NACET strongly increases the intracellular glutathione (GSH) content in RPE cells. ARPE-19 RPE cells were treated for 16 h with increasing concentrations of NAC or NACET and intracellular concentrations of GSH (a), cysteine (b), and NAC and NACET (d), and the extracellular concentrations of total GSH (tGSH) (c) were measured. The intracellular thiol concentrations were normalized to the concentration of total proteins (mean ± SD, number of replicates = 3). The p-values for the comparison of the treated vs. untreated cells (point 0) are indicated in black, while the p-values for the comparison between the cells treated with equal concentrations of NAC or NACET are indicated in red. * p < 0.05; *** p < 0.001; **** p < 0.0001.