Mitochondria-targeted penetrating cations as carriers of hydrophobic anions through lipid membranes

High negative electric potential inside mitochondria provides a driving force for mitochondria-targeted delivery of cargo molecules linked to hydrophobic penetrating cations. This principle is utilized in construction of mitochondria-targeted antioxidants (MTA) carrying quinone moieties which produce a number of health benefitting effects by protecting cells and organisms from oxidative stress. Here, a series of penetrating cations including MTA were shown to induce the release of the liposome-entrapped carboxyfluorescein anion (CF), but not of glucose or ATP. The ability to induce the leakage of CF from liposomes strongly depended on the number of carbon atoms in alkyl chain (n) of alkyltriphenylphosphonium and alkylrhodamine derivatives. In particular, the leakage of CF was maximal at n about 10-12 and substantially decreased at n = 16. Organic anions (palmitate, oleate, laurylsulfate) competed with CF for the penetrating cation-induced efflux. The reduced activity of alkylrhodamines with n = 16 or n = 18 as compared to that with n = 12 was ascribed to a lower rate of partitioning of the former into liposomal membranes, because electrical current relaxation studies on planar bilayer lipid membranes showed rather close translocation rate constants for alkylrhodamines with n = 18 and n = 12. Changes in the alkylrhodamine absorption spectra upon anion addition confirmed direct interaction between alkylrhodamines and the anion. Thus, mitochondria-targeted penetrating cations can serve as carriers of hydrophobic anions across bilayer lipid membranes.