The electrogenic proton/hexose carrier in the plasmalemma of Chenopodium rubrum suspension cells: effects of Δc, ΔpH and Δψ on hexose exchange diffusion

Abstract

The hexose carrier in the plasmalemma of a higher plant suspension cell ( Chenopodium rubrum L.) catalyzes not only electrogenic proton/hexose symport, but also electroneutral exchange diffusion of hexoses. The transport process is studied mainly through short-term efflux disturbance of labelled 3-O- methyl- d-glucose (3-OMG). This novel method singles out the transport system in situ and reliably detects small (>5%) changes of unidirectional hexose fluxes. The three driving forces for the electrogenic net uptake of hexose are the gradients of substrate (hexose), cosubstrate (H +) and the electric potential difference across the plasmalemma. Increasing substrate concentrations (outside or inside) led to increased unidirectional fluxes into the compartment where the substrate concentration was increased (e.g., trans stimulation of 3-OMG efflux by external hexose); however, an increase of the external H + concentration at non-saturating external hexose concentrations led to a trans inhibition of 3-OMG efflux. Depolarization of the transmembrane electric potential difference without influencing the surface potential led to only a small increase in hexose efflux at low hexose concentrations and had no measurable effect at saturating hexose concentrations. However, depolarization mainly due to a decrease of the surface and Donnan potentials mimicked the effect of alkalinization. These findings agree with a model based on a negatively charged carrier showing random-order binding of substrate and cosubstrate on the outside of the plasmalemma. It is not necessary to assume a recycling translocation step of the empty substrate-binding site. The low-voltage sensitivity of the hexose efflux observed is discussed with regard to possible electric potential profiles at the site of transport.