Tetrazolium compounds have the general formula:
An insoluble coloured formazan results on reduction.
Strictly speaking, tetrazolium compounds and their formazans are not dyes. Their characteristic structure is the five member ring of one carbon and four nitrogens atoms. They may contain one or two of these structures. Examples are iodonitrotetrazolium and nitro blue tetrazolium.
When this characteristic structure is reduced by the addition of a hydrogen atom, the nitrogen to nitrogen bond is broken with the formation of the corresponding formazan. Formazans are intensely coloured but water insoluble, so they precipitate as they form. They are valuable for the demonstration of enzymes that transport hydrogen, such as dehydrogenases, where they act as hydrogen acceptors.
R. D. Lillie.
Conn's Biological Stains
Williams & Wilkins, Baltimore, MD., U.S.A.
Enzyme Histochemistry and its application to the study of neoplasms
Academic Press, New York, NY, USA