Formazan

File:Tetrazolium reduction.png

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.

Formazan 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:

1. INT or 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-phenyl-2H-tetrazolium chloride, which is water-insoluble.
2. MTT or 3-(4,5-dimethyl-2-thiazolyl)-2, 5-diphenyl-2H-tetrazolium bromide, which is water-insoluble and used in the MTT assay.
3. XTT or 2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide, which is water-soluble.
4. MTS or 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, which is water-soluble and used in the MTS assay.
5. TTC or tetrazolium chloride or 2,3,5-triphenyl-2H-tetrazolium chloride, which is water-soluble.
6. NBT used in a diagnostic test, particularly for chronic granulomatous disease and other diseases of phagocyte function.

When reduced in a cell, either enzymatically or through direct reaction with NADH or NADPH, the classical tetrazolium salt, MTT, turns blue to purple and may form an insoluble precipitate. These formazan dyes are commonly used in cell proliferation and toxicity assays like the EpiDerm and EpiSkin tests since they only stain living, metabolically active cells.