Tetraphenylphosphonium cation (TPP⁺) is not a reliable membrane potential probe in the cyanobacterium Synechococcus R‐2 PCC 7942

Abstract

Uptake of the lipophilic cation, tetraphenylphosphonium (TPP + ), which is often used as a membrane potential probe, was studied in the cyanobacterium Synechococcus R-2 (PCC 7942) to determine if the technique was valid for these cells. Lipophilic cation probes are notorious for giving inconsistent estimates of membrane potential (∆Ψ) using the Nernst equation (E TPP + l ∆Ψ). It is necessary to correct for the large amounts of TPP + bound to the surface of the cells. The rate of equilibration of intracellular TPP + is very fast (t "/# $ 1-3 min), hence the permeability of the plasmalemma to TPP + (P TPP + $ 10-50 nm s −" ) is very high. There appears to be an active uptake of TPP + with a pH optimum at about pH o 7n5. TPP + does not readily uncouple Synechococcus cells. Carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) and dinitrophenol depolarized E TPP + , but monensin made E TPP + more negative. Accumulation of TPP + is not affected by [K + ] in the medium in a way consistent with it acting as a reporter of the membrane potential. Addition of K + , NH % + and HCO $ − altered the uptake rate of TPP + in a way consistent with electrogenic uptake of these ions but the effects were smaller than those found using the )'Rb+\valinomycin and #!"TI+ probes. A comparison has been made with ∆Ψ determinations using other membrane potential probes. E TPP + was often close to the value of the membrane potential obtained using other methods under control conditions but effects of experimental treatments are sometimes inconsistent with those found using other ∆Ψ probes. The TPP + method is therefore unreliable and could be seriously misleading.