Catechols and quercetin reduce MTT through iron ions: A possible artefact in cell viability assays

The reduction of MTT (d-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) by mitochondrial enzymes to its blue formazan product is a widely used colorimetric assay method to determine the viability of cells in culture. In the present study the reduction of MTT by several agents in the absence of living cells was investigated. Several catechols and the flavonoid quercetin were found to reduce MTT in serum-supplemented Dulbecco's modified Eagle's medium. All the agents tested failed to reduce MTT in Hank's balanced salt solution, but supplementing this buffer with serum or iron ions resulted in MTT reduction. The involvement of an iron-catalysed redox reaction in the reduction of MTT induced by catechols and flavonoids was demonstrated. Caution is needed in using MTT in cell viability assays involving such chemicals.