The Relationships Between Ferricyanide Reduction and Associated Proton Efflux in Mechanically Isolated Photosynthetically Competent Mesophyll Cells

Abstract

The plasma membrane of plant cells contains an outwardly directed, proton translocating, electrogenic ATPase. The activity of this enzyme establishes both an electrical gradient and pH gradient across the plasma membrane (Sze 1985). The resulting proton electrochemical gradient favours proton entry into the cell and this downhill flux of protons is coupled to and drives the accumulation of sugars and amino acids (Reinhold and Kaplan 1984). Substantial evidence also suggests that a plasma membrane redox enzyme can reduce exogenous electron acceptors using an endogenous electron donor. Associated with this redox activity is proton efflux (Crane et al 1985). The mechanism coupling the reduction of an exogenous oxidant to proton efflux is contentious. In particular it is not clear whether the redox enzyme transfers both electrons and protons to the external medium or whether the redox activity transfers electrons only and thereby activates the proton-translocating ATPase. Some workers suggest that the redox enzyme catalyses an electrogenic transfer of electrons, which depolarizes the plasma membrane reducing the proton electrochemical gradient and activating the plasma membrane ATPase (Rubinstein and Stern 1986, Lass et al 1986). Others have noted an apparent 1:1 stoichiometry between electron and proton movements and the apparent insensitivity of redox activity dependent proton efflux to ATPase inhibitors. They suggest that a single redox system is responsible for movement of both electron and protons (Crane et al 1985, Craig and Crane 1985, Neufeld and Bown 1987, Bown and Crawford 1988).