The most important energy source in the cells of living organisms is ATP, which is a key indicator of the level of cellular metabolism. In cells of the same type and under similar culture conditions, the ATP content has a good linear relationship with the number of viable cells. Therefore, the number of viable cells can be reflected by the ATP content. Firefly luciferase is a protein with a molecular weight of approximately 61 kDa that can catalyze luciferin into oxyluciferin in the presence of ATP, magnesium ions, and oxygen. During this oxidation process, it emits bioluminescence at a wavelength of around 560 nm. This reaction is an ATP-dependent luminescent reaction, and its light intensity is linearly related to the ATP content.
