Mechanism of thyroid peroxidase inhibition by ethylenethiourea

Determination of ethylene-bis-dithiocarbamates and their degradation product ethylenethiourea in dry herbs by UHPLC-MS/MS

Dithiocarbamates are a class of fungicides widely used in many countries. In this study, methods for determining the ethylene-bis-dithiocarbamate (EBDC) subclass, and their degradation product ethylenethiourea (ETU) were validated by UHPLC-MS/MS in different types of dry herbs, which can be used as food and/or medicinal purposes. Mancozeb was used in the validation of the EBDC method, where it was initially complexed with EDTA, derivatized, extracted with dimethyl sulfate in acetonitrile, magnesium sulfate (MgSO4), and sodium chloride (NaCl), and then purified using primary secondary amine (PSA). In the ETU method, L-cysteine hydrochloride monohydrate was added to the samples before extraction with acetonitrile, MgSO4, and NaCl, followed by purification with PSA. A pesticide-free blend of seven herbs (boldo, artichoke, "espinheira-santa", cat's claw, senna, chamomile, and cascara buckthorn) comprising distinct parts of the plants (leaves, bark, flowers and/or stems) was used as a control for method validation. Recoveries ranged from 79 to 113% for EBDC and 81 to 109% for ETU. Repeatability and intermediate precision were <20% for both methods. The limit of quantification was 0.03 mg kg-1 for EBDC (0.02 mg kg-1 of CS2) and ETU. The limit of detection (LOD) was set at 1/3 of the LOQ (0.01 mg kg-1 for both analytes). In total, 103 samples of 33 different dry herbs were analyzed, of which 19.4% were positive for EBDC (≥LOD), but no ETU residues were found in any of the analyzed samples. Given the absence of registered dithiocarbamates for use in the investigated herbs in Brazil, the positive results suggest potential illegal pesticide use or cross-contamination, especially considering the low concentrations detected in most samples. Although exposure to EBDC through the consumption of medicinal herbs from positive samples did not indicate a health risk to consumers, these plants must be monitored to prevent illicit pesticide usage, particularly when the herbs are intended for therapeutic purposes.

Fungicides as a risk factor for the development of neurological diseases and disorders in humans: a systematic review

Although studies show that pesticides, especially insecticides, may be toxic to humans, publications on the neurological effects of fungicides are scarce. As fungicides are used widely in Brazil, it is necessary to gather evidence to support actions aimed at safely using of these chemicals. We investigated through a systematic review of publications on the use of fungicides and consequences of exposure related to nervous system diseases or neurological disorders in humans. The protocol review was registered on PROSPERO and followed the guidelines of the PRISMA-Statement. As far as it is known, there is no apparent systematic review in the literature on this topic. The search was comprised of the following databases: PubMed; Web of Science; Scopus and EMBASE, using groups of Mesh terms and strategies specific to each database. Thirteen articles were selected for this review. Regarding the substances analyzed in the studies, some reported the use of fungicides in general, without separating them by type, while others summarized the categories of all pesticides by their function (insecticides, herbicides, fungicides, etc.) or chemical class (dithiocarbamate, dicarboximide, inorganic, etc.). However, most of the articles referred to fungicides that contain the metal manganese (Mn) in their composition. As for neurological disorders, articles addressed Parkinson's disease (PD), neurodevelopmental outcomes, extrapyramidal syndrome resembling PD, cognitive disorders, depression, neural tube defects, motor neurone disease, and amyotrophic lateral sclerosis. Most investigations pointed to exposure to fungicides, mainly maneb and mancozeb, leading to the development of at least one neurological disease, which suggests the need for further multicentric clinical trials and prospective studies for greater clarity of the research problem.

Oxidation of anti-thyroid drugs and their selenium analogs by ABTS radical cation

Lactoperoxidase was previously used as a model enzyme to test the inhibitory activity of selenium analogs of anti-thyroid drugs with 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) as a substrate. Peroxidases oxidize ABTS to a metastable radical ABTS•+, which is readily reduced by many antioxidants, including thiol-containing compounds, and it has been used for decades to measure antioxidant activity in biological samples. We showed that anti-thyroid drugs 6-n-propyl-2-thiouracil, methimazole, and selenium analogs of methimazole also reduced it rapidly. This reaction may explain the anti-thyroid action of many other compounds, particularly natural antioxidants, which may reduce the oxidized form of iodine and/or tyrosyl radicals generated by thyroid peroxidase thus decreasing the production of thyroid hormones. However, influence of selenium analogs of methimazole on the rate of hydrogen peroxide consumption during oxidation of ABTS by lactoperoxidase was moderate. Direct hydrogen peroxide reduction, proposed before as their mechanism of action, cannot therefore account for the observed inhibitory effects. 1-Methylimidazole-2-selone and its diselenide were oxidized by ABTS•+ to relatively stable seleninic acid, which decomposed slowly to selenite and 1-methylimidazole. In contrast, oxidation of 1,3-dimethylimidazole-2-selone gave selenite and 1,3-dimethylimidazolium cation. Accumulation of the corresponding seleninic acid was not observed.

AOP Report: Thyroperoxidase Inhibition Leading to Altered Visual Function in Fish Via Altered Retinal Layer Structure

Thyroid hormones (THs) are involved in the regulation of many important physiological and developmental processes, including vertebrate eye development. Thyroid hormone system-disrupting chemicals (THSDCs) may have severe consequences, because proper functioning of the visual system is a key factor for survival in wildlife. However, the sequence of events leading from TH system disruption (THSD) to altered eye development in fish has not yet been fully described. The development of this adverse outcome pathway (AOP) was based on an intensive literature review of studies that focused on THSD and impacts on eye development, mainly in fish. In total, approximately 120 studies (up to the end of 2021) were used in the development of this AOP linking inhibition of the key enzyme for TH synthesis, thyroperoxidase (TPO), to effects on retinal layer structure and visual function in fish (AOP-Wiki, AOP 363). In a weight-of-evidence evaluation, the confidence levels were overall moderate, with ample studies showing the link between reduced TH levels and altered retinal layer structure. However, some uncertainties about the underlying mechanism(s) remain. Although the current weight-of-evidence evaluation is based on fish, the AOP is plausibly applicable to other vertebrate classes. Through the re-use of several building blocks, this AOP is connected to the AOPs leading from TPO and deiodinase inhibition to impaired swim bladder inflation in fish (AOPs 155-159), together forming an AOP network describing THSD in fish. This AOP network addresses the lack of thyroid-related endpoints in existing fish test guidelines for the evaluation of THSDCs. Environ Toxicol Chem 2022;41:2632-2648. © 2022 SETAC.

