Efficiency of mitochondrial uncoupling by modified butyltriphenylphosphonium cations and fatty acids correlates with lipophilicity of cations: Protonophoric vs leakage mechanisms

Abstract

In order to determine the share of protonophoric activity in the uncoupling action of lipophilic cations a number of analogues of butyltriphenylphosphonium with substitutions in phenyl rings (C₄TPP-X) were studied on isolated rat liver mitochondria and model lipid membranes. An increase in the rate of respiration and a decrease in the membrane potential of isolated mitochondria were observed for all the studied cations, the efficiency of these processes was significantly enhanced in the presence of fatty acids and correlated with the octanol-water partition coefficient of the cations. The ability of C₄TPP-X cations to induce proton transport across the lipid membrane of liposomes loaded with a pH-sensitive fluorescent dye increased also with their lipophilicity and depended on the presence of palmitic acid in the liposome membrane. Of all the cations, only butyl[tri(3,5-dimethylphenyl)]phosphonium (C₄TPP-diMe) was able to induce proton transport by the mechanism of formation of a cation-fatty acid ion pair on planar bilayer lipid membranes and liposomes. The rate of oxygen consumption by mitochondria in the presence of C₄TPP-diMe increased to the maximum values corresponding to conventional uncouplers; for all other cations the maximum uncoupling rates were significantly lower. We assume that the studied cations of the C₄TPP-X series, with the exception of C₄TPP-diMe at low concentrations, cause nonspecific leak of ions through lipid model and biological membranes which is significantly enhanced in the presence of fatty acids.

Keywords: Butyltriphenylphosphonium; Fatty acids; Mitochondria uncoupling; Nonspecific leakage; Proton transport; Pyranine-loaded liposomes.