5-Methylphenazinium methylsulfate mediates cyclic electron flow and proton gradient dissipation in chromaffin-vesicle...

When 5-methylphenazinium methylsulfate and a reductant (ascorbate or NADH) are added together to a suspension of resealed chromaffin-vesicle membranes, the pH gradient (inside acidic) and the membrane potential (inside positive) established by the H(+)-translocating adenosine triphosphatase (ATPase) are rapidly dissipated. Dissipation of the pH gradient may be observed using either the optical probe acridine orange or the weak base methylamine. Dissipation of the membrane potential may be observed using the potential-dependent dye oxonol VI. A reductant and 5-methylphenazinium methylsulfate added in combination will also abolish a K+ diffusion potential across chromaffin-vesicle membranes but not across liposome membranes. 5-Methylphenazinium methylsulfate oxidizes cytochrome b561 in chromaffin-vesicle ghosts. Ascorbate readily reduces cytochrome b561, but reduction of cytochrome b561 by NADH is greatly enhanced in the presence of 5-methylphenazinium methylsulfate. These results are consistent with a mechanism in which proton gradient dissipation (a net efflux of H+) is caused by an influx of electrons through the membrane-protein cytochrome b561 coupled with an efflux of H carried by the reduced species 5-methyl-10-hydrophenazine. Although 5-methylphenazinium has been thought to accumulate within acidic vesicles as a weak base, this accounts for neither proton gradient dissipation nor for intravesicular accumulation of the compound.