<P> WSTs (water - soluble tetrazolium salts) are a series of other water - soluble dyes for MTT assays, developed to give different absorption spectra of the formed formazans . WST - 1 and in particular WST - 8 (2 - (2 - methoxy - 4 - nitrophenyl) - 3 - (4 - nitrophenyl) - 5 - (2, 4 - disulfophenyl) - 2H - tetrazolium), are advantageous over MTT in that they are reduced outside cells, combined with PMS electron mediator, and yield a water - soluble formazan . Finally, WST assays (1) can be read directly (unlike MTT that needs a solubilization step), (2) give a more effective signal than MTT, and (3) decrease toxicity to cells (unlike cell - permeable MTT, and its insoluble formazan that accumulate inside cells). </P> <P> Tetrazolium dye reduction is dependent on NAD (P) H - dependent oxidoreductase enzymes largely in the cytosolic compartment of the cell . Therefore, reduction of MTT and other tetrazolium dyes depends on the cellular metabolic activity due to NAD (P) H flux . Cells with a low metabolism such as thymocytes and splenocytes reduce very little MTT . In contrast, rapidly dividing cells exhibit high rates of MTT reduction . It is important to keep in mind that assay conditions can alter metabolic activity and thus tetrazolium dye reduction without affecting cell viability . In addition, the mechanism of reduction of tetrazolium dyes, i.e. intracellular (MTT, MTS) vs. extracellular (WST - 1), will also determine the amount of product . </P>

What does mtt stand for in mtt assay