Feed or food responsible for the presence of low-level thiouracil in urine of livestock and humans?

Improved Thyreostatic Drug Detection in Animal Tissues Using Liquid Chromatography-High-Field Asymmetric Waveform Ion Mobility Spectrometry-Mass Spectrometry

Thyreostatic drugs (thyreostats) interfere with thyroid function and have been used illegally in animals slaughtered for food. Thyreostat use leads to poorer quality meat, and the drug residues can cause adverse effects in humans. These drugs, with the exception of thiouracil, do not occur naturally and require sensitive methodologies for their detection in animal tissues. Because thyreostats are low-molecular-weight polar analytes, liquid chromatography-mass spectrometry (LC-MS) is typically used for detection and, in particular, triple quadrupole mass spectrometry with selective reaction monitoring (i.e., LC-SRM). However, LC-SRM thyreostat methods suffer from chemical background noise and endogenous interferences arising from the complex tissue matrix. An improved high-field asymmetric waveform ion mobility spectrometry interface (FAIMS Pro), which separates ions based on differential ion mobility, was combined with LC-SRM to minimize these interferences. Using the same samples and conditions, LC-FAIMS-SRM showed improvements in the signal-to-noise ratio (S/N) of up to 50 times compared with our validated LC-SRM method. In addition, wider linear ranges, including substantial improvements in the lower limit of quantification (approximately an order of magnitude for tapazole and methylthiouracil), were observed with LC-FAIMS-SRM.

Discrimination between the exogenous and endogenous origin of thiouracil in farm animals, the final chapter?

Thiouracil (2-thiouracil) is a thyreostatic compound that can be used as an illegal growth promoter. In bovine, porcine and other farm animals, low concentrations of thiouracil are detected in urine. There is much debate on which concentrations can be considered to originate from feed ('natural') and which concentrations are caused by the illegal administration of thiouracil for growth-promoting purposes. Currently, a threshold value of 10 µg/L in urine is applied. The threshold value is based on epidemiological data. Data on thiouracil from animals treated with thiouracil is scarce. We conducted a study whereby animals were fed with rapeseed, rapeseed with thiouracil, or regular feed with thiouracil (low and high concentration). It was determined that administration of thiouracil leads to concentrations higher than the current 10 µg/L threshold of thiouracil and its metabolites in urine during treatment. Animals fed with rapeseed showed higher thiouracil concentrations than the control group, mostly above 10 µg/L and in some cases above 30 µg/L. In the discovery study, several biomarkers for thiouracil treatment were tentatively identified and confirmed with reference standards. One metabolite was identified as indicative for thiouracil abuse, namely 6-methyl-thiouracil. Another metabolite, 4-thiouracil, was indicative for endogenous formation and did not increase during 2-thiouracil treatment. 6-Methyl-thiouracil was not found in urine samples from the Dutch routine control programmes that contained (endogenous) 2-thiouracil above the threshold value. However, 4-thiouracil was found at high concentrations in the same samples when 2-thiouracil was present. This study's overall conclusion is that the threshold value for thiouracil in bovine urine samples should be set at 10 µg/L and for porcine urine samples at 30 µg/L. Also, confirmation of 6-methyl-thiouracil and 4-thiouracil should be used as indicators for exogenous or endogenous origin in routine control monitoring programmes.

Determination of Thyreostats in Urine Using Supported Liquid Extraction and Mixed-Mode Cation-Exchange Solid-Phase Extraction: Screening and Confirmatory Methods

The application of thyreostats in livestock has been banned in the European Union since 1981, but these drugs are currently in the focus due to the natural occurrence of thiouracil (TU). Studies have been published on TU contamination in urine samples of animal and human origins without any drug administration of it. This paper presents new analytical methods to analyze thyreostats to support the legislation on the recommended concentration (RC) levels of these drugs. Both screening and confirmatory methods are developed for analyzing thyreostats in porcine and bovine urines using a liquid chromatography-tandem mass spectrometry technique. The new methods include a chemical derivatization with 3-iodobenzyl bromide, followed by novel purification approaches using supported liquid extraction and mixed-mode cation-exchange solid-phase extraction (SPE) for screening and confirmatory purposes, respectively. The optimized derivatization in combination with the cation-exchange SPE gives high sensitivity and reducing matrix effect of the analysis. The methods are validated in accordance with the guidelines for the validation of screening methods and European Commission Decision 2002/657/EC. The confirmatory method is used in the national monitoring plan. The detected levels of TU in urine samples are below the currently applicable RC level (10 μg L-1).

