High-performance liquid chromatography of the mycotoxin sterigmatocystin and its application to the analysis of mouldy rice for sterigmatocystin

Development of a polyclonal antibody-based indirect competitive ELISA for determination of sterigmatocystin in wheat and corn flours

Sterigmatocystin (STC) is a toxic secondary metabolite produced by more than 50 fungal species, including Aspergillus flavus, A. parasiticus, A. nidulans, and A. versicolor. The Joint FAO/WHO Expert Committee on Food Additives concluded that sterigmatocystin is genotoxic and carcinogenic with the critical effect determined to be carcinogenicity. The present study describes a simple method to prepare hapten and immunogens in order to generate polyclonal antibodies against this metabolite. We developed a sensitive and specific polyclonal antibody-based competitive indirect enzyme-linked immunosorbent assay (ciELISA) for monitoring STC in wheat and corn flours without the need for derivatisation of STC or clean-up of samples by immunoaffinity chromatography for quantification. The half inhibitory concentration (IC₅₀) of the established method was 4.52 ± 0.81 ng mL^-1, with the limit of detection (IC₁₀) being 0.19 ± 0.04 ng mL^-1 in wheat and corn flour matrices with the coefficient of variation of less than 22%.The assay was very specific to STC and showed no cross-reactivity with its analogue structures. The ELISA allowed for up to 5% methanol without significant influence on the IC₅₀ value. Validation of the assay was performed by spiking STC into a blank flour matrix and the recoveries were in the range of 75.3 % to 104.5 % with a coefficient of variation less than 15%. A small retail survey was conducted by purchasing wheat (n = 8) and corn flours (n = 2) from local grocery stores. All of these retail samples were negative for STC using the developed ELISA method and were confirmed by LC-MS/MS. We demonstrated a rapid, simple, and reliable method for screening STC in wheat and corn flours.

Determination of sterigmatocystin in grain using gas chromatography-mass spectrometry with an on-column injector

Sterigmatocystin (STC) is a mycotoxin produced by several species of Aspergillus and other fungi. STC is a precursor of the carcinogen aflatoxin B1 and exerts carcinogenic, teratogenic, and mutagenic effects. A reliable GC-MS-based analytical method using on-column injection was developed and validated to determine STC content in grain. In this method, STC was extracted with acetonitrile (84%), and the filtered extract was diluted with phosphate buffer before immunoaffinity column (IAC) cleanup. After elution and N2 evaporation, the sample containing STC was dissolved in acetone and injected into the GC-MS system without derivatization. Separation of STC was carried out by a capillary column (0.25 mm i.d.×30 m, 0.25 μm) with a deactivated pre-column (0.53 mm i.d.×0.6 m). The matrix effect was investigated in maize, wheat, and rice, and an insignificant matrix effect was observed after IAC cleanup. The calibration curve was linear in the range of 20-300 pg (equivalent to 8-120 μg/kg in grain) with a coefficient of determination (R2) of 0.999, and the relative standard deviation of repeatability (RSDr) was less than 10 %. The limit of detection (LOD) of the method was 6 pg (equivalent to 2.4 μg/kg in grain), and the limit of quantification (LOQ) was 20 pg (equivalent to 8 μg/kg). The method described in this report is sensitive, reliable, and can be used to monitor STC contamination in grain.

Determination of sterigmatocystin in beer by high performance liquid chromatography with ultraviolet detection

A new method for the determination of sterigmatocystin in beer was developed. This method consists of a solid phase extraction procedure of light and dark beer and reversed phase high performance liquid chromatography with ultraviolet detection. Recoveries of sterigmatocystin from beer samples spiked at levels from 0.5 to 100 µg/l ranged from 81 to 126% for light beer and from 88 to 126% for dark beer with coefficients of variation between 2.2 and 14%. From the total of 26 analysed Latvian beer samples only two (7.7%) were contaminated with sterigmatocystin.

