Surface Potential in Energized Mitochondria
Oxidative phosphorylation is widely believed to be mediated by proton transport (1). Substrate oxidation is coupled to proton extrusion from the mitochondrial matrix, while ATP synthesis is coupled to proton uptake into the matrix. According to the chemiosmotic hypothesis, the transmembranal proton electrochemical potential gradient is the direct and only driving force for ATP synthesis. The mitochondrial inner membrane contains several very active electroneutral proton-substrate co-carriers (2) which result in the reduction of the pH gradient. Therefore, in mitochondria, membrane potential is the major component of the proton electrochemical gradient, ΔμH (4). In fully coupled mitochondria the magnitude of membrane potential is over 150 mV. On both sides of the mitochondrial inner membrane there is also a negative surface potential. However, under physiological conditions the magnitude of the surface potential is small (5–20mV). Moreover, if protons equilibrate between the bulk and the membrane surface, the magnitude of the proton electrochemical potential should be identical at the bulk and at the adjacent surface. In recent years it was suggested that coupling by proton transfer may occur by more direct pathways in addition or instead of the bulk solutions (5–8). One possibility is that surface protons do not equilibrate with the bulk so that the coupling proton current is carried on the surface (7,9). If this was the case, we would expect that energy conversion would be associated with more positive external surface potential since the mitochondrial surface charge strongly depend on surface pH (see below). Indeed, recently several laboratories reported changes in surface charge on energization of mitochondria and other energy converting membranes (10–13). However, the reported changes for mitochondria indicated that the surface on the cytosilic side become more negative, which is opposite to the predictions of the above mentioned model. Because of our interest in the possible existence of direct coupling modes (5,8), we decided to examine the reports on energy dependent surface potential changes in mitochondria.
KeywordsSurface Charge Surface Potential Spin Probe Proton Electrochemical Gradient Phosphorescence Decay
Unable to display preview. Download preview PDF.
- 2.Klingenberg M. (1970) in Essays in Biochemistry (P.N. Campbell and F. Dickens, eds.) Vol. 6, Academic Press, N.Y. pp. 117–159.Google Scholar
- 4.Nichols D.G. (1982) Bioenergetics, Academic Press, London.Google Scholar
- 7.Kell D.B. (1979) Biochim. Biophys. Acta 544, 55–99.Google Scholar
- 8.Rottenberg, H. (1985) in Modern Cell Biol. Vol. 4 (B. Satir, ed.), Allen Liss, New York, in press.Google Scholar
- 13.ArchUnderline G.P.R., Farrington C.L., Lapin S.A., McKay M., Malpress F.H. (1980) Biochem. Int. 1, 422–427.Google Scholar
- 17.Slavic J. (1982) Biochim. Biophys. Acta 694, 1–24.Google Scholar
- 18.Robertson D.E. and Rottenberg H. (1983) J. Biol. Chem. 258, 11039–11048Google Scholar