Analytische Chemie
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Aflatoxins, a group of structurally related mycotoxins, are well known for their
toxic and carcinogenic effects in humans and animals. Aflatoxin derivatives and protein
conjugates are needed for diverse analytical applications. This work describes a reliable
and fast synthesis of novel aflatoxin derivatives, purification by preparative HPLC and
characterisation by ESI-MS and one- and two-dimensional NMR. Novel aflatoxin bovine
serum albumin conjugates were prepared and characterised by UV absorption and
MALDI-MS. These aflatoxin protein conjugates are potentially interesting as immunogens
for the generation of aflatoxin selective antibodies with novel specificities.
Mycotoxins can be found worldwide in foods and feed and cause a variety of mold-related health risks which makes it necessary to further examine their toxic effects and metabolic fate in human and other mammals. Beside standard in vitro and in vivo assays with liver cell preparations or rodents an increasing interest in new simulation methods are playing a growing role. Electrochemistry (EC) is one of these novel techniques and has been used successfully and efficiently in pharmacological and drug research for several years now.
The primary objective of this study was to determine the capability of EC as a supportive and versatile instrument to elucidate metabolic pathways of mycotoxins.
On the example of the food relevant mycotoxin Citrinin a coulometric flow through cell equipped with a carbon working electrode was used to oxidize Citrinin by applying potential between 0.7 and 2.5 V vs. Pd/H2. The electrochemically generated oxidation products were then analyzed by mass-spectrometric detection coupled online to EC (EC-MS) and compared with data from a standard in vitro model with human and rat liver microsomes preparations. To receive a comprehensive assessment of oxidative techniques chemical oxidation by Fenton´s reaction was performed as well.
The obtained LC-MS/MS data confirmed the production of Dihydrocitrinone by all of the three tested oxidation systems and demonstrates the potential of EC-MS for the successful prediction of the main phase I metabolic reactions of mycotoxins, since Dihydrocitrinone is the mainly formed metabolite by humans after intake of Citrinin.
Beside the identified Dihydrocitrinone from electrochemical, enzymatic and chemical oxidation of Citrinin there is still a number of yet unknown compounds. As the next step structural characterization of the generated oxidation products by NMR and X-ray analysis will be enabled by their large-scale production using preparative EC cells.
Mycotoxins are secondary metabolites produced by fungi contaminating food and feed worldwide. Intake of these foodborne toxins can cause several diseases in humans and animals highlighting the need to understand metabolic pathways of mycotoxins. Methods of choice have been in vitro and in vivo approaches, so far. Beside hundreds of documented mycotoxins numerous new ones need to be elucidated and enhance the demand for fast and reliable methods. Here, we present electrochemistry coupled to mass spectrometry (EC/MS set up Fig. 1) as novel and promising tool in mycotoxin research. Electrochemical oxidation of mycotoxins like zearalenone, citrinin or dihydroergocristine lead to several oxidation products known from phase I biotransformation as well as new interesting reaction products analyzed by EC/MS, LC MS/MS and ESI-HRMS. To ensure a comparative overview results obtained from electrochemical oxidation experiments were compared to Fenton reaction, UV irradiation and microsomal experiments. The presentation will point out the benefits and drawbacks of EC/MS in mycotoxin research on the basis of selected food relevant mycotoxins.
The priority ergot alkaloids ergometrine and ergometrinine are highly toxic mycotoxins naturally occurring in different types of grains (i.e. rye, wheat, rice), as well as grain-based foods and, therefore, have gained increasing importance for food safety over the last years. The application of HPLC-MS/MS for the analysis of ergot alkaloids in food presupposes the availability of isotopically labelled internal standards. Thus, a multistep synthesis was developed for ergometrine-(N-13CD3) and its epimer ergometrinine-(N-13CD3) with a mass shift of four units compared with the parent compounds. The synthesis is based on the preparation of stable isotope labelled lysergic acid that was coupled with (S)-alaninol. The chemical synthesis of both compounds has been achieved in six steps with an overall yield of 1 % (ergometrine-(N-13CD3)) and 0.6 % (ergometrinine-(N-13CD3)), respectively. Structural identification was performed by MS analysis as well as 1H and 13C NMR.
