Nitrite Test
Since nitric oxide (NO) was unstable and easy to form nitrates and nitrites in cells, the nitrite in the supernatant was measured as an indicator of NO production using an oxidized nitrite kit (Beyotime Institute of Biotechnology, Jiangsu, China). Briefly, according to the instructions, the supernatant was sequentially mixed with an equal volume of Gris reagent I and Gris reagent II, and then the concentration of nitrites was calculated according to the optical density (OD) value at 540 nm measured using a smart-spec plus spectrophotometer (Bio Rad laboratories, Hercules, CA, USA) (Stepuro, Chaikovskaya, 1997).