Evaluation of toxicities of 7-hydroxyisotrichodermin and 8-hydroxyisotrichodermin, shunt intermediates in the biosynthetic grid of deoxynivalenol, by using a sensitive yeast assay

Abstract

Fusarium graminearum causes a disease of wheat and barley known as Fusarium head blight. It contaminates the grains with trichothecene mycotoxins such as deoxynivalenol (DON). As shunt intermediates in the DON biosynthetic pathway, 7-hydroxyisotrichodermin (7-HIT) and 8-hydroxyisotrichodermin (8-HIT) are known. However, their activities have not been previously evaluated. In this study, we performed toxicity assays of these trichothecenes by using a sensitive yeast bioassay that we have recently established. The IC50 of 7-HIT and 8-HIT were in the range of 20-40 μg/ml, while the IC50 of DON was approximately 1.5 μg/ml. Although the toxicity of these shunt metabolites remains to be investigated in animal systems, our present data indicate that 7-HIT and 8-HIT may not be major issues that require regulation in agricultural products. Fusarium head blight (FHB) is a serious disease of wheat and barley caused mainly by Fusarium graminearum and Fusarium culmorum. The infected grains are often contaminated with trichothecenes such as deoxynivalenol (DON; Fig. 1), nivalenol (NIV), and acetylated derivatives thereof1),2),3). These mycotoxins, which are characterized by the presence of a ketogroup at C-8, are collectively termed type B trichothecenes. The toxicities of these mycotoxins are significantly affected by the side-chain oxygenation and acetylation patterns4),5). With regard to DON, a provisional limit (1.1 ppm in wheat in Japan) has been set in many countries to ensure the safety of cereal grains and processed products. DON biosynthesis starts with cyclization of alltrans-farnesyl pyrophosphate to give trichodiene, followed by four oxygenation steps and a second nonenzymatic cyclization yielding isotrichodermol. As major intermediates of the DON pathway, isotrichodermol, isotrichodermin, 15-deacetylcalonectrin, calonectrin, and 3,15-diacetyldeoxynivalenol are known6),7). The toxicities of these major intermediates were previously evaluated using either animal or plant systems8). However, the toxicities of shunt intermediates of DON, such as 7-hydroxyisotrichodermin (7-HIT) and 8hydroxyisotrichodermin (8-HIT) (see Fig. 1) have not been examined so far. Previously, we developed a sensitive yeast bioassay for trichothecenes by utilizing a gene deletion mutant of three resistance genes against trichothecenes: pdr5, erg6, and rpb4 in Saccharomyces cerevisiae BY47429). Fig. 1 Chemical structures of deoxynivalenol (DON), 7hydroxyisotrichodermin (7-HIT), and 8-hydroxyisotrichodermin (8-HIT).