Towards a science-based testing strategy to identify maternal thyroid hormone imbalance and neurodevelopmental effects in the progeny-part III: how is substance-mediated thyroid hormone imbalance in pregnant/lactating rats or their progeny related to neurodevelopmental effects?

This review investigated which patterns of thyroid- and brain-related effects are seen in rats upon gestational/lactational exposure to 14 substances causing thyroid hormone imbalance by four different modes-of-action (inhibition of thyroid peroxidase, sodium-iodide symporter and deiodinase activities, enhancement of thyroid hormone clearance) or to dietary iodine deficiency. Brain-related parameters included motor activity, cognitive function, acoustic startle response, hearing function, periventricular heterotopia, electrophysiology and brain gene expression. Specific modes-of-action were not related to specific patterns of brain-related effects. Based upon the rat data reviewed, maternal serum thyroid hormone levels do not show a causal relationship with statistically significant neurodevelopmental effects. Offspring serum thyroxine together with offspring serum triiodothyronine and thyroid stimulating hormone appear relevant to predict the likelihood for neurodevelopmental effects. Based upon the collated database, thresholds of ≥60%/≥50% offspring serum thyroxine reduction and ≥20% and statistically significant offspring serum triiodothyronine reduction indicate an increased likelihood for statistically significant neurodevelopmental effects; accuracies: 83% and 67% when excluding electrophysiology (and gene expression). Measurements of brain thyroid hormone levels are likely relevant, too. The extent of substance-mediated thyroid hormone imbalance appears more important than substance mode-of-action to predict neurodevelopmental impairment in rats. Pertinent research needs were identified, e.g. to determine whether the phenomenological offspring thyroid hormone thresholds are relevant for regulatory toxicity testing. The insight from this review shall be used to suggest a tiered testing strategy to determine whether gestational/lactational substance exposure may elicit thyroid hormone imbalance and potentially also neurodevelopmental effects.

Exposure to antithyroid drugs and ethylenethiourea and risk of thyroid cancer: a systematic review of the epidemiologic evidence

INTRODUCTION

The thyroid peroxidase inhibiting compounds methimazole, methylthiouracil, propylthiouracil, thiouracil (i.e. 'antithyroid' drugs) and ethylenethiourea have been associated to thyroid tumours in rodents. According to a systematic review by the International Agency for Research on Cancer (IARC) published in 2000, evidence for the human carcinogenicity was inadequate.

METHODS

We performed an up-to-date systematic review of human epidemiological studies on the association between such compounds and thyroid cancer incidence or mortality.

RESULTS

The literature research (1999-March 2020) identified four relevant articles. Considering also reports from the previous IARC review, this systematic review considered seven reports (five distinct studies) on antithyroid drugs and two on ethylenethiourea. As for antithyroid drugs, three reports based on different follow-ups gave results from a cohort of patients treated for hyperthyroidism in 1946-1964. In the earlier report, thyroid cancer incidence was higher in patients primarily treated with antithyroid drugs (3.2/1000) than in those originally treated with thyroidectomy (0.34/1000) or radioactive iodine (0.88/1000), which can be explained by the higher frequency of subsequent thyroidectomy, and hence the higher chance of cancer detection, in that group (30 vs. 0.5 and 1.2%). The two subsequent reports found no deaths from thyroid cancer among patients treated exclusively with antithyroid drugs through 1990 and 2014. A nested case-control study found an odds ratio (OR) of thyroid cancer of 2.79 [95% confidence interval (CI), 0.78-10.02, from a 2-year lag analysis] for ≥3 vs. no propylthiouracil prescriptions. The increased risk can be attributed to advanced diagnosis of an underlying cancer, as suggested by the stronger association observed in a no-lag analysis (OR, 8.03). In a historical cohort of newly diagnosed hyperthyroid patients, the hazard ratio for treatment with radioactive iodine vs. thionamides only was 0.45 (95% CI, 0.21-0.99), possibly due to the closer surveillance of patients receiving thionamides only. Two case-control studies did not find any association with the use of antithyroid drugs. As for ethylenethiourea, no thyroid cancer cases were found in a historical cohort of 1929 workers occupationally exposed in a 15-year period and no association with proxies of mancozeb exposure (a fungicide whose main metabolite is ethylenethiourea) was detected in a cohort of >236 000 farmers.

CONCLUSION

There is no evidence for a relevant role of either antithyroid drugs or ethylenethiourea on thyroid cancer.

An assay for screening xenobiotics for inhibition of rat thyroid gland peroxidase activity

1. A number of chemicals have been shown to produce disruption of the thyroid gland, resulting in reduced thyroid hormone synthesis, by a mechanism involving inhibition of thyroid peroxidase (TPO) activity (EC 1.11.1.8).2. An assay was developed for rat thyroid gland microsomal TPO activity, employing L-tyrosine as the physiological substrate, with analysis of the formation of the 3-iodo-L-tyrosine (3MIT) metabolite by ultra-performance liquid chromatography-mass spectrometry-mass spectrometry.3. Formation of 3MIT was linear with respect to both rat thyroid gland microsomal protein concentration and incubation time, whereas only small quantities of 3,5-diodo-L-tyrosine were formed.4. Studies were performed with nine known TPO inhibitors. The most potent inhibitors were 3-amino-1,2,4-triazole, ethylene thiourea, methimazole and 6-propyl-2-thiouracil which had IC₅₀ values (i.e. concentration to produce a 50% inhibition of enzyme activity) of 0.059, 0.791, 1.07 and 1.96 μM, respectively, whereas the least potent inhibitor was sodium perchlorate which had an IC₅₀ value of 13,800 µM.5. For five inhibitors, where literature data were available, the observed IC₅₀ values obtained in this study employing rat thyroid gland microsomes and L-tyrosine as substrate were similar to those previously reported using the spectrophotometric guaiacol oxidation assay.

