The PromoKine Total Antioxidant Capacity (TAC) Assay Kit is a colorimetric assay that is based on copper reduction by antioxidants. It measure either the combination of both small molecule antioxidants and proteins or small molecules alone in the presence of a proprietary Protein Mask. In the presence of antioxidants, the Cu2 ion is converted to Cu (by both small molecules and proteins). However, the Protein Mask prevents Cu2 reduction by proteins, enabling the analysis of only the small molecule antioxidants. The reduced Cu ion is chelated with a colorimetric probe giving a broad absorbance peak around 570 nm, proportional to the total antioxidant capacity.