Subcritical water extraction of thyreostats from bovine muscle followed by liquid chromatography-tandem mass spectrometry

Thyreostats can be used fraudulently to promote a rapid increase in weight of breeding animals at low cost. Their severe toxicological effects impose the development of reliable analytical methods to be used in monitoring plans. This work describes an alternative approach to isolate residues of thiouracil, methyl-thiouracil, propyl-thiouracil, phenyl-thiouracil, tapazole and mercaptobenzimidazole from bovine muscle tissue. The developed procedure is based on the following three steps: i) matrix solid-phase dispersion with C₁₈ for the preliminary sample preparation; ii) subcritical water extraction (SWE) at 160°C and 100 bar; iii) clean-up on an Oasis HLB cartridge. The quantitative determination was performed by LC-MS/MS in dual polarity ionization by using internal standardization. The SWE-LC-MS/MS method was validated according to the identification criteria of the Commission decision 2002/657/EC. The relative recoveries ranged from 72 to 97%; within-lab reproducibility was less than 18%. The decision limit and the detection capability of all analytes were below the recommended concentration, set at 10 µg kg^-1, but the validation results demonstrated that this method could only be applied for screening of thiouracil and methyl-thiouracil. Besides the analytical advantages related to the use of water as solvent extraction, the procedure allowed significant removal of lipids, whose detrimental effects on instrumentation and MS sensitivity are well-known.

Discovery of urinary biomarkers to discriminate between exogenous and semi-endogenous thiouracil in cattle: A parallel-like randomized design

In the European Union, the use of thyreostats for animal fattening purposes has been banned and monitoring plans have been established to detect potential abuse. However, this is not always straightforward as thyreostats such as thiouracil may also have a semi-endogenous origin. Therefore, this study aimed at defining urinary metabolites, which may aid in defining the origin of detected thiouracil. Hereto, a parallel-like randomized in vivo study was conducted in which calves (n = 8) and cows (n = 8) were subjected to either a control treatment, rapeseed-enriched diet to induce semi-endogenous formation, or thiouracil treatment. Urine samples (n = 330) were assessed through metabolic fingerprinting, employing liquid-chromatography and Q-Exactive^TM Orbitrap mass spectrometry. Urinary fingerprints comprised up to 40,000 features whereby multivariate discriminant analysis was able to point out significant metabolome differences between treatments (Q²(Y) ≥ 0.873). Using the validated models, a total of twelve metabolites (including thiouracil) were assigned marker potential. Combining these markers into age-dependent biomarker panels rendered a tool by which sample classification could be improved in comparison with thiouracil-based thresholds, and this during on-going thiouracil treatment (specificities ≥ 95.2% and sensitivities ≥ 85.7%), post-treatment (sensitivities ≥ 80% for ≥ 24 h after last administration), and simulated low-dose thiouracil treatment (exogenous thiouracil below 30 ng μL^-1). Moreover, the metabolic relevance of revealed markers was supported by the suggested identities, for which a structural link with thiouracil could be determined in most cases. The proposed biomarker panels may contribute to a more justified decision-making in monitoring thiouracil abuse.

Discrimination between Synthetically Administered and Endogenous Thiouracil Based on Monitoring of Urine, Muscle, and Thyroid Tissue: An in Vivo Study in Young and Adult Bovines

Thiouracil (TU), synthesized for its thyroid-regulating capacities and alternatively misused in livestock for its weight-gaining effects, is acknowledged to have an endogenous origin. Discrimination between low-level abuse and endogenous occurrence is challenging and unexplored in an experimental setting. Therefore, cows (n = 16) and calves (n = 18) were subjected to a rapeseed-supplemented diet or treated with synthetic TU. Significant higher urinary TU levels were recorded after TU administration (<CC_α, 15 642 μg L^-1) compared to rapeseed supplementation (<CC_α, 65.8 μg L^-1), however, with overlapping values. TU was not detected in the edible meat; however, concentrations between the CC_α and 10 μg kg^-1 were noted in thyroid tissue of calves and cows following rapeseed supplementation. The latter concentrations were significantly higher in thyroid tissue of calves (22.9-41.8 μg kg^-1) and cows (16.9-36.7 μg kg^-1) after synthetic TU administration. These results strongly point toward thyroid analysis as a discriminatory tool.