The Colorful Diversity of Thyroid Hormone Metabolites

Since the discovery of L-thyroxine, the main secretory product of the thyroid gland, and its major metabolite T3, which exerts the majority of thyroid hormone action via ligand-dependent modulation of the function of T3 receptors in nuclei, mitochondria, and other subcellular compartments, various other T4-derived endogenous metabolites have been identified in blood and tissues of humans, animals, and early protochordates. This review addresses major historical milestones and experimental findings resulting in the discovery of the key enzymes of thyroid hormone metabolism, the three selenoprotein deiodinases, as well as the decarboxylases and amine oxidases involved in formation and degradation of recently identified endogenous thyroid hormone metabolites, i.e. 3-iodothyronamine and 3-thyroacetic acid. The concerted action of deiodinases 2 and 3 in regulation of local T3 availability is discussed. Special attention is given to the role of the thyromimetic "hot" metabolite 3,5-T2 and the "cool" 3-iodothyronamine, especially after administration of pharmacological doses of these endogenous thyroid hormone metabolites in various animal experimental models. In addition, available information on the biological roles of the two major acetic acid derivatives of thyroid hormones, i.e. Tetrac and Triac, as well as sulfated metabolites of thyroid hormones is reviewed. This review addresses the consequences of the existence of this broad spectrum of endogenous thyroid hormone metabolites, the "thyronome," beyond the classical thyroid hormone profile comprising T4, T3, and rT3 for appropriate analytical coverage and clinical diagnostics using mass spectrometry versus immunoassays for determination of total and free concentrations of thyroid hormone metabolites in blood and tissues.

Contaminant and Environmental Influences on Thyroid Hormone Action in Amphibian Metamorphosis

Aquatic and terrestrial environments are increasingly contaminated by anthropogenic sources that include pharmaceuticals, personal care products, and industrial and agricultural chemicals (i. e., pesticides). Many of these substances have the potential to disrupt endocrine function, yet their effect on thyroid hormone (TH) action has garnered relatively little attention. Anuran postembryonic metamorphosis is strictly dependent on TH and perturbation of this process can serve as a sensitive barometer for the detection and mechanistic elucidation of TH disrupting activities of chemical contaminants and their complex mixtures. The ecological threats posed by these contaminants are further exacerbated by changing environmental conditions such as temperature, photoperiod, pond drying, food restriction, and ultraviolet radiation. We review the current knowledge of several chemical and environmental factors that disrupt TH-dependent metamorphosis in amphibian tadpoles as assessed by morphological, thyroid histology, behavioral, and molecular endpoints. Although the molecular mechanisms for TH disruption have yet to be determined for many chemical and environmental factors, several affect TH synthesis, transport or metabolism with subsequent downstream effects. As molecular dysfunction typically precedes phenotypic or histological pathologies, sensitive assays that detect changes in transcript, protein, or metabolite abundance are indispensable for the timely detection of TH disruption. The emergence and application of 'omics techniques-genomics, transcriptomics, proteomics, metabolomics, and epigenomics-on metamorphosing tadpoles are powerful emerging assets for the rapid, proxy assessment of toxicant or environmental damage for all vertebrates including humans. Moreover, these highly informative 'omics techniques will complement morphological, behavioral, and histological assessments, thereby providing a comprehensive understanding of how TH-dependent signal disruption is propagated by environmental contaminants and factors.

Prenatal Mancozeb Exposure, Excess Manganese, and Neurodevelopment at 1 Year of Age in the Infants' Environmental Health (ISA) Study

BACKGROUND

Although growing evidence suggests that early-life excess manganese (Mn) impairs neurodevelopment, data on the neurodevelopmental effects of mancozeb, a fungicide containing Mn, and its main metabolite ethylenethiourea (ETU) are limited.

OBJECTIVE

We examined whether prenatal mancozeb exposure and excess Mn were associated with neurodevelopment in 355 1-y-old infants living near banana plantations with frequent aerial mancozeb spraying in Costa Rica.

METHODS

We measured urinary ETU, hair Mn, and blood Mn concentrations in samples collected 1-3 times during pregnancy from mothers enrolled in the Infants' Environmental Health (ISA) study. We then assessed neurodevelopment in their 1-y-old infants using the Bayley Scales of Infant and Toddler Development, 3rd edition (BSID-III). We estimated exposure-outcome associations using linear regression models adjusted for maternal education, parity, gestational age at birth, child age, Home Observation for Measurement of the Environment score, and location of neurodevelopmental assessment.

RESULTS

Median (P25-P75) urinary ETU, hair Mn, and blood Mn measured during pregnancy were 3.3 μg/L (2.4-4.9; specific gravity-corrected), 1.7 μg/g (0.9-4.1), and 24.0 μg/L (20.3-28.0), respectively. Among girls, higher ETU was associated with lower social-emotional scores [β per 10-fold increase=-7.4 points (95% CI: -15.2, 0.4)], whereas higher hair Mn was associated with lower cognitive scores [-3.0 (-6.1, 0.1)]. Among boys, higher hair Mn was associated with lower social-emotional scores [-4.6 (-8.5, -0.8)]. We observed null associations for blood Mn, language, and motor outcomes.

CONCLUSIONS

Our findings indicate that maternal exposure to mancozeb and excess Mn during pregnancy may have adverse and sex-specific effects on infant neurodevelopment. https://doi.org/10.1289/EHP1955.

Deleterious Effects From Occupational Exposure to Ethylene Thiourea in Pregnant Women

Human exposure to endocrine disrupting chemicals (EDCs) has become common as a result of widespread application of these chemicals to the food supply, environmental contamination, and occupational exposures (Caserta et al., 2011). However, relatively little is known about the effects of EDCs such as ethylene thiourea (ETU) in developing fetuses and the lasting implications of this disruption on human development from birth through adulthood. Of highest concern are chronic, low-dose exposures among industrial and agricultural workers. Current knowledge regarding the significance of endocrine thyroid signaling on normal human development raises serious concerns about the possible deleterious effects of EDCs in the developing fetus, children, and mature adults. Occupational health nurses are critical in identifying women and families at increased risk of ETU exposure and mitigating early exposures in pregnancy.

Iodine nutritional status and thyroid effects of exposure to ethylenebisdithiocarbamates

INTRODUCTION

Italy is still characterized by a mild iodine deficiency and is among the most intensive users of chemical products for agriculture in Europe. The aim of this study was i) to evaluate thyroid effects of exposure to mancozeb, a fungicide widely used in agriculture, in a sample of Italian grapevine workers, and ii) to verify whether the iodine intake may modulate the risk of thyroid disruption due to the mancozeb metabolite ethylenthiourea (ETU).

