2.15. Oxidative Stress: Nitrite (NO2−) Concentration Assay

The Griess Reagent Kit for Nitrite Determination (Invitrogen™, Carlsbad, CA, USA) was used according to the manufacturer’s instructions to measure nitrite (NO₂⁻), one of two primary stable and nonvolatile breakdown products of nitric oxide (NO). The procedure was performed 6 and 24 h after the end of the incubation of HaCaT cells. The level of nitrite in the cell culture medium was analyzed spectrophotometrically. The absorbance was measured at 548 nm using a multi-well plate reader (BioTek TM, Epoch, BioTek Instruments Inc., Winoosky, VT, USA). In parallel, complete medium was used as a vehicle and control.