4.6. Cell Viability Assays

The MTT (Sigma-Aldrich, St. Louis, MO, USA) cell viability assay was used to determine the viability of cells following treatment with licorice TE- and compounds-only and the pre-treatment of cells with licorice TE and/or compounds and MPP⁺. Cells were seeded in 96-well plates and treated as stated above, after which the MTT assay was performed. After treatment, 10 or 20 µL (depending on well volume) of the 5 mg/mL MTT solution in phosphate-buffered saline (PBS) (Lonza Group Ltd., Verviers, Belgium) was added to each well and left to incubate in the dark at 37 °C for 4 h. After incubation, the medium containing the MTT dye was discarded, and the MTT formazan was solubilized with 100 μL of DMSO for an absorbance reading using a microplate reader (BMG Labtech Omega^® POLARStar, Offenburg, Germany) at a wavelength of 570 nm. Cell viability was calculated and expressed as a percentage of control.