Formazan

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:

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.

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