Mycotoxins cause a variety of mold-related health risks which makes it necessary to further examine their metabolic pathways in human and other mammals. Beside standard in vitro assays with liver cell microsomes an increasing interest in new and rapid simulation techniques are playing a growing role in mycotoxin research.
Herein, the coupling of electrochemistry with liquid chromatography and mass spectrometry (EC/LC/MS) is presented as fast and simple method to investigate the oxidative fate of mycotoxins. For this case study, two food relevant mycotoxins (zearalenone and citrinin) were selected. Experiments were performed by using an electrochemical flow through cell integrated in the flow path of the autosampler of the chromatographic system. The reaction mixture was separated by a RP-C18 column and analyzed by a single quadrupole MS (Figure 1). Oxidation products were generated by applying potentials of 400, 800, 1200 and 1600 mV vs Pd/H2 using a glassy carbon working electrode. Different oxidation reactions like hydroxylation, dehydrogenation and dimerization lead to a diverse product pattern of the investigated mycotoxins.
In a comparative study, electrochemical generated reaction products were compared with metabolites produced by human and rat liver microsomes in vitro. The obtained data show that EC/LC/MS is a versatile and promising tool in mycotoxin research to support metabolic investigations of known and unknown mycotoxins.
Zearalenone (ZEN) and its metabolites such as the phase II sulfate and glucoside conjugates have been detected in food and feed commodities1. In addition to these naturally occurring ZEN conjugates, food fermentation by fungi may also lead to conjugate formation. Fungal strains of the genera Rhizopus and Aspergillus are used in tempeh fermentation, and it is known that some strains can conjugate ZEN2. Moreover, during this process the reductive phase I metabolites α- and β-zearalenol (α-/β-ZEL) and their conjugates are formed by the same microorganisms in vitro. As the conjugates can be hydrolysed by human intestinal microbiota, also sulfates and glucosides represent a relevant source for the human exposure to ZEN. If the unprocessed grains for tempeh fermentation like soybeans and/or maize are contaminated with ZEN, upon analysis ZEN content of the final tempeh product may be underestimated, because ZEN has been conjugated during food processing. Very recently, the EU-CONTAM Panel found it appropriate to set a group tolerable daily intake (TDI) expressed as ZEN equivalents for ZEN and its modified forms (phase I and phase II metabolites)3.
In this study a total of 70 strains of fungal species commonly found in tempeh fermentation were screened for ZEN metabolite formation in vitro. The fungal strains were isolated from inocula or fresh tempeh from different tempeh producers and market areas in Indonesia. ZEN (c = 0.3 µM) was added to liquid cultures of the fungi. After two days of incubation the media were analyzed for metabolite formation by HPLC-MS/MS. The conjugate formation varied with the fungal strain. In most cultures ZEN-14-sulfate was the major metabolite accompanied by rapid disappearance of ZEN; also, formation of α- and β-ZEL, ZEN-16-glucoside and two ZEL-sulfates has been observed.
The in vitro-screening showed that fungal strains used in tempeh fermentation have the potential for ZEN conversion leading to ZEN and ZEL conjugate formation. Therefore, reference substances of ZEN conjugates are urgently needed for monitoring food safety of tempeh and other fermented products manufactured from grains that can be contaminated with ZEN.
Development of certified reference material and analytical method for zearalenone in edible oil
(2017)
Quality and safety of food products require their reliable analysis. Contaminants, in particular mycotoxins, are key-components for food safety. About 25 % of the world's food crops are contaminated with mycotoxins posing a severe health risk to humans. In order to strengthen food safety and consumer protection the European Commission (EC) set maximum levels for priority mycotoxins in certain foods for human consumption. In 2013, the EC and CEN (European Committee for Standardization) started an initiative to standardize analytical methods for mycotoxins in food which gained increasing relevance, e.g. zearalenone (ZEN). ZEN, an estrogenic mycotoxin produced by several Fusarium species, contaminates cereal crops worldwide. Due to its lipophilic nature ZEN is often found in edible oils (particularly in maize germ oils) derived from contaminated plants. Therefore, an European maximum level of 400 µg/kg is currently in force. To perform reliable food analysis a sustainable metrological infrastructure is of major importance enabling the quantification of priority mycotoxins (here: ZEN). To achieve this goal, an integrated approach is needed targeted at the development of validated analytical methods and certified reference materials (CRM).
