The role of plasma membrane in bioreduction of two tetrazolium salts, MTT, and CTC

Despite widespread use of various tetrazolium assays, the mechanisms of bioreduction of these compounds have not been fully elucidated. We investigated the capacity of tetrazolium salts to penetrate through intact cell plasma membranes. 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) and 3-(4,5-dimethylthiazol-2-yl)-2,5 diphenyltetrazolium bromide (MTT) tetrazolium salts appear to represent examples of species that are reduced by different mechanisms. We provide evidence suggesting that MTT readily crosses intact plasma membranes and is reduced intracellularly. MTT appears to be reduced by both plasma membrane and intracellular reductases; reducing cells are not damaged and remain metabolically active for at least 45 min. In contrast, CTC remains extracellular with respect to viable cells and thus requires plasma membrane permeable electron carrier to be reduced efficiently. However, reduction of CTC in the presence of an electron carrier inflicts damage on plasma membranes. The intracellular vs extracellular sites of reduction of tetrazolium salts were established on the basis of deposition of formazans. Crystals of formazan were detected using fluorescence or backscattered light confocal laser microscopy. We postulate that the capacity of a tetrazolium salt to cross intact plasma membranes constitutes an important experimental variable which needs to be controlled in order to correctly interpret the outcome of tetrazolium assays designed to measure cellular production of oxygen radicals, activity of mitochondrial, cytosolic, or outer membrane reductases, etc.