METHODS

One hundred seventy-seven occupationally exposed male workers (29 from Chianti, a mild iodine deficient area, and 148 from Bolzano an iodine sufficient province) and 74 non-occupationally exposed male controls (34 from Chianti and 40 from Bolzano) were enrolled in the study. Serum biomarkers of thyroid function, as well as urinary iodine and ETU concentrations were assessed. Moreover all the recruited subjects underwent clinical examination and thyroid ultrasound.

RESULTS

Multivariate comparisons showed lower mean serum levels of FT4 in Chianti-workers as compared to Bolzano-workers. Moreover, an increased urinary iodine excretion (>250µg/L) was more frequently found among more exposed workers (ETU>20µg/L) than among less exposed ones and this effect was more pronounced in Chianti- than in Bolzano-workers. Chianti-workers also showed a significantly higher frequency of very low thyroid volume (≤6.0ml) as compared to controls.

CONCLUSIONS

These findings showed a mild thyroid disrupting effect due to occupational exposure to mancozeb, more pronounced in workers residing in an area characterized by a mild to moderate iodine deficiency as compared to workers residing in an area covered by a long-lasting iodine prophylaxis program.

Pesticide exposure and neurodevelopment in children aged 6-9 years from Talamanca, Costa Rica

Certain pesticides may affect children's neurodevelopment. We assessed whether pesticide exposure was associated with impaired neurobehavioral outcomes in children aged 6-9 years. We conducted a cross-sectional study in 140 children living near banana plantations and plantain farms in the Talamanca County, Costa Rica and assessed their neurobehavioral performance. Exposure was determined by analyzing urinary metabolites of chlorpyrifos (3,5,6-trichloro-2-pyridinol, TCPy), mancozeb (ethylenethiourea, ETU), and pyrethroids (3-phenoxybenzoic acid, 3-PBA). Repeated urine samples were obtained for 36 children. We estimated associations of pesticide concentrations with neurobehavioral outcomes using multivariable linear and logistic regression models. Median (25th-75th percentiles) TCPy, ETU, and 3-PBA concentrations were 1.4 (.7-3.1), 1.2 (.7-3.0), and .8 (.5-1.5) μg/L, respectively. Intraclass correlation coefficients (ICC) ranged between .32 and .67. After adjustment for potential confounders, higher urinary TCPy concentrations were associated with poorer working memory in boys (n = 59) (β per 10-fold increase in TCPy concentrations = -7.5, 95% CI: -14.4, -.7); poorer visual motor coordination (β = -1.4, 95% CI: -2.7, -.1); increased prevalence of parent-reported cognitive problems/inattention (adjusted OR per 10-fold increase in urinary concentrations = 5.8, 95% CI: 1.6, 22.9), oppositional disorders (aOR = 3.9, 95% CI: 1.0, 16.0), and ADHD (aOR = 6.8, 95% CI: 1.8, 28.6), and; decreased ability to discriminate colors (aOR = 6.6, 95% CI: 1.6, 30.3; the higher the score the worse). Higher ETU concentrations were associated with poorer verbal learning outcomes (β = -7.0, 95% CI: -12.7, -1.3). Higher 3-PBA concentrations were associated with poorer processing speed scores, particularly in girls (β = -8.8, 95% CI: -16.1, -1.4). Our findings indicate that children living near banana and plantain plantations are exposed to pesticides that may affect their neurodevelopment, which for certain domains may differ between boys and girls. We recommend the implementation of measures to reduce pesticide exposure in children living nearby banana plantations.

Identifying reference chemicals for thyroid bioactivity screening

Reference chemicals were selected based on thyroid bioactivity in 'Tier 1' screening assays used by the U.S. EPA's Endocrine Disruptor Screening Program. Active reference chemicals had significant effects on thyroid-responsive endpoints in the amphibian metamorphosis assay, and the male and female pubertal rat assays. In the absence of thyroid weight or histopathological effects, additional published studies providing mechanistic data on thyroid activity were required for active chemicals. Inactive reference chemicals had no significant effects on thyroid-responsive endpoints in Tier 1 assays, or in amphibian or rodent studies from several online databases. The 34 reference chemicals (29 active and five inactive) will be useful for performance-based validation of alternative, high throughput screening assays for thyroid bioactivity.

The neonicotinoid pesticide imidacloprid and the dithiocarbamate fungicide mancozeb disrupt the pituitary-thyroid axis of a wildlife bird

Thyroid is an important homeostatic regulator of metabolic activities as well as endocrine mechanisms including those of reproduction. Present investigation elucidated the thyroid disrupting potential of a neonicotinoid imidacloprid and a dithiocarbamate mancozeb in a seasonally breeding wildlife bird, Red Munia (Amandava amandava) who is vulnerable to these two pesticides through diet (seed grains and small insects). Adult male birds were exposed to 0.5% LD50 mgkg(-1)bwd(-1) of both the pesticides through food for 30days during the preparatory and breeding phases. Weight, volume and histopathology of thyroid gland were distinctly altered. Disruption of thyroid follicles reflected in nucleus-to-cytoplasm ratio (N/C) in epithelial and stromal cells, epithelial cell hypertrophy and altered colloid volume. Impairment of thyroid axis was pesticide and phase specific as evident from the plasma levels of thyroid (T4 and T3) and pituitary (TSH) hormones. In preparatory phase, plasma TSH was increased in response to decrease of T4 on mancozeb exposure showing responsiveness of the hypothalamic-pituitary-thyroid (HPT) axis to feedback regulation. On imidacloprid exposure, however, plasma levels of both T4 and TSH were decreased indicating non-functioning of negative feedback mechanism. Increased plasma T3 in response to both the pesticides exposure might be due to synthesis from non-thyroidal source(s) in a compensatory response to decrease level of T4. In breeding phase, impairment of HPT axis was more pronounced as plasma T4, T3 and TSH were significantly decreased in response to both mancozeb and imidacloprid. Thus, low dose pesticide exposure could affect the thyroid homeostasis and reproduction.

Effects of pesticide chemicals on the activity of metabolic enzymes: focus on thiocarbamates

INTRODUCTION

Thiocarbamates are chemicals widely used as pesticides. Occupational exposure is associated with acute intoxication. Populations can be exposed through food and water. Moreover, certain thiocarbamates are used clinically. The widespread use of thiocarbamates raises many issues regarding their toxicological and pharmacological impact.

AREAS COVERED

Thiocarbamates and their metabolites can modify biological macromolecules functions, in particular enzymes, through modification of cysteine residues, chelation of metal ions or modulation of the oxidative stress. Loss of enzyme activity can lead to the disruption of metabolic pathways, and explain, at least in part, the effects of these pesticides. Additionally, their reactivity and ability to easily cross biological barrier confer them a great interest for development of clinical applications.

EXPERT OPINION

Many advances in the study of thiocarbamates metabolism and reactivity have led to a better knowledge of biological effects of these compounds. However, more data are needed on the determination of targets and specificity. Only few data concerning the exposure to a cocktail of pesticides/chemicals are available, raising the need to evaluate the toxic side effects of representative pesticides mixtures. Moreover, the dithiocarbamate Disulfiram has shown great potential in therapeutic applications and leads to the development of pharmacological thiocarbamates derivatives, highly specific to their target and easily distributed.

Combined repeated-dose and reproductive/developmental toxicity screening test of 3-amino-5-mercapto-1,2,4-triazole in rats

The substance 3-amino-5-mercapto-1,2,4-triazole (AMT, CAS No. 16691-43-3) was daily administered by gavage to Crl:CD (SD)IGS rats at doses of 0 (control), 10, 50, and 250 mg/kg bw/day. Males (12/group) were treated for a total of 42 days beginning 14 days before mating. Females (12/group) were treated beginning 14 days before mating to day 4 of lactation throughout the mating and gestation periods. No deaths occurred in males but three females died on day 23 of gestation at 250 mg/kg/day. Only temporary decreases in body weight and food intake were found in both sexes at 250 mg/kg/day. There were no considerable changes in general appearance, the functional battery tests, biochemical analysis or urinalysis. Anemia was observed in both sexes at 250 mg/kg/day. The relative weight of thyroid glands was significantly increased in both sexes at 250 mg/kg/day and hypertrophy of thyroid follicular cells was observed in 50 and 250 mg/kg/day males and 250 mg/kg/day females. As this effect on thyroid glands was considered to be the major toxicity, the possible mechanism was discussed comparing with the toxicity of structural similar analogs. Other histopathological changes in males were hypertrophy of centrilobular hepatocytes at 250 mg/kg/day, and anterior pituitary glands at 50 mg/kg/day and more. Vacuolization in renal tubular epithelium of females was observed at 50 and 250 mg/kg/day. For reproduction, the gestation period was prolonged and the delivery index was decreased at 250 mg/kg/day. The number of pups born and the birth index were also reduced. It was thus concluded that the NOAEL for repeated-dose toxicity was 10 mg/kg/day based on the thyrotoxicity and renal toxicity, and that the NOAEL for reproductive/developmental toxicity was 50 mg/kg/day based on the reduced number of offspring, etc.

Inhibition of lactoperoxidase-catalyzed oxidation by imidazole-based thiones and selones: a mechanistic study

Herein, we describe the synthesis and biomimetic activity of a series of N,N-disubstituted thiones and selones that contain an imidazole pharmacophore. The N,N-disubstituted thiones do not show any inhibitory activity towards LPO-catalyzed oxidation reactions, but their corresponding N,N-disubstituted selones exhibit inhibitory activity towards LPO-catalyzed oxidation reactions. Substituents on the N atom of the imidazole ring appear to have a significant effect on the inhibition of LPO-catalyzed oxidation and iodination reactions. Selones 16, 17, and 19, which contain methyl, ethyl, and benzyl substituents, exhibit similar inhibition activities towards LPO-catalyzed oxidation reactions with IC50 values of 24.4, 22.5, and 22.5 μM, respectively. However, their activities are almost three-fold lower than that of the commonly used anti-thyroid drug methimazole (MMI). In contrast, selone 21, which contains a N-CH2CH2OH substituent, exhibits high inhibitory activity, with an IC50 value of 7.2 μM, which is similar to that of MMI. The inhibitory activity of these selones towards LPO-catalyzed oxidation/iodination reactions is due to their ability to decrease the concentrations of the co-substrates (H2O2 and I2), either by catalytically reducing H2O2 (anti-oxidant activity) or by forming stable charge-transfer complexes with oxidized iodide species. The inhibition of LPO-catalyzed oxidation/iodination reactions by N,N-disubstituted selones can be reversed by increasing the concentration of H2O2. Interestingly, all of the N,N-disubstituted selones exhibit high anti-oxidant activities and their glutathione peroxidase (GPx)-like activity is 4-12-fold higher than that of the well-known GPx-mimic ebselen. These experimental and theoretical studies suggest that the selones exist as zwitterions, in which the imidazole ring contains a positive charge and the selenium atom carries a large negative charge. Therefore, the selenium moieties of these selones possess highly nucleophilic character. The (77)Se NMR chemical shifts for the selones show large upfield shift, thus confirming the zwitterionic structure in solution.

New parameter that supports speculation on the possible mechanism of hypothyroidism induced by chemical substances in repeated-dose toxicity studies

Hypothyroidism induced by xenobiotic treatment was analyzed for possible underlying mechanism(s) on the basis of different responses of the thyroid gland and the liver, using a newly-created database of repeated-dose toxicity of 500 chemicals. Two mechanisms are proposed: direct inhibition of thyroid hormone biosynthesis in the thyroid gland, and stimulated degradation of thyroid hormone by induction of hepatic drug-metabolizing enzymes. In the database there were 10 chemicals inducing hypertrophy/hyperplasia of follicular cells in the thyroid gland and having data on thyroid glands. On the basis of the chemical structure and information available in the literature, we judged three chemicals to be typical thioamide derivatives that act directly on the thyroid gland, and the others as non-thioamide derivatives that were unlikely to have any direct action on the thyroid gland. All these chemicals were classified into two groups using the ratios of relative weight increase rate of thyroid gland versus that of the liver. These values were at least 1.7, but 3.2 or more in the most of the cases for thioamide derivatives, and 1.2 or less for non-thioamide derivatives. This background analysis suggests the feasibility of parameter-supported speculation on the possible underlying mechanism when new repeated-dose toxicity data on hypothyroidism becomes available.

