Electron donation to the plasma membrane redox system of cultured carrot cells stimulates proton release.

Membrane-permeable electron donors, duroquinol, diphenylcarbazide, pyrocatechol and tert-octylcatechol, promoted both reduction of an impermeant electron acceptor and proton transport with cultured carrot cells. These cells were preloaded with electron donors for 15, 30, 45 and 60 min. Aliquots of cells were removed at various times, washed free of excess electron donors and assayed for their effect on transplasma membrane redox with impermeable hexacyanoferrate (HCF III) as the electron acceptor and for simultaneous H+ excretion in the presence of hexacyanoferrate. All four electron donors stimulated HCF III reduction and associated H+ excretion. Below a rate of hexacyanoferrate reduction of 6 mumol/g dry wt. per min, the ratios of H+/e- were between 0.3 and 1 with low concentrations (0.1 mM) of the added electron donors. When hexacyanoferrate reduction exceeded 6 mumol/g dry wt. per min, proton release began to cascade to give ratios of 1 to 3, suggesting activation of an H(+)-ATPase or a proton transporter. This behavior by cultured carrot cells indicates that a certain threshold of proton concentration in a limited membrane domain must be reached in order for the proton channel to be opened.