Mutagenicity and toxicity of aflatoxin precursors

Retrospective and Prospective Look at Aflatoxin Research and Development from a Practical Standpoint

Among the array of structurally and toxicologically diverse mycotoxins, aflatoxins have attracted the most interest of scientific research due to their high toxicity and incidence in foods and feeds. Despite the undeniable progress made in various aspects related to aflatoxins, the ultimate goal consisting of reducing the associated public health risks worldwide is far from being reached due to multiplicity of social, political, economic, geographic, climatic, and development factors. However, a reasonable degree of health protection is attained in industrialized countries owing to their scientific, administrative, and financial capacities allowing them to use high-tech agricultural management systems. Less fortunate situations exist in equatorial and sub-equatorial developing countries mainly practicing traditional agriculture managed by smallholders for subsistence, and where the climate is suitable for mould growth and aflatoxin production. This situation worsens due to climatic change producing conditions increasingly suitable for aflatoxigenic mould growth and toxin production. Accordingly, it is difficult to harmonize the regulatory standards of aflatoxins worldwide, which prevents agri-foods of developing countries from accessing the markets of industrialized countries. To tackle the multi-faceted aflatoxin problem, actions should be taken collectively by the international community involving scientific research, technological and social development, environment protection, awareness promotion, etc. International cooperation should foster technology transfer and exchange of pertinent technical information. This review presents the main historical discoveries leading to our present knowledge on aflatoxins and the challenges that should be addressed presently and in the future at various levels to ensure higher health protection for everybody. In short, it aims to elucidate where we come from and where we should go in terms of aflatoxin research/development.

Mycotoxins in corn and wheat silage in Israel

Silage is an important feed source for intensive dairy herds worldwide. Fungal growth and mycotoxin production before and during silage storage is a well-known phenomenon, resulting in reduced nutritional value and a possible risk factor for animal health. With this in mind, a survey was conducted to determine for the first time the occurrence of mycotoxins in corn and wheat silage in Israel. A total of 30 corn and wheat silage samples were collected from many sources and analysed using a multi-mycotoxin method based on LC-MS/MS. Most mycotoxins recorded in the present study have not been reported before in Israel. Overall, 23 mycotoxins were found in corn silage; while wheat silage showed a similar pattern of mycotoxin occurrence comprising 20 mycotoxins. The most common post-harvest mycotoxins produced by the Penicillium roqueforti complex were not found in any tested samples, indicative of high-quality preparation and use of silage. Moreover, none of the European Union-regulated mycotoxins--aflatoxin B1, ochratoxin, T-2 toxin, diacetoxyscirpenol and deoxynivalenol--were found above their limits of detection (LODs). The Alternaria mycotoxins--macrosporin, tentoxin and alternariol methyl ether--were highly prevalent in both corn and wheat silage (>80%), but at low concentrations. The most prominent (>80%) Fusarium mycotoxins in corn silage were fusaric acid, fumonisins, beauvericin, monilifomin, equisetin, zearalenone and enniatins, whereas in wheat silage only beauvericin, zearalenone and enniatins occurred in more than 80% of the samples. The high prevalence and concentration of fusaric acid (mean = 765 µg kg⁻¹) in Israeli corn silage indicates that this may be the toxin of highest potential concern to dairy cow performance. However, more data from different harvest years and seasons are needed in order to establish a more precise evaluation of the mycotoxin burden in Israeli silage.

Selective aflatoxin B1-induced sister chromatid exchanges and cytotoxicity in differentiating B and T lymphocytes in vivo

The purpose of this study was to assess the genotoxic and cytotoxic effects of the fungal metabolite aflatoxin B1 (AfB1) on the developing immune system of the chick embryo, a model in vivo system. Of particular interest was the assessment of AfB1-mediated selective toxicity toward developing B lymphocytes as compared to T lymphocytes. In vivo bromodeoxyuridine (BrdU) labelling of DNA was used to detect the induction of sister chromatid exchanges (SCE) in lymphocytes and to assess the progression of these cells through successive cell cycles. Cytotoxicity was also assessed by studying the entrance and maintenance of cells in mitosis (mitotic index). Graded doses of AfB1 (1.09-17.4 micrograms/g embryo) were applied to chick embryos at 18 days of incubation (DI). Embryos also received two doses of BrdU at 3 mg/200 microliters (3 hr apart) to provide continuous labelling of B and T lymphocyte replicating DNA. B and T lymphocytes were harvested 20 hr post-AfB1/BrdU exposure from the bursa and thymus, respectively, and were processed for cytogenetic analyses. AfB1 induced dose-related increases in SCE in B lymphocytes; this induction was 6- to 8-fold that of controls at the higher doses tested. AfB1-mediated induction of SCE in T cells was just 2-fold that of controls at the highest dose tested. AfB1 reduced the progression of B cells and to a lesser extent T cells through successive rounds of replication. Furthermore, AfB1 dramatically reduced the mitotic index of B cells but not of T cells. These data indicate both selective genotoxicity and cytotoxicity of AfB1 toward B cells in the late stage embryo.

Quantum-chemical studies of aflatoxin B1, sterigmatocystin and versicolorin A, and a comparison with their mutagenic activity

The INDO atomic charges q and Wiberg bond indices p (in electrons) were calculated for aflatoxin B1, sterigmatocystin and versicolorin A. The C-2-C-3 bond in these compounds has the same bond order and is predicted to be the most reactive towards epoxidation. The electronic effects do not explain the observed differences in mutagenicity and toxicity.

Embryonic exposure to aflatoxin-B1: mutagenicity and influence on development and immunity

Chick embryos were mutagenized in ovo in order to study developmentally related alterations in immune functions in survivors of this prenatal toxicant insult. In this experimental system, a single exposure of 6-day chick embryos to 0.1 microgram aflatoxin-B1 (AF-B1) in 10 microliters of acetone was employed, and the control embryos received 10 microliters of solvent alone. This dosage of AF-B1 administered to 6-day embryos was found to increase the incidence of sister chromatid exchanges in blood cells approximately fivefold above the baseline observed in solvent controls. A second sham control, where no solvent was administered, was included in some experiments. The cell cycle times in blood increased slightly during the initial exposure to AF-B1. However, a majority of the AF-B1 and acetone exposed embryos survived and hatched without incident. Losses occurred mainly in the latter part of embryogenesis. After hatching, no significant differences were observed in body weight between different treatment groups up to 26 weeks of age and no change in primary humoral immunity was detected. In contrast, two parameters of cell-mediated immunity, graft vs host (GvH), and cutaneous basophil hypersensitivity (CBH) reactions were both depressed as a result of exposure to AF-B1. The AF-B1 treatment group was significantly reduced in the GvH reaction compared with sham-treated controls. In the CBH assay, AF-B1-exposed chicks showed reduced immunity compared with acetone controls. These results suggest that long-term selective immune depression can occur following embryonic exposure to AF-B1.