

3-Bromotyrosine and 3,5-dibromotyrosine are major products of protein oxidation by eosinophil peroxidase: potential markers for eosinophil-dependent tissue injury in vivo

Detection of specific reaction products is a powerful approach for dissecting out pathways that mediate oxidative damage in vivo. Eosinophil peroxidase (EPO), an abundant protein secreted from activated eosinophils, has been implicated in promoting oxidative tissue injury in conditions such as asthma, allergic inflammatory disorders, cancer, and helminthic infections. This unique heme protein amplifies the oxidizing potential of H2O2 by utilizing plasma levels of Br- as a cosubstrate to form potent brominating agents. Brominated products might thus serve as powerful tools for identifying sites of eosinophil-mediated tissue injury in vivo; however, structural identification and characterization of specific brominated products formed during EPO-catalyzed oxidation have not yet been reported. Here we explore the role of EPO and myeloperoxidase (MPO), a related leukocyte protein, in promoting protein oxidative damage through bromination and demonstrate that protein tyrosine residues serve as endogenous traps of reactive brominating species forming stable ring-brominated adducts. Exposure of the amino acid L-tyrosine to EPO, H2O2, and physiological concentrations of halides (100 mM Cl-, </=100 microM Br-) produced two new major products with distinct retention times on reverse phase HPLC. The products were identified as 3-bromotyrosine and 3, 5-dibromotyrosine by electrospray ionization mass spectrometry and multinuclear (1H and 15N) NMR spectroscopy. Formation of the ring-brominated forms of the amino acid occurred readily at neutral pH with the enzymatic system and a variety of reactive brominating species, including HOBr/OBr-, N-bromoamines, and N,N-dibromoamines. Addition of primary amines (e.g., Nalpha-acetyllysine and taurine) to L-tyrosine exposed to either HOBr/OBr- or the EPO-H2O2-Br- system enhanced phenolic ring bromination, suggesting N-bromoamines are preferred brominating intermediates in these reactions. Reduction of N-bromoamines (e.g., Nalpha-acetyl,Nepsilon-bromolysine) by L-tyrosine was shown to result in the loss of reactive halogen with a near stoichiometric increase in the level of tyrosine ring bromination (i.e., carbon-bromine bonds). Although both EPO and MPO could use Br- to halogenate protein tyrosine residues in vitro, only EPO effectively brominated the aromatic amino acid at physiological levels of halides and H2O2. Collectively, these results suggest that 3-bromotyrosine and 3,5-dibromotyrosine are attractive candidates for serving as molecular markers for oxidative damage of proteins by reactive brominating species in vivo. They also suggest that in biological mixtures where amine groups are abundant, the trapping of EPO-generated HOBr/OBr- as N-bromoamines will serve to effectively "funnel" reactive brominating equivalents to stable ring-brominated forms of tyrosine.

Expression of the proliferating cell nuclear antigen (PCNA) in the rat thyroid gland after exposure to bromide

Analysis of expression of the proliferating cell nuclear antigen (PCNA) was used to determine the presumed hyperplastic character of morphological changes in the rat thyroid evoked by bromide administration. Male rats fed by a standard diet with determined iodine and bromine content were given potassium bromide. Control animals received no bromide. Experimental animals were given 10, 50 or 100 mg Br- per 11 drinking water for 16 and 66 days, or 100, 200, 400 mg Br-/l drinking water for 133 days. The thyroids of treated animals showed activation of growth of the epithelial follicular component as well as diffuse and focal microfollicular rearrangement of the parenchyma with higher follicular cells accompanied by a decrease of the amount of colloid even at low bromine concentrations (10-100 mg Br-/l drinking water). Using the PCNA-LI index (PCNA-positive nuclei.100/total number of follicular cell nuclei in the section), immunohistochemical analysis of PCNA in the nuclei of the follicular cells was carried out in parrafin sections. The index was significantly higher in bromide exposed animals (P < 0.01) and correlated well with the histological changes, with bromide concentration and with a increased mitotic activity of the follicular cells. PCNA analysis showed that morphological changes resembling a parenchymatic goitre reflect a microfollicular rearrangement of the thyroid of rats exposed to bromide and have the character of hyperplasia owing to the increased mitotic activity of the follicular epithelium.

Potassium bromide and the thyroid gland of the rat: morphology and immunohistochemistry, RIA and INAA analysis

The increasing environmental concentration of bromine has resulted in attempts to obtain information on its possibly deleterious effect on humans, particularly on a major target organ of this halogen i.e. the thyroid gland. In order to establish the morphological and functional effects of bromine on the thyroid, we have performed experiments on male rats which, in addition to a standard diet with an estimated iodine/bromine content, were fed for periods of 16 and 66 days with the small quantities of bromide expected to be encountered in the environment (10, 50 and 100 mg of Br-/l in drinking water). This treatment induced growth of the follicular epithelial component and microfollicular tissue rearrangement, a reduction of intrafollicular colloid, an increase in the height of the follicular cells and the number of mitoses, and it enhanced vascularization. Image analysis revealed a significant reduction in the volume of colloid, despite the accompanying rise in the number of minute follicles. The immunohistochemical positivity of the thyroglobulin fell in the microfollicular colloid of the exposed animals, although this was affected to a lesser extent in the larger follicles. The concentration of bromine in the thyroid increased with the amount of bromine intake, while at the same time the molar ratio of iodine/bromine decreased. The plasma level of T4 was lowered after both 16 and 66 days of treatment, but the T3 level only after 66 days treatment. The level of TSH did not exhibit any significant change. The observed changes, which have a parenchymatous goitre-like character, may have a direct relevance for human medicine, since the concentrations of bromide chosen in these experiments are readily encountered in the environment.

