This assay measures the metabolic activity of cells, which is
considered a reflection of cell viability. 3-(4,5-dimethylthiazol-2-
yl)-2,5-diphenyltetrazolium bromide (MTT) is a pale yellow substrate
that is reduced by succinate dehydrogenase in living cells
to a dark blue formazan product, which precipitates as crystals.
Succinate dehydrogenase is part of complex II at the inner mitochondrial
membrane and of Krebs cycle. The assay is considered
to reflect both the number of cells and the activity status of the
mitochondria. Non-respiring mitochondria in dead cells are unable
to reduce significant amounts of MTT. Alternative substrates, such
as (2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-
carboxanilide (XTT), 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-
2-(4-sulfophenyl)-2H-tetrazolium (MTS) and
water soluble tetrazolium salts (WSTs) are also often used. They
have advantages over MTT in that they can be directly quantified.
The major disadvantage of these viability assays, especially at the
early stages of apoptotic cell death, is that dead cells often still
have partially intact succinate dehydrogenase activity, which is
erroneously scored as survival.