AMINE UNIPORT AT THE PLASMALEMMA OF CHAROPHYTE CELLS .2. RATIO OF MATTER TO CHARGE TRANSPORTED AND PERMEABILITY OF FREE BASE

We have previously reported that inward positive current flows across the plasmalemma of voltage-clamped Chara cells when ammonia or methylamine is added to the medium. This is attributed to (inward) uniport of the cation. We have measured the stoichiometric ratio of the quantity of methylamine transported to the quantity of positive charge transported. We find 0.9 mol/faraday from pH 5.7 to 8.5, as expected if the cation flux is much larger than that of free base. The ratio increases progressively above pH 9 as the concentration of free base becomes comparable with that of cation: the fluxes fit those predicted if neutral methylamine has a permeability of 1.8 × 10-5 m/sec. This is comparable with the permeability of the methylammonium ion, 6×10-6 m/sec, at low concentration and -200 mV, as previously reported. Low concentrations of NH4+ are found to inhibit entry of CH3NH3+ when membrane PD is constant. Half maximum inhibition is found at ