Structure–activity relationships for the fluorescence of ochratoxin A: Insight for detection of ochratoxin A metabolites

Comprehensive Insights into Ochratoxin A: Occurrence, Analysis, and Control Strategies

Ochratoxin A (OTA) is a toxic mycotoxin produced by some mold species from genera Penicillium and Aspergillus. OTA has been detected in cereals, cereal-derived products, dried fruits, wine, grape juice, beer, tea, coffee, cocoa, nuts, spices, licorice, processed meat, cheese, and other foods. OTA can induce a wide range of health effects attributable to its toxicological properties, including teratogenicity, immunotoxicity, carcinogenicity, genotoxicity, neurotoxicity, and hepatotoxicity. OTA is not only toxic to humans but also harmful to livestock like cows, goats, and poultry. This is why the European Union and various countries regulate the maximum permitted levels of OTA in foods. This review intends to summarize all the main aspects concerning OTA, starting from the chemical structure and fungi that produce it, its presence in food, its toxicity, and methods of analysis, as well as control strategies, including both fungal development and methods of inactivation of the molecule. Finally, the review provides some ideas for future approaches aimed at reducing the OTA levels in foods.

Cryo-EM structure and rational engineering of a superefficient ochratoxin A-detoxifying amidohydrolase

Ochratoxin A (OTA) is among the most prevalent mycotoxins detected in agroproducts, posing serious threats to human and livestock health. Using enzymes to conduct OTA detoxification is an appealing potential strategy. The recently identified amidohydrolase from Stenotrophomonas acidaminiphila, termed ADH3, is the most efficient OTA-detoxifying enzyme reported thus far and can hydrolyze OTA to nontoxic ochratoxin α (OTα) and L-β-phenylalanine (Phe). To elucidate the catalytic mechanism of ADH3, we solved the single-particle cryo-electron microscopy (cryo-EM) structures of apo-form, Phe- and OTA-bound ADH3 to an overall resolution of 2.5-2.7 Å. The role of OTA-binding residues was investigated by structural, mutagenesis and biochemical analyses. We also rationally engineered ADH3 and obtained variant S88E, whose catalytic activity was elevated by 3.7-fold. Structural analysis of variant S88E indicates that the E88 side chain provides additional hydrogen bond interactions to the OTα moiety. Furthermore, the OTA-hydrolytic activity of variant S88E expressed in Pichia pastoris is comparable to that of Escherichia coli-expressed enzyme, revealing the feasibility of employing the industrial yeast strain to produce ADH3 and its variants for further applications. These results unveil a wealth of information about the catalytic mechanism of ADH3-mediated OTA degradation and provide a blueprint for rational engineering of high-efficiency OTA-detoxifying machineries.

Investigation of ochratoxin A biomarkers in biological materials obtained from patients suffering from renal cell carcinoma

Ochratoxin A (OTA) exposure can result in chronic renal diseases and cancer. The incidence of kidney, renal pelvis, and ureter malignant neoplasms in the Czech Republic is approximately 29.5 renal tumours per 100,000 inhabitants. The question arises whether mycotoxins are also involved in kidney disease and cancer. A sensitive validated analytical methodology, based on an immunoaffinity clean-up followed by HPLC with fluorescence detection, was developed to explore whether OTA accumulates in clear renal cell carcinoma-adenocarcinoma in Czech patients. Simultaneously, DNA-adducts and OTA metabolites were qualitatively analysed in tissues and urine. OTA was analysed in 33 kidney and tumour samples from 26 men and 7 women collected during nephrectomy from patients of the East Bohemian region from 2015 to 2017. OTA was found in 76% of the analysed samples. Its concentrations ranged from not detectable to 390 ng/kg with a median of 167 ng/kg in kidney samples and from not detectable to 430 ng/kg with a median of 122 ng/kg in tumour samples. Urinary OTA metabolites and DNA adducts were qualitatively analysed for the corresponding 20 patients. The presence of some OTA metabolites such as ochratoxin A hydroquinone and/or decarboxylated ochratoxin A hydroquinone correlate with the presence of OTA-DNA adducts.

13C-Satellite Decoupling Strategies for Improving Accuracy in Quantitative Nuclear Magnetic Resonance

Quantitative ¹H nuclear magnetic resonance (qHNMR) with an appropriate internal standard is a well-established quantitation method for assigning purity to organic molecules. For accurate measurements, the premise of qHNMR relies on the careful selection of integrals, for both the analyte and the standard, in such a way that the selected integrals are free from interferences. The ¹³C-satellite signals of adjacent integrals, low-level impurities, and tautomer signals are among the common integral interferences that are typically encountered. One of the simplest ways to identify and avoid these interferences is to decouple the ¹³C-satellites. Two decoupling schemes were explored to illustrate the benefits of ¹³C-decoupling for qHNMR or qH{¹³C}NMR: GARP and bilevel adiabatic broadband decoupling. Unwanted sample heating and nuclear Overhauser effect (NOE) enhancements are the two main drawbacks of decoupling schemes. We show that with careful optimization of acquisition parameters and decoupling power, no excessive sample heating occurred during acquisition at 400 MHz. At 900 MHz, only bilevel adiabatic decoupling could be safely implemented. Furthermore, any undesirable NOE enhancements were completely avoided if acquisition was executed with an inverse-gated pulse sequence. We explored and confirmed the benefits of qH{¹³C}NMR through the quantitation of a diverse set of compounds, namely, small molecules (dimethyl terephthalate and zearalenone), a ¹³C-labeled compound (¹³C₆-ochratoxin A), and an octapeptide (angiotensin II). Statistical comparisons confirmed that qH{¹³C}NMR produced comparable data to qHNMR. However, with qH{¹³C}NMR data providing added clarity about the presence of overlapping ¹³C-satellites, impurities, and tautomers, it has an edge over qHNMR for accurate measurements.

Evaluation of Two Fully Automated Setups for Mycotoxin Analysis Based on Online Extraction-Liquid Chromatography-Tandem Mass Spectrometry

Mycotoxins are secondary metabolites of fungi species widely known for their potentially toxic effects on human health. Considering their frequent presence in crops and their processed food, monitoring them on food-based matrices is now an important topic. Within such a context, the sample preparation step is usually mandatory before the chromatographic analysis, due to the complexity of matrices such as nuts, cereals, beverages, and others. For these reasons, we herein present the evaluation of two greener setups, based on the automation and miniaturization of the sample preparation step for mycotoxin analysis in different beverages. Firstly, we describe an analytical method based on a multidimensional assembly, coupling a lab-made microextraction column (508 µm i.d. × 100 mm) to a UPLC–MS/MS for the analysis of ochratoxin A in beverages. This configuration used a synthesized sorbent phase containing C18-functionalized graphene–silica particles, which exhibited excellent extraction performance, as well as being reusable and cheaper than commercially available extractive phases. Sequentially, a second setup, based on a multidimensional capillary LC coupled to MS/MS, was assessed for the same purpose. In this case, a graphene oxide-based capillary extraction column (254 µm i.d. × 200 mm) was used as the first dimension, while a C18 analytical capillary column performed the mycotoxin separation in beverages. Although this second one has similarities with the first, we focused mainly on the benefits related to the link between a miniaturized/automated sample preparation device with a capillary LC–MS/MS system, which made our analysis greener. Additionally, the chromatographic efficiency could even be enhanced.

The registration of aptamer-ligand (ochratoxin A) interactions based on ligand fluorescence changes

The fluorescent properties of ligands can change when they bind to specific receptors. Modulated by the transition of the ligand from the free to the bound state, fluorescence makes it possible both to detect this ligand and quantitatively register its binding. We characterized the interaction of ochratoxin A (OTA) with the specific G-quadruplex aptamer through excitation-emission matrix fluorescence spectroscopy. It was shown that the formation of the complex changes the OTA fluorescence spectrum both in the region of the main peak at λ_ex/λ_em 380/430 nm and in the region of peak at λ_ex/λ_em 265/425 nm. At pH 8.5 and OTA concentration of 30 nM, this peak is smaller in intensity than the main peak of fluorescence. The formation of the complex with the aptamer leads to an increase of the fluorescence at λ_ex/λ_em 265/425 nm up to 6.5 times, which makes it up to 4.9 times more intense than fluorescence at 380/430 nm. Fluorescence of the G-quadruplex aptamer (donor) takes part in increasing of the OTA (acceptor) emission at λ_ex/λ_em 265/425 nm due to the resonance energy transfer. The concentration regularities of the modulated fluorescence of OTA at λ_ex/λ_em 265/425 nm have been studied. Their correspondence to the calculations of complexation conducted on the basis of the dissociation constant is shown.

Determination of Ochratoxin A traces in foodstuffs: Comparison of an automated on-line two-dimensional high-performance liquid chromatography and off-line immunoaffinity-high-performance liquid chromatography system

Automated on-line two-dimensional high-performance liquid chromatography method (2D-HPLC) is proposed to determine Ochratoxin A (OTA) in food samples as an alternative to OTA immunoaffinity column (IAC). An on-line 2D-HPLC system is designed for the analysis of OTA using an affinity-based monolithic column in the first dimension and reversed-phase C18 column in the second dimension. Initially, optimal OTA separation efficiency is determined through traditional HPLC system consisting of a P(HEMAPA) monolithic column coupled with HPLC system. Secondly, after providing optimum conditions, OTA determination was investigated through the 2D-HPLC system. According to results, 2D-HPLC system showed good linearity in the range 0.5 to 20 ng/mL with limit of detection (LOD) and limit of quantification (LOQ) values of 21.2 pg/mL and 64.3 pg/mL, respectively. The P(HEMAPA)-4 monolithic column displayed good recovery of OTA ranging from 104.34% to 107.33%. Relative standard deviations (RSD) varied in the range 0.21% to 1.31% thus indicating the efficiency of P(HEMAPA)-4 monolithic column developed for OTA.

DNA Aptamer-Target Binding Motif Revealed Using a Fluorescent Guanine Probe: Implications for Food Toxin Detection

DNA aptamers are single-stranded oligonucleotides that are generated by an in vitro selection method to bind targets with high affinity and specificity. Understanding molecular recognition by DNA aptamers is of fundamental importance in the development of biosensor applications. The small molecule ochratoxin A (OTA) is a fungal-derived food toxin, and OTA DNA aptamers have been established for the development of rapid detection platforms required for food safety. One such OTA aptamer (OTAA) is a guanine-rich DNA oligonucleotide that folds into an antiparallel G-quadruplex (GQ) upon OTA binding, although structural details of the GQ fold and its interaction with OTA are currently unknown. In the present study, the fluorescent nucleobase analogue, 8-thienyl-2'-deoxyguanosine (ThdG), was inserted into various G sites of OTAA to determine the probe impact on GQ folding and OTA binding affinity. Our results suggest that OTAA contains three lateral (l) loops connecting two stacked G-tetrads with an anticlockwise loop progression to afford a -(lll) GQ topology. The phenolic ring system of OTA undergoes π-stacking interactions with the G-tetrads of OTAA. Our results also demonstrate aptamer sites that can be modified with ThdG to afford a fluorescent light-up signal upon OTA binding.

Fluorescent natural products as probes and tracers in biology

Fluorescence is a remarkable property of many natural products in addition to their medicinal and biological value. Herein, we provide a review of these peculiar secondary metabolites to stimulate prospecting of them as original fluorescent tracers, endowed with unique photophysical properties and with applications in most fields of biology.

Heat Stability of Ochratoxin A in an Aqueous Buffered Model System

Ochratoxin A (OTA) represents one of the most widespread mycotoxins in agricultural commodities in the world and is considered a possible human carcinogen with its potent nephrotoxicity. OTA is stable under most food processing conditions; however, higher-temperature treatment may reduce OTA content in foods. Since OTA can be found in processed products destined for both human and animal consumption, factors affecting its stability or reduction during thermal processes were investigated here. The reduction of OTA was measured during various heating times (up to 60 min) at different temperatures (100, 125, 150, 175, and 200°C) in aqueous buffer solutions at different pHs (pH 4, 7, and 10). Quantification of OTA was carried out using high-performance liquid chromatography with fluorescence detection. The results showed that the rate and extent of OTA reduction were dependent on pH, processing time, and temperature; greater than 90% OTA reduction was achieved at 200°C for all treatments except pH 4. After processing under an alkaline condition (pH 10) at 100°C for 60 min, about 50% of the OTA was lost, while after 60 min under neutral and acidic conditions at 100°C, significant reductions of OTA were not shown.

Ochratoxin A: 50 Years of Research

Since ochratoxin A (OTA) was discovered, it has been ubiquitous as a natural contaminant of moldy food and feed. The multiple toxic effects of OTA are a real threat for human beings and animal health. For example, OTA can cause porcine nephropathy but can also damage poultries. Humans exposed to OTA can develop (notably by inhalation in the development of acute renal failure within 24 h) a range of chronic disorders such as upper urothelial carcinoma. OTA plays the main role in the pathogenesis of some renal diseases including Balkan endemic nephropathy, kidney tumors occurring in certain endemic regions of the Balkan Peninsula, and chronic interstitial nephropathy occurring in Northern African countries and likely in other parts of the world. OTA leads to DNA adduct formation, which is known for its genotoxicity and carcinogenicity. The present article discusses how renal carcinogenicity and nephrotoxicity cause both oxidative stress and direct genotoxicity. Careful analyses of the data show that OTA carcinogenic effects are due to combined direct and indirect mechanisms (e.g., genotoxicity, oxidative stress, epigenetic factors). Altogether this provides strong evidence that OTA carcinogenicity can also occur in humans.

In vitro glucuronidation of ochratoxin a by rat liver microsomes

Ochratoxin A (OTA), one of the most toxic mycotoxins, can contaminate a wide range of food and feedstuff. To date, the data on its conjugates via glucuronidation request clarification and consolidation. In the present study, the combined approaches of ultra high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS), UHPLC-Orbitrap-high resolution mass spectrometry (HRMS) and liquid chromatography-multiple stage mass spectrometry (LC-MS(n)) were utilized to investigate the metabolic profile of OTA in rat liver microsomes. Three conjugated products of OTA corresponding to amino-, phenol- and acyl-glucuronides were identified, and the related structures were confirmed by hydrolysis with β-glucuronidase. Moreover, OTA methyl ester, OTα and OTα-glucuronide were also found in the reaction solution. Based on these results, an in vitro metabolic pathway of OTA has been proposed for the first time.

Impact of pH on the stability and the cross-reactivity of ochratoxin A and citrinin

Mycotoxins are secondary metabolites produced by several fungi contaminating crops. In several countries, the maximum permitted levels of mycotoxins are found in foodstuffs and feedstuffs. The common strategy of mycotoxin analysis involves extraction, clean-up and quantification by chromatography. In this paper, we analyzed the reasons of underestimation of ochratoxin A (OTA) content in wine, and overestimation of OTA in wheat, depending on the pH of the clean-up step and the simultaneous presence of citrinin (CIT). We demonstrated that the increase of pH by adding polyethylene glycol (PEG) to wine led to an underestimation of OTA by conversion of OTA into open ring ochratoxin A OP-OA. In comparing three methods of extraction and clean-up for the determination of OTA and CIT in wheat--(i) an inter-laboratory validated method for OTA in cereals using immunoaffinity column clean-up (IAC) and extraction by acetonitrile/water; (ii) a validated method using IAC and extraction with 1% bicarbonate Na; and (iii) an in-house validated method based on acid liquid/liquid extraction--we observed an overestimation of OTA after immunoaffinity clean-up when CIT is also present in the sample, whereas an underestimation was observed when OTA was alone. Under neutral and alkaline conditions, CIT was partially recognized by OTA antibodies.

Structure-activity relationships imply different mechanisms of action for ochratoxin A-mediated cytotoxicity and genotoxicity

Ochratoxin A (OTA) is a fungal toxin that is classified as a possible human carcinogen based on sufficient evidence for carcinogenicity in animal studies. The toxin is known to promote oxidative DNA damage through production of reactive oxygen species (ROS). The toxin also generates covalent DNA adducts, and it has been difficult to separate the biological effects caused by DNA adduction from that of ROS generation. In the current study, we have derived structure-activity relationships (SAR) for the role of the C5 substituent of OTA (C5-X = Cl) by first comparing the ability of OTA, OTBr (C5-X = Br), OTB (C5-X = H), and OTHQ (C5-X = OH) to photochemically react with GSH and 2'-deoxyguanosine (dG). OTA, OTBr, and OTHQ react covalently with GSH and dG following photoirradiation, while the nonchlorinated OTB does not react photochemically with GSH and dG. These findings correlate with their ability to generate covalent DNA adducts (direct genotoxicity) in human bronchial epithelial cells (WI26) and human kidney (HK2) cells, as evidenced by the (32)P-postlabeling technique. OTB lacks direct genotoxicity, while OTA, OTBr, and OTHQ act as direct genotoxins. In contrast, their cytotoxicity in opossum kidney epithelial cells (OK) and WI26 cells did not show a correlation with photoreactivity. In OK and WI26 cells, OTA, OTBr, and OTB are cytotoxic, while the hydroquinone OTHQ failed to exhibit cytotoxicity. Overall, our data show that the C5-Cl atom of OTA is critical for direct genotoxicity but plays a lesser role in OTA-mediated cytotoxicity. These SARs suggest different mechanisms of action (MOA) for OTA genotoxicity and cytotoxicity and are consistent with recent findings showing OTA mutagenicity to stem from direct genotoxicity, while cytotoxicity is derived from oxidative DNA damage.

Ochratoxin A: in utero exposure in mice induces adducts in testicular DNA

Ochratoxin A (OTA) is a nephrotoxin and carcinogen that is associated with Balkan endemic nephropathy and urinary tract tumors. OTA crosses the placenta and causes adducts in the liver and kidney DNA of newborns. Because the testis and kidney develop from the same embryonic tissue, we reasoned that OTA also may cause adducts transplacentally in the testis. We tested the hypothesis that acute exposure to OTA, via food and via exposure in utero , causes adducts in testicular DNA and that these lesions are identical to those that can be produced in the kidney and testis by the consumption of OTA. Adult mice received a single dose of OTA (from 0–1,056 µg/kg) by gavage. Pregnant mice received a single i.p. injection of OTA (2.5 mg/kg) at gestation day 17. DNA adducts were determined by 32P-postlabeling. Gavage-fed animals sacrificed after 48 hours accumulated OTA in kidney and testis and showed DNA adducts in kidney and testis. Some OTA metabolites isolated from the tissues were similar in both organs (kidney and testis). The litters of mice exposed prenatally to OTA showed no signs of overt toxicity. However, newborn and 1-month old males had DNA adducts in kidney and testis that were chromatographically similar to DNA adducts observed in the kidney and testis of gavage-fed adults. One adduct was identified previously as C8-dG-OTA adduct by LC MS/MS. No adducts were observed in males from dams not exposed to OTA. Our findings that in utero exposure to OTA causes adducts in the testicular DNA of male offspring support a possible role for OTA in testicular cancer.

Structures of covalent adducts between DNA and ochratoxin a: a new factor in debate about genotoxicity and human risk assessment

The potent renal carcinogenicity of ochratoxin A (OTA) in rats, principally in the male, raises questions about mechanism. Chromatographic evidence of DNA adducts after (32)P-postlabeling analysis contrasts with experimental attempts to demonstrate the absence of OTA in such adducts. Proffered schemes for alternative epigenetic mechanisms in OTA carcinogenicity remain unsatisfying, while structural data substantiating DNA-OTA adducts has also been lacking. We report refined (32)P-postlabeling methodology revealing one principal adduct isolated in small amounts from the kidneys of all five Fischer and five Dark Agouti rats to which OTA had been given on four consecutive days. We also describe structural data for the principal adduct from OTA/DNA interaction in vitro and its subsequent preparative isolation by the postlabeling methodology (as C-C8 OTA 3'dGMP), essentially creating an ochratoxin B-guanine adduct. Reasoning for the unsuitability of experimental protocols in published evidence claiming nongenotoxicity of OTA is given. In vivo exposure of renal DNA to cycles of adduction with OTA, necessarily protracted for carcinogenesis to occur, can reasonably explain an occasional focal neoplasm from which metastasizing carcinoma could develop.