4.2. Cell Counting Kit-8 (CCK-8) Cell Viability Assay

The cell viability was examined using Cell Counting Kit-8 (CCK-8) assay kits (#96992, Sigma, Hiroshima, Japan) according to the manufacturer’s instructions. CCK8 contains the tetrazolium salt WST-8, which is metabolized by living cells to form a soluble in cell culture medium dye formazan, which can be detected by optical density. The amount of this dye is directly proportional to the number of living cells. On day 10 of differentiation after 24 h treatment of cells with peptides, 10 µL of CCK-8 reagent was added to each well and incubated at 37 °C in a humidified atmosphere with 5% CO₂ for 3 h. The absorbance at 450 nm was determined, using an Infinite F50 Tecan microplate photometer (Tecan, Grödig, Austria). Three wells of cells were used for each condition in every independent experiment. The cell viability of untreated cells was presented as a percentage of untreated control SH-SY5Y cells. The cell viability in peptide-treated cells was presented as a percentage of that in control or untreated cells in parallel experiments.