Tetrazolium

Tetrazolium refers to derivatives containing the tetrazolium ring, a five-membered heterocycle with four nitrogen atoms that forms positively charged salts when paired with common counterions such as chloride or bromide. The core structure enables the compounds to act as redox-active electron acceptors in a variety of chemical and biological assays.

Common tetrazolium salts include MTT, XTT, MTS, and the WST series (such as WST-1 and WST-8), as well as nitroblue tetrazolium (NBT) and tetrazolium chloride (TTC). In many biology laboratories, tetrazolium salts are used to assess cellular metabolic activity. In living cells, NADH- or NADPH-dependent dehydrogenases reduce the salt to a colored formazan compound. Depending on the specific reagent, the formazan is either insoluble (e.g., MTT) and requires solubilization for measurement, or soluble (e.g., XTT, MTS, WST-1, WST-8), allowing direct spectrophotometric readouts.

Applications are widespread: viability and cytotoxicity assays in cell culture, estimation of metabolic activity, and respiration measurements in microorganisms. MTT and its derivatives are among the most used tools in cell biology, providing a visible or measurable readout proportional to viable cell number. NBT and TTC have been employed in histochemistry and microbiology to detect enzymatic activity or respiratory processes, often forming distinctive colored formazans in situ.

Potential limitations include interference by reducing agents, variability in formazan solubility, and the need for appropriate controls and solubilization steps to ensure accurate quantification.