Tetraphenylphosphonium (TPP+) is not suitable for the assessment of electrical potentials in Chlorella emersonii

Abstract. For Chlorella emersonii , plausible membrane potentials between –80 and –120 mV were calculated from the distribution of the lipophilic cation tetraphenylphosphonium (TPP⁺) between the cells and the medium. Furthermore, these calculated membrane potentials were influenced in a way expected from the literature, by different metabolic conditions induced by light or dark, anaerobiosis, glucose, and by inhibition or uncoupling of electron transport.
Nevertheless, the experiments presented here indicate that TPP⁺ is unsuitable as a probe for electrical potentials, at least in Chlorella emersonii. The reasons for this conclusion are as follows:

• 1. 

  Much of the incorporated TPP⁺-¹⁴C could not be exchanged against unlabelled TPP⁺.
• 2. 

  The uptake of TPP⁺-¹⁴C was very slow and exhibited complex rather than simple saturation kinetics.
• 3. 

  A large adsorption of TPP⁺-¹⁴C took place even after the cells were killed; the adsorption by living cells was only 20–60% higher than with killed cells. Furthermore, the adsorption by killed cells showed kinetics similar to living cells.