Pr. Rich et al., The sites of interaction of triphenyltetrazolium chloride with mitochondrial respiratory chains, FEMS MICROB, 202(2), 2001, pp. 181-187
The inability of cells and microorganisms to reduce the colourless electron
acceptor triphenyltetrazolium chloride (TTC) to a red formazan precipitate
is commonly used as a means of screening for cells that have a dysfunction
al respiratory chain. The site of reduction of TTC is often stated to be at
the level of cytochrome c oxidase where it is assumed to compete with oxyg
en for reducing equivalents. However, we show here that TTC is reduced not
by cytochrome c oxidase but instead by dehydrogenases, particularly complex
I, probably by accepting electrons directly from low potential cofactors.
The reduction rate is fastest in coupled membranes because of accumulation
in the matrix of the positively charged TTC+ cation. However, the initial p
roduct of TTC reduction is rapidly reoxidised by molecular oxygen, so that
generation of the stable red formazan product from this intermediate occurs
only under strictly anaerobic conditions. Colonies of mutants defective in
cytochrome oxidase do not generate sufficiently anaerobic conditions to al
low the intermediate to form the stable red formazan. This revision of the
mode of interaction of TTC with respiratory chains has implications for the
types of respiratory-defective mutants that might be detected by TTC scree
ning. (C) 2001 Federation of European Microbiological Societies. Published
by Elsevier Science B.V. All rights reserved.