The acceptance and application of whole-cell bioassays in routine chemical food contaminant analysis is progressing slowly but steadily. Typical examples are the DR-CALUX® bioassay and hormone reporter gene bioassay. The principle of such assays is illustrated in Figure 5: Genetically modified cells are exposed to the sample extract containing the contaminant(s). The cells are modified in such a way that they express the receptor protein of interest, in this example the aryl hydrocarbon (Ah) receptor which binds to dioxins and PCBs. Upon binding the receptor-ligand complex travels into the cell nucleus and binds onto the modified DNA at specific responsive elements which act as a switch for the activation of a gene encoding for the production of a marker protein such as luciferase or green fluorescent protein (GFP); as a result the cells will generate light upon exposure, even at trace levels [54]. Cell assays can be performed in parallel in 96-well plates and require hardly any reagents. Only the suspect samples will require confirmatory analysis using expensive instrumental analysis methods such as GC/HRMS in the case of dioxins. A key issue in the application to food and feed samples is the stability of the cells. In general, mammalian cells seem to be more vulnerable to cell toxicity caused by matrix components and require a more stringent sample clean-up in contaminant and residue analysis. Yeast cells on the other hand are inherently more robust, allowing application to

Figure 5 Schematic representation of the mechanism of the DR-CALUX®. Reproduced and kindly provided by T.F.H. Bovee [54].

residue analysis of estrogens in feed and urine samples following a simple solidphase extraction step [54].

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