A highly selective method for ZEN in edible oils will be presented, based on solid phase extraction (SPE) using hydrazine-functionalized particles. This method was developed for manual application using commercial SPE cartridges as well as for automated SPE-HPLC online coupling. While ZEN is covalently coupled to the solid phase by means of a hydrazone bond, undesired matrix components can be removed very efficiently. Finally, ZEN is decoupled from the solid phase, leading to highly purified extracts which are measured by HPLC-FLD.
The development of the first European Reference Material (ERM®) for ZEN in maize germ oil (ERM®-BC715) will be presented and discussed. This ERM®-project underpins the urgent need for mycotoxin-CRMs to support food safety and public health.
Zearalenone (ZEN) and its phase II sulfate and glucoside metabolites have been detected in food and feed commodities. After consumption, the conjugates can be hydrolyzed by the human intestinal microbiota leading to liberation of ZEN that implies an underestimation of the true ZEN exposure. To include ZEN conjugates in routine analysis, reliable standards are needed, which are currently not available. Thus, the aim of the present study was to develop a facilitated biosynthesis of ZEN-14-sulfate, ZEN-14-glucoside and ZEN-16-glucoside. A metabolite screening was conducted by adding ZEN to liquid fungi cultures of known ZEN conjugating Aspergillus and Rhizopus strains. Cultivation conditions and ZEN incubation time were varied. All media samples were analyzed for metabolite formation by HPLC-MS/MS. In addition, a consecutive biosynthesis was developed by using Fusarium graminearum for ZEN biosynthesis with subsequent conjugation of the toxin by utilizing Aspergillus and Rhizopus species. ZEN-14-sulfate (yield: 49%) is exclusively formed by Aspergillus oryzae. ZEN-14-glucoside (yield: 67%) and ZEN-16-glucoside (yield: 39%) are formed by Rhizopus oryzae and Rhizopus oligosporus, respectively. Purities of ≥73% ZEN-14-sulfate, ≥82% ZEN-14-glucoside and ≥50% ZEN-16-glucoside were obtained by 1H-NMR. In total, under optimized cultivation conditions, fungi can be easily utilized for a targeted and regioselective synthesis of ZEN conjugates.
A new dimer of the food-relevant mycotoxin zearalenone was isolated after electrochemical and chemical oxidation. The structure was determined as a 16-O-15'-biaryl ether-linked dimer based on spectroscopic analyses (1H- and 13C-NMR, COSY, HMBC, and HSQCAD) and high-Resolution mass spectrometry analysis (Q-TOF).
Rapid, cost-efficient, and eco-friendly methods are desired today for routine analysis of the Fusarium mycotoxin zearalenone (ZEN) in edible vegetable oils. Liquid chromatography with fluorescence detection (HPLC-FLD) is commonly used to reliably control the specified ZEN maximum levels, which requires efficient sample clean-up to avoid matrix interferences. Therefore, a highly selective extraction and clean-up method based on reversible covalent hydrazine chemistry (RCHC) using hydrazine-functionalized silica was developed. This efficient solid-phase extraction (SPE) involves reversible hydrazone formation of ZEN with the hydrazine moiety covalently bound to a solid phase. Optimal conditions were achieved with 1 mL SPE cartridges filled with 400 mg of hydrazine-functionalized silica. The developed RCHC-SPE method was validated in an interlaboratory comparison study (ILC) with twelve participants analyzing six edible vegetable oils with a focus on maize oils. The derived method parameters (ZEN recovery 83%, repeatability 7.0%, and reproducibility 18%) meet the performance criteria of Commission Regulation (EC) No 401/2006. The developed RCHC-SPE-based HPLC-FLD method allows the reliable quantification of ZEN in the range of 47–494 μg/kg for different types of edible vegetable oils, also for matrix-reach native oils. Due to the high efficiency, the significantly reduced matrix load helps to extend the lifetime of analytical equipment. Furthermore, the re-useability of the RCHC-SPE cartridges contributes to an eco-friendly approach and reduced analysis costs. To our knowledge, this is the first report on ZEN quantification in edible vegetable oils based on manual RCHC-SPE cartridges. Due to its high performance, the developed RCHC-SPE method is a promising alternative to the current European standard method EN 16924:2017 (HPLC-FLD part).