Masked Mycotoxins of Deoxynivalenol and Zearalenone – Unpredicted Toxicity

Masked Mycotoxins of Deoxynivalenol and Zearalenone – Unpredicted Mycotoxins are secondary metabolites produced by fungal species. Detoxification processes of plants which are infected with the toxigenic fungi are responsible for the conversion of them to mycotoxin derivatives, called masked mycotoxins. Masked mycotoxins are less toxic metabolites for plants according to their free forms. Both of these metabolites have toxic effects on animals and humans. Masked mycotoxins are hydrolyzed into their free forms by human and animal intestinal microbiomes. Here, we reviewed the unforeseen toxicity of masked trichothecenes, especially focused on deoxynivalenol (DON) and zearalenone (ZEN), which are classified as major mycotoxins of Fusarium species.


Introduction
Since ancient times the importance of food in human health has been known and their relationship was highlighted by Hippocrates in the 5th Century BC with the statement "Let thy food be thy medicine and medicine be thy food" [1]. Every year approximately a million people experience a foodborne illness.

Masked forms of DON and ZEN, and their Toxicity
Masked mycotoxins are generated as a result of the transformation of the main type of mycotoxins by living plants in order to protect themselves from xenobiotic compounds [19].
They were detected in early 1990 and termed as masked due to the inability to be screened in food and feed through conventional analytical methods [20]. The most frequently detected masked forms of Fusarium toxins are deoxynivalenol-3-glucoside (DON-3G) and zearalenone-14-D-glucopyranoside (ZEN-14G). The isolation of DON-3G was first reported from naturally contaminated maize and wheat [21]. Then, the DON-3G also has been found in barley, oat and cereal-based food and feeds [22][23][24][25]. The toxicity of DON is provided by three free hydroxyl groups (-OH). DON is converted into DON-3G as a result of glucose transferring to its hydroxyl group at carbon 3 [6,26]. ZEN-14G is another common masked mycotoxin that has occurred in naturally infected cereals such as wheat, barley and maize. ZEN is reduced to phase I metabolites named α-zearalenol (α-ZEL) and β-zearalenol (β-ZEL). The glucose conjugates of these compounds are also produced ( Figure 1).  Table 1. In addition to masked forms, the modified forms of mycotoxins could occur by transformation or degradation of mycotoxins during food processing and the production of industrial goods. Due to their stable structure and features, mycotoxins tend to remain in the ultimate products [5]. While all these forms can be called as hidden, conjugated, and bound, the term masked is used only for plant metabolites [3,12,[27][28][29][30][31][32][33][34].
Mycotoxins have various effects like mutagenic, teratogenic, carcinogenic on eukaryotic organisms [5,35,36]. Epidemiological can disrupt cell signaling, differentiation and proliferation; and even can cause death [40][41][42]. ZEN, on the other hand, has a toxic effect on the endocrine and reproductive system and also shows developmental toxicity. Due to its estrogenic activity, it most affects the reproductive organs and their functions [43,44] Interestingly, these mycotoxins also appear in hen eggs, cow and sheep milks, chicken meat, fish [45,46], which are consumed as a food. Therefore, people are exposed to mycotoxins not only through plants but also through animals.
Intestinal transfer of two glucosylated masked forms of ZEN (ZEN-14G and ZEN-16G) revealed that intestinal cells took these masked forms up, and then ZEN was released by enzymatic cleavage [50,51]. ZEN-14G showed no effect on human breast cancer (MCF-7) cells. Also, these cells converted of ZEN-14G to several common aglycones with xenoestrogen activity and all of these metabolites ultimately occasioned the toxic impact [1].
Besides, in vitro toxicity studies were conducted through imitating organism's fluids [52]. Artificial digestive juices (saliva, be also included in the consideration during the evaluation of the population exposed to mycotoxin [55,60].
The intrinsic toxicity study revealed that oral uptake of DON-3G in rats was hardly hydrolyzed in the stomach, whereas ZEN-14G was rather unstable in the stomach and formed the main form ZEN.
The intense hydrolysis of DON-3G, ZEA-14G to their main forms was detected in the intestine [61]. Studies on pigs (in vivo) related to conversion of masked mycotoxins (DON-3G) to their main forms indicated the hydrolyzing of DON-3G in the distal small and large intestine [62]. Likewise, it was exhibited that this masked form was cleaved during digestion in rats and pigs (in vivo), and recovered as DON, which subsequently excreted in feces and urine [63,64].
Additionally, oral uptake of ZEN-14G and ZEN-16G in piglets showed that these masked forms were converted in the digestive tract to the ZEN, which then was detected in feces and urine [65].
After intravenous and oral administration of α-ZEL and α-ZEL-14G

Conclusion
Food safety has great importance in the maintenance of human health and wellbeing. For this reason, it is essential to prevent the contamination with DON and ZEN, the foodborne Fusarium endotoxins, which is a major concern all over the world. Originating