F. H. Malpress

Proton movements and adenosine triphosphate synthesis in rat liver mitochondria.

In a previous communication (Archbold et al., 1974) we have described experiments the results of which are not readily reconciled with some aspects of the chemiosmotic hypothesis. Our interpretation required a distinction between the distributive nature ofelectroneutral (EN) and eiectrogenic (EG) proton movements across the mitochondria1 inner membrane. This distinction has not been made hitherto in the formulation of the energy-transduction relationship. Mitchell (1968) debated, but ultimately disregarded, the significance of the double electrical layer on wi, the electrical potential in the mitochondria] matrix, but entertained no corresponding effect on iy0, the electrical potential in the medium bulk phase. Our experiments suggested an influence of the fixed charge on the outer surface of the coupling membrane, not only on ri/, (Coster, 1973), but also on electrogenic proton movement. Acceptance of this fact leads to a reformulation of the equation for the protonmotive force in which maximum values of the potentials occurring across the space-charge areas, or p-zones, are substituted for the bulk-phase potentials existing between the medium and the matrix:

On the chemiosmotic hypothesis and the nature of the mitochondrial protonmotive force

The chemiosmotic hypothesis (Mitchell, 1961, 1966) has proved one of the most rewarding, and one of the most provocative concepts in contemporary biological studies. The utilization of a transmembrane electrochemical gradient as an intermediate in energy transduction has been demonstrated by a wealth of transport studies; and for a large consensus of workers it defines the path by which substrate energy, released in the electron transport chain, is transferred to effect the synthesis of adenosine triphosphate (ATP). The hypothesis, represented by the developing views of Mitchell (1979), has never been without its critics, and in general this criticism has been of three kinds: first through alternative theories (Williams, 1979; Boyer, 1977) that, in their origins at least, have denied any significant role for a transmembrane protonmotive force; next, by doubts expressed against important aspects of the hypothesis: stoicheiometric values (Brand, Reynafarje & Lehninger, 1976; Alexandre & Lehninger, 1979) or the precise mechanisms of proton production (Wikstriim & Krab, 1978), that nevertheless leave the basic premise of a transmembrane protonmotive force intact; and lastly criticism that, while accepting the full importance of a protonmotive force, maintains that in synthesis it has characteristics radically different from those that the chemiosmotic hypothesis propounds (Archbold, Farrington & Malpress, 1975a, Archbold, Farrington, Lappin, McKay & Malpress, 1979; Kell, 1979). It is the last argument which is discussed here. The question is a specific one: what are the characteristics of the protonmotive force when ATP synthesis is taking place? It might also be put more generally: do the electrochemical forces acting on a membrane have the same distribution under all circumstances? Insofar as mitochondrial ATP synthesis is concerned the chemiosmotic hypothesis makes the assumption that they do, and that while the electrical and chemical components of the protonmotive force may be present in

Proton turnover at mitochondrial coupling sites

The claim (Brand et al. , 1978) that steady-state experiments indicatedissimilar H + /2e quotients and therefore different rates of ATP synthesis for the three mitochondrial coupling sites has been reconsidered. The data, which conflict with the requirements of the chemiosmotic hypothesis, do not necessarily lead to the conclusions drawn by these authors. Their findings are prdeicted by the ρ-zone interpretation of mitochondrial energy transduction which reconciles them with the presence of constant H * /2e values at all sites.