Energy transduction, ion transport and protonmotive force: are they linked?

Abstract

The chemiosmotic hypothesis is approximately 20 years old. It has been enormously useful, since it has offered a perspective from which we were able to ask questions never asked before. For the first time proton as well as other ion fluxes through membranes were examined from the common point of view of energy transduction. Despite this usefulness, the time has come to examine the tenets of this hypothesis in light of the available data and decide which ideas are still useful and should be retained and which have not withstood the test of time and should be discarded. It is clear that ion fluxes and proton fluxes must be fundamental processes in energy transduction. To argue otherwise would be foolish. However, it is equally clear that a central role of the electrochemical proton gradient, the socalled ‘protonmotive force’ is not substantiated as a general, primary mechanism of energy transduction in mitochondria. Data which will be presented here indicate that the two proposed components of the protonmotive force, i.e. the proton gradient and the electrical potential across the mitochondrial membrane, do not play a significant role in energy transduction. The various values for the difference in pH between the inside and the outside of mitochondria obtained in several laboratories show that the proton gradient alone does not have a significant role (e.g., Addanki et al.. 1968; Rottenberg, 1973; Kinnally & Tedeschi, 1976; Ogawa etal., 1980; Dodgson et al., 1982). Below we report similar values for giant mitochondria using a technique involving the micro-injection of a fluorescent pH indicator. It is obvious that at least in rat-liver mitochondria a role of the proton gradient would be limited to providing no more than about 10% of the necessary protonmotive force. Therefore, if the protonmotive force plays a role in energy transduction, a large metabolically induced membrane potential must be present. However, to our knowledge, there is no evidence at this time for a significant membrane potential in mitochondria. In the most frequently used approach, the potential is calculated from the distribution of ions of strong acids or bases, most frequently cations. Since the responses of electrochromic dyes depend on their distribution, and they are charged, any criticism of the use of ionic probes applies to the electrochromic dyes as well. These probes can be and have been used successfully to calculate electric potentials across membranes in other systems and the approach is valid as long as the distribution depends solely on these