Uncoupling and toxic action of alkyltriphenylphosphonium cations on mitochondria and the bacterium Bacillus subtilis as a function of alkyl chain length

A series of permeating cations based on alkyl derivatives of triphenylphosphonium (C-n-TPP+) containing linear hydrocarbon chains (butyl, octyl, decyl, and dodecyl) was investigated in systems of isolated mitochondria, bacteria, and liposomes. In contrast to some derivatives (esters) of rhodamine-19, wherein butyl rhodamine possessed the maximum activity, in the case of C-n-TPP a stimulatory effect on mitochondrial respiration steadily increased with growing length of the alkyl radical. Tetraphenylphosphonium and butyl-TPP+ at a dose of several hundred micromoles exhibited an uncoupling effect, which might be related to interaction between C-n-TPP+ and endogenous fatty acids and induction of their own cyclic transfer, resulting in transport of protons across the mitochondrial membrane. Such a mechanism was investigated by measuring efflux of carboxyfluorescein from liposomes influenced by C-n-TPP+. Experiments with bacteria demonstrated that dodecyl-TPP+, decyl-TPP+, and octyl-TPP+ similarly to quinone-containing analog (SkQ1) inhibited growth of the Gram-positive bacterium Bacillus subtilis, wherein the inhibitory effect was upregulated with growing lipophilicity. These cations did not display toxic effect on growth of the Gram-negative bacterium Escherichia coli. It is assumed that the difference in toxic action on various bacterial species might be related to different permeability of bacterial coats for the examined triphenylphosphonium cations.