Formazan

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Formazan dyes are artificial chromogenic substrates for dehydrogenases and reductases. They are formed from reduction of tetrazolium salts, including INT (2-(4-Iodophenyl)-3-(4-nitrophenyl)-5-phenyl-2H-tetrazolium chloride), MTT (3-(4, 5-dimethyl-2-thiazolyl)-2, 5-diphenyl-2H-tetrazolium bromide) (used in the MTT assay), and the more water soluble XTT (2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide). When reduced in a cell, either enzymatically or through direct reaction with NADH or NADPH 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.

Formazans are produced when tetrazolium salts such as MTS, MTT, INT and XTT are reduced by naturally occurring enzymes. They have a variety of colors from dark blue to deep red to orange, depending on the tetrazolium salt. They can be water soluble or non-water soluble.