Reduction of a tetrazolium salt, CTC, by intact HepG2 human hepatoma cells: subcellular localisation of reducing systems

Cell-mediated reduction of tetrazolium salts, including MTT, XTT, MTS, NET, NTV, INT, in the presence or absence of intermediate electron carriers is used as a convenient test for animal or bacterial cell viability. Bioreduct ion of tetrazolium is considered an alternative to a clonogenic assay and a thymidine incorporation assay. However, correlation between clonogenic pot ential and capacity to reduce tetrazolium has not been demonstrated convinc ingly. Moreover, despite a wide use of tetrazolium viability assays, the me chanism and subcellular localisation of reducing systems or species in viab le intact cells have not been fully elucidated. We report evidence indicati ng that a tetrazolium salt CTC can be reduced in the presence as well as in the absence of an electron carrier by viable HepG2 human hepatoma cells. C TC-formazan is formed within or at the outer surface of plasma membranes. W e hypothesise that in the presence of an electron carrier the electron dono rs active in the reduction of CTC are located in the intracellular compartm ent, as well as in plasma membranes. However, in the absence of an electron carrier, the reduction occurs primarily via a plasma membrane-associated e nzymatic system or species. (C) 1999 Elsevier Science B.V. All rights reser ved.