**1. Introduction**

Studying mycotoxins is topical and relevant for ensuring food and biological safety [1–3]. Analytical approaches to the detection and identification of mycotoxins [4] are being actively developed, and so are strategies for mycotoxin control and detoxification [5–8].

These are currently the most actively pursued areas: (i) elucidating the mechanisms of toxic effects of mycotoxins on living organisms [5]; (ii) development and testing of effective selective and sensitive analytical methods for the detection of mycotoxins in food, agricultural feed, and raw materials for the pharmaceutical industry [9,10]; (iii) search for sorbents capable of removing mycotoxins from raw materials [11]; (iv) the search for new methods for the destruction of mycotoxins, especially those involving

**Citation:** Efremenko, E.; Maslova, O.; Stepanov, N.; Ismailov, A. Using Cholinesterases and Immobilized Luminescent Photobacteria for the Express-Analysis of Mycotoxins and Estimating the Efficiency of Their Enzymatic Hydrolysis. *Toxins* **2021**, *13*, 34. https://doi.org/10.3390/ toxins13010034

Received: 30 November 2020 Accepted: 4 January 2021 Published: 6 January 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

bio-destructors [12,13]. The last of the above directions is especially important today, since it implies the development of means, including combined action, that eliminate mycotoxins not only due to their sorption, but also due to their catalytic decomposition. Various enzymes are considered as such biocatalytic detoxifiers [13,14].

In fundamental and applied research in the field of mycotoxins, liquid chromatography (LC) and enzyme-linked immunosorbent assay (ELISA) are most widely used today. An LC run is followed by mass spectrometry (MS), sensitive fluorescence detection (FLD), or ultraviolet (UV) detection [9,10,12]. High analytical accuracy and selectivity are the main advantages of the abovementioned methods. However, in this case, the duration and complexity of the sample preparation are obvious limiting factors for the express use of LC. ELISA kits allow for express analysis; however, they are distinguished by their high cost, since special antibodies are required for each mycotoxin.

When developing approaches to detoxification of mycotoxins using biocatalysts, selectivity and accuracy are not always priority indicators at the stage of screening and selection of primary candidates. Analytical express methods are more popular at this stage, because they allow the rapid assessment the residual toxicity of the test samples after their enzymatic or cellular treatment. Thus, ineffective candidates can be quickly eliminated from the study, whereas more promising biocatalysts can be chosen for a deeper study of their characteristics using more accurate analytical instruments based on LC or ELISA.

The initial choice of perspective biocatalysts for in-depth study is based on the published data. For example, it is known that enzymes of some classes, including hexahistidine-tagged organophosphorus hydrolase (His6-OPH), exhibit destructive activity towards various mycotoxins [3,9,10,13–17]. Computer design, and, in particular, the molecular docking method [18,19], is ye<sup>t</sup> another technique which has been proved useful for the initial selection of promising potential candidates from a number of enzymes for the decomposition of mycotoxins. The next stages of research already imply practical experimental research.

To quickly screen out candidates selected as a result of docking, but which do not efficiently detoxify mycotoxins, luminescent photobacterial cells can be successfully used for the rapid assessment of the toxicity of samples [17]. These cells sensitively react to the presence of mycotoxins via changing the level of their bioluminescence. It is important that when the cells are used in an immobilized form, such analyzes become possible both in discrete and continuous modes and the analytical signal is stable enough [17,20]. It appears possible to find ways of increasing the sensitivity of mycotoxin detection with this technique by varying the strains of photobacteria immobilized by the same method. We have not identified such comparative studies conducted earlier.

The search for other sensitive analytical agents that can be used for affordable rapid analysis essential for controlling mycotoxins is of obvious scientific and practical importance. In particular, cholinesterases can be considered among the promising candidates, which have proven themselves well in the rapid analysis of many other toxic compounds [21]. Our previous results showing mycotoxins docking to the surface of cholinesterases [13] indicated that inactivation of these enzymes under the action of mycotoxins is feasible. Therefore, cholinesterases (acetyl- (AChE) and/or butyrylcholinesterase (BChE)) can be used for assessing the concentrations of mycotoxins and the effectiveness of the action of destructors on these substances. However, we were unable to find any reports on systematic studies of the inhibition of the cholinesterases by various mycotoxins and on the possibility of implementing an analytical technique based on this effect.

The aim of the present work was to compare the characteristics of cholinesterases and luminescent photobacterial strains for use in rapid quantitative analysis of mycotoxins using cholinesterases and luminescent photobacterial strains. We also studied the applicability of this technique for assessing the effectiveness of mycotoxin biodegradation in the case of zearalenone and the His6-OPH.
