Date Published: December 20, 2018
Publisher: Springer Berlin Heidelberg
Author(s): Ke Bi, Wen Zhang, Zhizhuang Xiao, Dawei Zhang.
A gene named zenc, encoding a zearalenone lactonase from Neurospora crassa, was over-expressed in Pichia pastoris. The zenc gene is 888-bp in length, encoding a 295-residue polypeptide. Purified ZENC has maximal activity at pH 8.0 and 45 °C, and is highly stable at pH 6.0–8.0 for 1 h at 37 °C. The activity of the secreted enzyme in shaken-flask fermentation was 40.0 U/ml. A high-density fermentation of the ZENC-producing recombinant strain was performed in a 30-l fermenter and the maximal enzyme activity reached 290.6 U/ml. The Km, Vmax and specific activity toward zearalenone are 38.63 μM, 23.8 μM/s/mg and 530.4 U/mg, respectively. ZENC can resist metal ions and inhibitors to some extent. We applied the enzyme into three different kinds of animal feed. On addition of ZENC (800 U) to distillers dried grains with solubles (DDGS), maize by-products and corn bran (25 g), the concentration of zearalenone was reduced by 70.9%, 88.9% and 94.7% respectively. All these properties of ZENC are promising for applications in the animal feed and food industries.
Zearalenone (ZEN) is a nonsteroidal estrogenic mycotoxin which was first isolated by Stob et al. in 1962 and its structure was determined by Urry (Perry et al. 1970; Urry et al. 1966). It was initially used as a growth promoter in animal feed (Wilson et al. 1972), but now it is considered to have adverse effects, mainly on the reproduction of animals (Kleinova et al. 2002). This mycotoxin is a secondary metabolite of Fusarium species including F. graminearum, F. culmorum, F. equiseti, F. semitectum and F. crookwellense, particularly F. graminearum. All these species are soil fungi and common in temperate regions (Bennett and Klich 2003). ZEN contamination mainly occurs in maize and its co-products, but it has also been detected in some other cereals, including barley, wheat and broomcorn (Hussein and Brasel 2001). Cereals are contaminated during the cultivation period due to the Fusarium in soil. Contamination can also be caused by improper storage of cereals.
Methods of degrading ZEN in cereal have attracted increasing attention in the past two decades (Higa et al. 2003; McCormick 2013). Currently, most biodegradation methods of ZEN applied in practice involve adding a microbial strain that can metabolize ZEN to a nontoxic compound (Fu et al. 2016). Cell-free biodegradation systems have seldom been used to deal with ZEN contamination. Extensive work is required to screen for strains having a particular function, and genetically modified organisms are strictly regulated. All these factors increase the practical cost, so enzymatic degradation of ZEN is considered as a promising method to solve problems of ZEN contamination.