Toward a new European threshold to discriminate illegally administered from naturally occurring thiouracil in livestock

Thiouracil is a thyrostat inhibiting the thyroid function, resulting in fraudulent weight gain if applied in the fattening of livestock. The latter abuse is strictly forbidden and monitored in the European Union. Recently, endogenous sources of thiouracil were identified after frequently monitoring low-level thiouracil positive urine samples and a "recommend concentration" (RC) of 10 μg/L was suggested by the EURL to facilitate decision-making. However, the systematic occurrence of urine samples exceeding the RC led to demands for international surveys defining an epidemiologic threshold. Therefore, six European member states (France, Poland, The Netherlands, United Kingdom, Norway, and Belgium) have shared their official thiouracil data (2010-2012) collected from bovines, porcines, and small livestock with 95 and 99% percentiles of 8.1 and 18.2 μg/L for bovines (n = 3894); 7.4 and 13.5 μg/L for porcines (n = 654); and 7.4 μg/L (95% only) for small livestock (n = 85), respectively. Bovine percentiles decreased with the animal age (nonadults had significantly higher levels for bovines), and higher levels were observed in male bovines compared to female bovines.

Thiouracil-Forming Bacteria Identified and Characterized upon Porcine In Vitro Digestion of Brassicaceae Feed

In recent years, the frequent detection of the banned thyreostat thiouracil (TU) in livestock urine has been related to endogenous TU formation following digestion of glucosinolate-rich Brassicaceae crops. Recently, it was demonstrated that, upon in vitro digestion of Brassicaceae, fecal bacteria induce TU detection in livestock (porcine livestock > bovines). Therefore, the present study was intended to isolate and identify bacteria involved in this intestinal TU formation upon Brassicaceae digestion and to gain more insight into the underlying mechanism in porcine livestock. Twenty porcine fecal inocula (gilts and multiparous sows) were assessed through static in vitro colonic-digestion simulations with rapeseed. After derivatization and extraction of the fecal suspensions, TU was analyzed using liquid chromatography-tandem mass spectrometry (LC-MS(2)). On average, lower TU concentrations were observed in fecal colonic simulations in gilts (8.35 ng g(-1) rapeseed ± 3.42 [mean ± standard deviation]) than in multiparous sows (52.63 ng g(-1) ± 16.17), which correlates with maturation of the gut microbial population with age. Further exploration of the mechanism showed cell-dependent activity of the microbial conversion and sustained TU-forming activity after subjection of the fecal inoculum to moderate heat over a time span of up to 30 min. Finally, nine TU-producing bacterial species were successfully isolated and identified by a combination of biochemical and molecular techniques as Escherichia coli (n = 5), Lactobacillus reuteri (n = 2), Enterococcus faecium (n = 1), and Salmonella enterica subsp. arizonae (n = 1). This report demonstrates that endogenous formation of TU is Brassicaceae induced and occurs under colonic conditions most likely through myrosinase-like enzyme activity expressed by different common intestinal bacterial species.

Intestinal microbiota contribute to the endogenous formation of thiouracil in livestock

In recent years, the frequent detection of the banned substance thiouracil (TU) in livestock urine has been related to its endogenous formation following consumption of glucosinolate-rich Brassicaceae crops. Besides, TU residues have been recovered in these crops upon plant-derived myrosinase hydrolysis. Through in vitro bovine and porcine static digestive simulations, the influence of gastrointestinal digestion of Brassicaceae-derived matrixes on TU formation was assessed. Following derivatization and LC-MS(2) analysis, TU was detected in colonic suspensions with traditional rapeseed, coarse colza "00" meal, cauliflower, and broccoli ranging from 3.47 to 30.96 μg kg(-1) (bovine) and from 3.55 to 26.34 μg kg(-1) (porcine). In stomach and small intestinal fluids, TU remained unfound, whereas upon rumen simulation TU was detected for coarse colza "00" meal (4.43 μg kg(-1)) and grounded traditional rapeseed (0.35 μg kg(-1)). The origin of this detection was investigated through filter-sterilizing and autoclaving the fecal inoculum causing a significant decrease in TU concentration, thereby reinforcing the possibility of an active bacterial involvement, which however was characterized with a high interanimal variation. In conclusion, these results support the previously proven endogenous origin of TU and acknowledge the active role of the gastrointestinal bacteria in TU formation, through production of an extracellular component.