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A comprehensive understanding of deoxynivalenol detoxification through epimerization by Devosia mutans 17-2-E-8

Zhou, T. 2017. A comprehensive understanding of deoxynivalenol detoxification through epimerization by Devosia mutans 17-2-E-8. 39th Mycotoxin Workshop, June, 19th-21st 2017, Bydgoszcz, Poland

Abstract

Microbial detoxification provides a new strategy for the mitigation of mycotoxin deoxynivalenol (DON). A bacterium originally isolated from an agriculture soil sample, was capable of reducing DON to the undetectable level in medium containing high levels of the mycotoxin aerobically. The new bacterial species has been designated as Devosia mutans 17-2-E-8. In the incubation medium, 3-epi-DON (major) and 3-keto-DON (minor) were detected, resulting in a hypothesis that DON is epimerized via a two-step process, i.e. DON was oxidized to 3-keto-DON followed by reduction to 3-epi-DON. The hypothesis was confirmed in several ways. 1). Comparison of the biotransformation rate achieved by intact bacteria with the theoretical ratio of DON and 3-epi-DON in a thermodynamic equilibrium showed that the transformation is coupled to yet unknown exothermic reaction. This excluded the involvement of a deprotonating epimerase with a reactive sp2 intermediate such as epimerases of carbohydrates. 2). the activity responsible for the reduction of 3-keto-DON to 3-epi-DON was separated from the oxidation of DON to 3-keto-DON by ammonium sulfate fractionation of cell-free extracts. 3). Comparison of activities of seven Devosia species towards DON and 3-keto-DON showed that all species were able to reduce 3-keto-DON into 3-epi-DON but only a single species, D. mutans, was also able to oxidize DON into 3-keto-DON, accomplishing the complete epimerization. 4). Most significantly, the enzymes responsible for the epimerization system have been identified and showed great effectiveness when tested in vitro. The DON epimerization has been confirmed to be a detoxification reaction. Cytotoxicities of the products were demonstrated by two assays: a MTT bioassay assessing cell viability and a BrdU assay assessing DNA synthesis. Compared with DON, the IC50 values of 3-epi-DON and 3-keto-DON were respectively 357 and 3.03 times higher in the MTT bioassay, and were respectively 1181 and 4.54 times higher in the BrdU bioassay. Toxicological effects of 14-day oral exposure of the B6C3F1 mouse to DON and 3-epi-DON were also investigated, proving that 3-epi-DON is at least 50 fold less toxic than DON. Different molecular and chemistry models also support the toxicity reduction. Further studies are underway to optimize the enzymes for more sustainable applications in mitigating Fusarium toxin contaminations. Considering the involvement of DON in the pathogenesis of different Fusarium species, it looks very plausible to develop plants that are resistant/tolerant Fusarium diseases by utilizing the epimerization process of DON. The enzymes would also have the capacity to minimize the possible passage of DON contamination to food/feed value chains when applied in post-harvest treatments such as the wet-milling of grains or semi-liquid feeding of animals.

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