High-throughput sample preparation for the quantitation of acephate, methamidophos, omethoate, dimethoate, ethylenethiourea, and propylenethiourea in human urine using 96-well-plate automated extraction and high-performance liquid chromatography-tandem mass spectrometry

Acephate, methamidophos, o-methoate, and dimethoate are organophosphorus pesticides, and ethylenethiouria and propylenethiourea are two metabolites from the bisdithiocarbamate fungicide family. They are some of the most widely used pesticides and fungicides in agriculture both domestically and abroad. The existing high-performance liquid chromatography (HPLC)-tandem mass spectrometry (MS/MS) method for the measurement of these compounds in human urine was improved by using a 96-well plate format sample preparation; the use of HPLC-MS/MS was comparable with a concentration range of 0.125 to 50 ng/ml. Deuterium-labeled acephate, ethylenethiouria, and methamidophos were used as internal standards. The sample preparation procedure, in the 96-well format with a 0.8-ml urine sample size, uses lyophilization of samples, followed by extraction with dichloromethane. The analytes were chromatographed on a Zorbax SB-C3 (4.6 × 150 mm, 5.0-μm) column with gradient elution by using 0.1% formic acid in aqueous solution (solvent A) and 0.1% formic acid in methanol (solvent B) mobile phase at a flow rate of 1 ml/min. Quantitative analysis was performed by atmospheric pressure chemical ionization source in positive ion mode using multiple-reaction monitoring of the precursor-to-product ion pairs for the analytes on a TSQ Quantum Ultra HPLC-MS/MS. Repeated analyses of urine samples spiked with high (15 ng/ml), medium (5 ng/ml), and low (1 ng/ml) concentrations of the analytes gave relative SDs of <13%. The limits of detection were in the range of 0.004-0.01 ng/ml. The method also has high accuracy, high precision, and excellent extraction recovery. Furthermore, the improved sample preparation method decreased the cost and labor required while effectively doubling the analytic throughput with minimal matrix effect.

Predictive Modeling of a Mixture of Thyroid Hormone Disrupting Chemicals That Affect Production and Clearance of Thyroxine

Thyroid hormone (TH) disrupting compounds interfere with both thyroidal and extrathyroidal mechanisms to decrease circulating thyroxine (T4). This research tested the hypothesis that serum T4 concentrations of rodents exposed to a mixture of both TH synthesis inhibitors (pesticides) and stimulators of T4 clearance in the liver (polyhalogenated aromatic hydrocarbons, PHAHs) could be best predicted by an integrated addition model. Female Long-Evans rats, 23 days of age, were dosed with dilutions of a mixture of 18 PHAHs (2 dioxins, 4 dibenzofurans, and 12 PCBs, including dioxin-like and non-dioxin like PCBs) and a mixture of 3 pesticides (thiram, pronamide, and mancozeb) for four consecutive days. Serum was collected 24 hours after the last exposure and T4 concentrations were measured by radioimmunoassay. Animals exposed to the highest dose of the mixture experienced a 45% decrease in serum T4. Three additivity model predictions (dose addition, effect addition, and integrated addition) were generated based on single chemical data, and the results were compared. Effect addition overestimated the effect produced by the combination of all 21 chemicals. The results of the dose- and integrated-addition models were similar, and both provided better predictions than the effect-addition model. These results support the use of dose- and integrated additivity models in predicting the effects of complex mixtures.

Perchlorate and ethylenethiourea induce different histological and molecular alterations in a non-mammalian vertebrate model of thyroid goitrogenesis

Despite evidence for a conserved role of thyroid-stimulating hormone (TSH) in regulating vertebrate thyroid function, molecular data on thyroid responses to TSH are mainly limited to mammalian species. In this study, we examined histological and molecular changes in the thyroid of Xenopus laevis tadpoles during a 12-day treatment with 20mg/l perchlorate (PER) and 50mg/l ethylenethiourea (ETU). Inhibition of thyroid hormone (TH) synthesis by PER and ETU was evident from developmental retardation, reduced expression of TH-regulated genes and up-regulation of tshb-A mRNA. Thyroid histopathology revealed goiters with strikingly different follicular morphologies following PER and ETU treatment. Using real-time PCR, we analyzed thyroids sampled on day 12 for differential expression of 60 candidate genes. Further temporal analyses were performed for a subset of 14 genes. Relative to the control, PER and ETU treatment modulated the expression of 51 and 49 transcripts, respectively. Particularly genes related to TH synthesis and protein metabolism were similarly affected by PER and ETU. However, several genes were differentially expressed in PER- and ETU-treated tadpoles. Specifically, goiter formation in the PER treatment was associated with low expression of genes related to DNA replication but high expression of negative growth regulators. Results from this work provide for the first time a characterization of gene expression profiles during goitrogenesis in a non-mammalian vertebrate model. Overall, our data suggest that, in addition to TSH over-stimulation, further mechanisms related to the mode of goitrogen action contribute to the regulation of thyroid gene expression.

Potential developmental neurotoxicity of pesticides used in Europe

Pesticides used in agriculture are designed to protect crops against unwanted species, such as weeds, insects, and fungus. Many compounds target the nervous system of insect pests. Because of the similarity in brain biochemistry, such pesticides may also be neurotoxic to humans. Concerns have been raised that the developing brain may be particularly vulnerable to adverse effects of neurotoxic pesticides. Current requirements for safety testing do not include developmental neurotoxicity. We therefore undertook a systematic evaluation of published evidence on neurotoxicity of pesticides in current use, with specific emphasis on risks during early development. Epidemiologic studies show associations with neurodevelopmental deficits, but mainly deal with mixed exposures to pesticides. Laboratory experimental studies using model compounds suggest that many pesticides currently used in Europe--including organophosphates, carbamates, pyrethroids, ethylenebisdithiocarbamates, and chlorophenoxy herbicides--can cause neurodevelopmental toxicity. Adverse effects on brain development can be severe and irreversible. Prevention should therefore be a public health priority. The occurrence of residues in food and other types of human exposures should be prevented with regard to the pesticide groups that are known to be neurotoxic. For other substances, given their widespread use and the unique vulnerability of the developing brain, the general lack of data on developmental neurotoxicity calls for investment in targeted research. While awaiting more definite evidence, existing uncertainties should be considered in light of the need for precautionary action to protect brain development.