Sterigmatocystin presence in typical Latvian grains

Ninety five samples of different Latvian grains (wheat, buckwheat, barley, oats and rye) from the year 2006 and 120 samples from the year 2007 were analyzed for Aspergillus ssp. mycotoxin-sterigmatocystin (STC) content. 13.7% of the analyzed 2006 year samples were positive for STC with the concentration levels ranging from <0.7 to 83μg/kg and 35% of the analyzed 2007 year samples were positive for STC with the concentration levels ranging from <1 to 47μg/kg. A previously developed sensitive LC - Electrospray Positive Ionization - MS/MS method was applied for the analysis of STC in grains. Method includes sample extraction with acetonitrile/water solution, solid phase extraction (SPE) on Strata X SPE column, separation on reversed phase C18 column and STC detection by LC-MS/MS.

Analytical method for the determination of sterigmatocystin in grains using high-performance liquid chromatography-tandem mass spectrometry with electrospray positive ionization

A sensitive LC-electrospray positive ionization (ESI+) MS/MS method was developed and validated for the determination of the Aspergillus mycotoxin, sterigmatocystin, in grains at levels down to less than 0.5 microg/kg. Recovery was found to be greater than 97% for wheat grains.

Immunochemical assay applied to mycotoxin biosynthesis: ELISA comparison of sterigmatocystin production by Aspergillus versicolor and Aspergillus nidulans

Conventional thin layer and instrumental methods for analyzing mycotoxins and their precursors are time-consuming and make the investigation of mycotoxin biosynthesis particularly difficult. As an alternative, sensitive enzyme-liked immunosorbent assays (ELISAs) can be utilized to analyze for these compounds. In this report, sterigmatocystin production in test tube cultures of Aspergillus versicolor ATCC 18643 and Aspergillus nidulans ATCC 32610 were compared using competitive ELISA. Polyclonal antiserum that was prepared against a sterigmatocystin hemiacetal-bovine serum albumin conjugate exhibited greatest specificity for sterigmatocystin hemiacetal and sterigmatocystin with less reactivity for O-methylsterigmatocystin. The antiserum could be used to detect as little as 50 ng/ml sterigmatocystin in ELISA. Direct ELISA could be performed on diluted culture broth and on mycelial extracts solubilized with N,N-dimethylformamide. Aspergillus versicolor ATCC 18643 produced more sterigmatocystin in SLS medium than in YES medium, and showed maximal levels at between 9 to 12 days incubation. Approximately 75% of sterigmatocystin was detectable in mycelium (254 micrograms/ml culture) compared to the extracellular fraction (87 micrograms/ml culture). Aspergillus nidulans exhibited qualitatively similar patterns of growth and toxigenesis in SLS medium but accumulated maximal levels of only 15 micrograms mycelial sterigmatocystin/ml culture and 5 micrograms extracellular sterigmatocystin/ml broth, respectively.

Chromatographic and spectroscopic properties of hemiacetals of aflatoxin and sterigmatocystin metabolites

Improved fluorescence detection of aflatoxin B1 by chromatographic analysis is accomplished by conversion to the corresponding hemiacetal, aflatoxin B2a. Because the metabolites aflatoxin M1, aflatoxin P1, aflatoxin Q1, sterigmatocystin, and O-methylsterigmatocystin have the same molecular conversion site, we investigated the chromatographic and spectroscopic properties of hemiacetals of these compounds to assist in confirming aflatoxins and sterigmatocystins in human urine. Nuclear magnetic resonance and infrared absorbance were used to confirm the hemiacetal structure for aflatoxin B1 and sterigmatocystin. The ultraviolet absorbance, fluorescence, and chromatographic properties of the metabolites were investigated. Using these data, we optimized the detection and solvent conditions for high-performance liquid chromatography. We determined that, of the conditions studied, maximum sensitivity and resolution for the native aflatoxins were achieved with a mobile phase of methanol, tetrahydrofuran, and water, a C8 column in series with a C18 column, and fluorescence detection with 365 nm excitation and 430 nm emission wavelengths for aflatoxins B1 and M1 and with 500 nm emission wavelength for aflatoxins P1 and Q1. For the analysis of the hemiacetals, a mobile phase of methanol, acetonitrile, and water provided better chromatography and fluorescence detection. Sterigmatocystin and O-methylsterigmatocystin were readily converted to the hemiacetal forms, which, like the aflatoxins, were more polar and, therefore, earlier eluting by reversed-phase HPLC (methanol, acetonitrile, and water, 236 nm absorbance). These data are important to maximize the sensitivity and confidence for detecting the mycotoxin metabolites in biological specimens.