Reduction of a tetrazolium to a formazan. The "Rs" stand-in for various organic groups that define the various tetrazolium salts and provide their unique chemical characteristics.
dyes are artificial
chromogenic products of the reduction of
tetrazolium salts by
dehydrogenases and reductases. They have a variety of colors from dark blue to deep red to orange, depending on the original tetrazolium salt used as the substrate for the reaction.
Leading examples of tetrazolium salts, include:
- INT (2-(4-Iodophenyl)-3-(4-nitrophenyl)-5-phenyl-2H-tetrazolium chloride), (water insoluble)
- MTT (3-(4, 5-dimethyl-2-thiazolyl)-2, 5-diphenyl-2H-tetrazolium bromide) (used in the MTT assay), (water insoluble)
- XTT (2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide) (water soluble)
- MTS (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium) (used in the MTS assay) (water soluble).
When reduced in a cell, either
enzymatically or through direct reaction with
NADH or
NADPH the classical tetrazolium salt, MTT, turns bright blue and may form an insoluble precipitate. These formazan dyes are commonly used in cell proliferation and toxicity
assays like the
EpiSkin test since they only stain living cells.