Method for determination of acephate, methamidophos, omethoate, dimethoate, ethylenethiourea and propylenethiourea in human urine using high-performance liquid chromatography-atmospheric pressure chemical ionization tandem mass spectrometry

Because of increasing concern about widespread use of insecticides and fungicides, we have developed a highly sensitive analytical method to quantify urine-specific urinary biomarkers of the organophosphorus pesticides acephate, methamidophos, omethoate, dimethoate, and two metabolites from the fungicides alkylenebis-(dithiocarbamate) family: ethylenethiourea and propylenethiourea. The general sample preparation included lyophilization of the urine samples followed by extraction with dichloromethane. The analytical separation was performed by high-performance liquid chromatography (HPLC), and detection by a triple quadrupole mass spectrometer with and atmospheric pressure chemical ionization source in positive ion mode using multiple reaction monitoring and tandem mass spectrometry (MS/MS) analysis. Two different Thermo-Finnigan (San Jose, CA, USA) triple quadrupole mass spectrometers, a TSQ 7,000 and a TSQ Quantum Ultra, were used in these analyses; results are presented comparing the method specifications of these two instruments. Isotopically labeled internal standards were used for three of the analytes. The use of labeled internal standards in combination with HPLC-MS/MS provided a high degree of selectivity and precision. Repeated analysis of urine samples spiked with high, medium and low concentration of the analytes gave relative standard deviations of less than 18%. For all compounds the extraction efficiency ranged between 52% and 63%, relative recoveries were about 100%, and the limits of detection were in the range of 0.001-0.282 ng/ml.

Dietary exposure of Brazilian consumers to dithiocarbamate pesticides—A probabilistic approach

A probabilistic estimation of the exposure of the Brazilian population to the dithiocarbamate pesticides was performed using the Monte Carlo Risk Assessment program (MCRA 3.5). Residue data, as CS2, for 3821 samples were obtained from the Brazilian national monitoring program on pesticide residues and from the monitoring program conducted in the Distrito Federal on rice, beans and nine fruits and vegetables. Food consumption data were obtained from a Brazilian household budget survey conducted between 2002 and 2003. Processing factors for washing, peeling or cooking were applied to the residues found in the crops. Daily intakes at the highest percentiles for the general population reached a maximum of 2.0 microg CS2/kg body weight per day (upper band of the 95% confidence interval at P99.99). Tomato, rice, apple and lettuce were the commodities which contributed most to the intake. Based on the registered uses and the toxicological profile of dithiocarbamates, the risk from exposure was evaluated assuming that all residues came from the use of ethylene-bis-dithiocarbamate (EBDC) or that a fraction of it came from the use of propineb. For this last scenario, a cumulative risk assessment was conducted. In the first scenario, the highest intake reached up to 11.9% EBDC ADI for the general population and up to 31.1% ADI for children. When 30% of the residues were considered as coming from propineb use, the values were 15.2% and 39.7% ADI, respectively.

Evaluation of Histological and Molecular Endpoints for Enhanced Detection of Thyroid System Disruption in Xenopus laevis Tadpoles

Amphibian metamorphosis represents a promising model for the identification of thyroid system-disrupting chemicals due to the pivotal role played by thyroid hormones for the initiation and regulation of metamorphosis. An important aspect of bioassay development is the identification and evaluation of sensitive and diagnostic endpoints. In this study, several morphological, histological, and molecular endpoints were evaluated for their utility to detect alterations in thyroid system function after exposure of stage 51 Xenopus laevis tadpoles to various concentrations (1.0, 2.5, 10, 25, and 50 mg/l) of the anti-thyroidal compound ethylenethiourea (ETU). Analysis of developmental stages on exposure day 20 and monitoring of time to fore limb emergence (FLE) revealed retardation and complete arrest of tadpole development at 25 mg/l and 50 mg/l ETU, respectively. Development was not affected by 1.0, 2.5, and 10 mg/l ETU. Histological alterations in the thyroid gland were observed in FLE-displaying tadpoles after exposure to 2.5, 10, and 25 mg/l ETU, as well as in developmentally arrested tadpoles exposed to 50 mg/l ETU. Prevalence and severity of histological changes increased in a concentration-dependent manner. Semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) showed increased mRNA expression of the alpha- and beta-subunits of thyroid-stimulating hormone (TSHalpha, TSHbeta) in pituitary tissue of tadpoles exposed to 25 and 50 mg/l ETU. Results demonstrate the successful detection of anti-thyroidal effects of ETU in Xenopus laevis tadpoles using various endpoints and highlight the particular sensitivity of thyroid gland histology to detect thyroid system disruption in tadpoles.

Bioinorganic chemistry in thyroid gland: effect of antithyroid drugs on peroxidase-catalyzed oxidation and iodination reactions

Propylthiouracil (PTU) and methimazole (MMI) are the most commonly used antithyroid drugs. The available data suggest that these drugs may block the thyroid hormone synthesis by inhibiting the thyroid peroxidase (TPO) or diverting oxidized iodides away from thyroglobulin. It is also known that PTU inhibits the selenocysteine-containing enzyme ID-1 by reacting with the selenenyl iodide intermediate (E-SeI). In view of the current interest in antithyroid drugs, we have recently carried out biomimetic studies to understand the mechanism by which the antithyroid drugs inhibit the thyroid hormone synthesis and found that the replacement of sulfur with selenium in MMI leads to an interesting compound that may reversibly block the thyroid hormone synthesis. Our recent results on the inhibition of lactoperoxidase (LPO)-catalyzed oxidation and iodination reactions by antithyroid drugs are described.

Anti-Thyroid Drugs and Thyroid Hormone Synthesis: Effect of Methimazole Derivatives on Peroxidase-Catalyzed Reactions

Syntheses and characterization of the selenium analogue (MSeI) of anti-thyroid drug methimazole and a series of organoselenium compounds bearing N-methylimidazole pharmacophore are described. In contrast to the sulfur compound that exists predominantly in its thione form, the selenium analogue exists in a selenol form, which spontaneously oxidizes in air to produce the corresponding diselenide. The reduction of the diselenide by GSH or NaBH(4) affords the biologically active selenol, which effectively inhibits the lactoperoxidase (LPO) activity in vitro. The monoselenides having N-methylimidazole moiety are found to be much less active than the selenol, suggesting that the presence of a selenol moiety is important for the LPO inhibition. The kinetic and mechanistic studies reveal that MSeI inhibits the LPO activity by reducing the H(2)O(2), providing a novel method to reversibly inhibit the enzyme. Although MSeI strongly inhibits LPO, the enzyme's activity can be completely recovered by increasing the H(2)O(2) concentration. On the other hand, the inhibition by methimazole (MMI), the sulfur analogue, cannot be reversed by increasing the H(2)O(2) concentration, leading to a complete inactivation of the enzyme. The reversible inhibition of LPO by some of the selenium derivatives is correlated with their glutathione peroxidase (GPx) activity, and the high GPx activity of the selenium compounds as compared with their sulfur analogues suggests that the selenium derivatives may protect the thyroid gland from oxidative damage.