Oxidation of bromide by the human leukocyte enzymes myeloperoxidase and eosinophil peroxidase. Formation of bromamines

Myeloperoxidase and eosinophil peroxidase catalyzed the oxidation of bromide ion by hydrogen peroxide (H2O2) and produced a brominating agent that reacted with amine compounds to form bromamines, which are long-lived oxidants containing covalent nitrogen-bromine bonds. Results were consistent with oxidation of bromide to an equilibrium mixture of hypobromous acid (HOBr) and hypobromite ion (OBr-). Up to 1 mol of bromamine was produced per mole of H2O2, indicating that bromamine formation prevented the reduction of HOBr/OBr- by H2O2 and the loss of oxidizing and brominating activity. Bromamines differed from HOBr/OBr- in that bromamines reacted slowly with H2O2, were not reduced by dimethyl sulfoxide, and had absorption spectra similar to those of chloramines, but shifted 36 nm toward higher wavelengths. Mono- and di-bromo derivatives (RNHBr and RNHBr2) of the beta-amino acid taurine were relatively stable with half-lives of 70 and 16 h at pH 7, 37 degrees C. The mono-bromamine was obtained with a 200-fold excess of amine over the amount of HOBr/OBr- and the di-bromamine at a 2:1 ratio of HOBr/OBr- to the amine. In the presence of physiologic levels of both bromide (0.1 mM) and chloride (0.1 M), myeloperoxidase and eosinophil peroxidase produced mixtures of bromamines and chloramines containing 6 +/- 4% and 88 +/- 4% bromamine. In contrast, only the mono-chloramine derivative (RNHCl) was formed when a mixture of hypochlorous acid (HOCl) and hypochlorite ion (OCl-) was added to solutions containing bromide and excess amine. The rapid formation of the chloramine prevented the oxidation of bromide by HOCl/OCl-, and the chloramine did not react with bromide within 1 h at 37 degrees C. The results indicate that when enzyme-catalyzed bromide or chloride oxidation took place in the presence of an amine compound at 10 mM or higher, bromamines were not produced in secondary reactions such as the oxidation of bromide by HOCl/OCl- and the exchange of bromide with chlorine atoms of chloramines. Therefore, the amount of bromamine produced by myeloperoxidase or eosinophil peroxidase was equal to the amount of bromide oxidized by the enzyme. Bromide was preferred over chloride as the substrate for both enzymes.

76Br-labeled monoclonal anti-CEA antibodies for radioimmuno positron emission tomography

For the application of anti-tumor monoclonal antibodies (MAbs) in positron emission tomography (PET), labeling radionuclides with half-lives allowing a suitable time frame for imaging are required. The anti-CEA MAb 38S1 was labeled with the positron emitting nuclide 76Br (t1/2 16 h) using bromoperoxidase (BPO), and subsequently affinity purified. A procedure was devised to allow reproducible production of MAb-preparations of high immunoreactivity and with acceptable bromination yield. The biological activity of 76Br-38S1 was retained and comparable to that of chloramine-T labeled 125I-38S1, as tested in vitro.

Peroxidase-catalyzed bromination of tyrosine, thyroglobulin, and bovine serum albumin: comparison of thyroid peroxidase and lactoperoxidase

A recent paper (Buchberger, W., 1988, J. Chromatogr. 432, 57) on lactoperoxidase-catalyzed bromination of tyrosine and thyroglobulin stated, without evidence, that thyroid peroxidase (TPO) is able to use bromide as a substrate. This was in disagreement with unpublished experiments previously performed in this laboratory, and we undertook, therefore, to examine this subject further. Highly purified porcine TPO was compared with lactoperoxidase (LPO) and chloroperoxidase (CPO) for ability to catalyze bromination of tyrosine, thyroglobulin, and bovine serum albumin (BSA). The incubation mixture contained 50-100 nM peroxidase, 10-500 microM 82Br-, tyrosine (150 microM), thyroglobulin (0.3 or 1 microM), or BSA (7.5 microM), and a source of H2O2. The latter was either generated by glucose (1 mg/ml)-glucose oxidase (0.5 or 1 micrograms/ml), or added initially as a bolus (100 microM). With TPO, formation of organically bound 82Br was undetectable under all conditions in the pH range 5.4-7.0. Lactoperoxidase and CPO, on the other hand, displayed considerable brominating activity. Lactoperoxidase was much more active at pH 5.4 than at pH 7.0 and was more active with BSA as acceptor than with tyrosine or thyroglobulin. The distribution of 82Br among the various amino acids in LPO-brominated thyroglobulin and BSA was determined by HPLC. As expected, monobromotyrosine and dibromotyrosine together comprised the greatest part of the bound 82Br. However, a surprisingly high percentage (20-25%) was present as monobromohistidine. Evidence was also obtained for the presence of a small percentage of the bound 82Br as tetrabromothyronine. Peroxidase-catalyzed bromination probably depends on the oxidation of Br- to Br+ by the Compound I form of the enzyme. Since oxidation of Br- to Br+ requires a stronger oxidant than oxidation of I- to I+, our results suggest that Compound I of LPO and of CPO has a higher oxidation potential than Compound I of TPO. In vivo experiments with rats on a low iodine diet injected with 82Br- showed that even under conditions of high stimulation by thyrotropic hormone, there is negligible formation of organic bromine in the thyroid. Measurements of thyroid:serum concentration ratios for 82Br- in similar rats provided no evidence that Br- is a substrate for the iodide transport system of the thyroid.