Inactivation of thyroid peroxidase by soy isoflavones, in vitro and in vivo

Soy-containing foods and dietary supplements are widely consumed for putative health benefits (e.g. cancer chemoprevention, beneficial effects on serum lipids associated with cardiovascular health, reduction of osteoporosis, relief of menopausal symptoms). However, studies of soy isoflavones in experimental animals suggest possible adverse effects as well (e.g. enhancement of reproductive organ cancer, modulation of endocrine function, anti-thyroid effects). This paper reviews the evidence in humans and animals for anti-thyroid effects of soy and its principal isoflavones, genistein and daidzein.

Inhibition of thyroid peroxidase by dietary flavonoids

Flavonoids are widely distributed in plant-derived foods and possess a variety of biological activities including antithyroid effects in experimental animals and humans. A structure-activity study of 13 commonly consumed flavonoids was conducted to evaluate inhibition of thyroid peroxidase (TPO), the enzyme that catalyzes thyroid hormone biosynthesis. Most flavonoids tested were potent inhibitors of TPO, with IC50 values ranging from 0.6 to 41 microM. Inhibition by the more potent compounds, fisetin, kaempferol, naringenin, and quercetin, which contain a resorcinol moiety, was consistent with mechanism-based inactivation of TPO as previously observed for resorcinol and derivatives. Other flavonoids inhibited TPO by different mechanisms, such as myricetin and naringin, showed noncompetitive inhibition of tyrosine iodination with respect to iodine ion and linear mixed-type inhibition with respect to hydrogen peroxide. In contrast, biochanin A was found to be an alternate substrate for iodination. The major product, 6,8-diiodo-biochanin A, was characterized by electrospray mass spectrometry and 1H-NMR. These inhibitory mechanisms for flavonoids are consistent with the antithyroid effects observed in experimental animals and, further, predict differences in hazards for antithyroid effects in humans consuming dietary flavonoids. In vivo, suicide substrate inhibition, which could be reversed only by de novo protein synthesis, would be long-lasting. However, the effects of reversible binding inhibitors and alternate substrates would be temporary due to attenuation by metabolism and excretion. The central role of hormonal regulation in growth and proliferation of thyroid tissue suggests that chronic consumption of flavonoids, especially suicide substrates, could play a role in the etiology of thyroid cancer.

Porphyrin pi-cation and protein radicals in peroxidase catalysis and inhibition by anti-thyroid chemicals

1. Thyroid peroxidase (TPO) catalyses the iodination and phenolic coupling reactions in the biosynthesis of thyroid hormones. 2. The two-electron oxidation of TPO by H2O2 produces an oxoferryl porphyrin pi-cation radical compound I that isomerizes spontaneously to a form of compound I that contains an oxoferryl haem and the second oxidizing equivalent as an amino acid radical. 3. The pi-cation radical compound I is the catalytic species that effects iodide ion oxidation and the protein radical compound I is most likely the catalytic species that catalyses coupling. 4. Methimazole, a therapeutic, anti-hyperthyroid drug, is a suicide substrate for TPO and effects irreversible inactivation by TPO-mediated S-oxygenation to a reactive sulphenic acid that binds covalently to the prosthetic haem. 5. Sulphamethazine and other arylamines containing electron-withdrawing substituents inhibit TPO compound I-mediated reactions by reversible, mixed-type inhibition. 6. Ethylenethiourea, a fungicide metabolite, blocks TPO-mediated iodination by reacting with the catalytic iodinating species as an alternate substrate. 7. Resorcinol and related dietary flavonoids are suicide substrates for TPO and act by covalent binding to amino acid residues, presumably those radical sites present in the compound I isomer. 8. Nitrosobenzene, a known radical-trapping agent, blocks TPO-mediated coupling but not iodination or phenolic oxidations presumably by interception of the 3,5-diiodotyrosyl radical species generated during the coupling reaction.

Gas chromatography-mass spectrometry determination of ethylenethiourea hemoglobin adducts: a possible indicator of exposure to ethylene bis dithiocarbamate pesticides

Ethylenebisdithiocarbamates (EBDC) are an important class of fungicides used to control crop diseases and prevent mold. Ethylenethiourea (ETU), reported to be their main degradation and metabolic product in animals and man, may have teratogenic and carcinogenic properties. The feasibility of monitoring exposure to ETU on the basis of the formation of adducts to hemoglobin (Hb) was investigated. Rats given a single oral dose of ETU (from 62.5 to 500 mg/kg body wt) formed stable covalent ETU-Hb adducts. Mild acid hydrolysis of the protein regenerated ETU, allowing its detection by isotope dilution gas chromatography-mass spectrometry (GC-MS). The amount of released ETU increased with the dose. The dose-response curve fitted a linear model only between 62.5 mg/kg and 250 mg/kg. Acid-releasable ETU was also positively identified in the hemoglobin of workers exposed to Mancozeb, an EBDC formulation. In the exposed group, 40% had ETU-Hb adducts levels ranging from 0.5 to 1.42 pmol ETU/mg Hb. Such adducts might be useful for measuring EBDC exposure in humans.

Peroxidase-catalyzed S-oxygenation: mechanism of oxygen transfer for lactoperoxidase

The mechanism of organosulfur oxygenation by peroxidases [lactoperoxidase (LPX), chloroperoxidase, thyroid peroxidase, and horseradish peroxidase] and hydrogen peroxide was investigated by use of para-substituted thiobenzamides and thioanisoles. The rate constants for thiobenzamide oxygenation by LPX/H2O2 were found to correlate with calculated vertical ionization potentials, suggesting rate-limiting single-electron transfer between LPX compound I and the organosulfur substrate. The incorporation of oxygen from 18O-labeled hydrogen peroxide, water, and molecular oxygen into sulfoxides during peroxidase-catalyzed S-oxygenation reactions was determined by LC- and GC-MS. All peroxidases tested catalyzed essentially quantitative oxygen transfer from 18O-labeled hydrogen peroxide into thiobenzamide S-oxide, suggesting that oxygen rebound from the oxoferryl heme is tightly coupled with the initial electron transfer in the active site. Experiments using H2(18)O2, 18O2, and H2(18)O showed that LPX catalyzed approximately 85, 22, and 0% 18O-incorporation into thioanisole sulfoxide oxygen, respectively. These results are consistent with a active site controlled mechanism in which the protein radical form of LPX compound I is an intermediate in LPX-mediated sulfoxidation reactions.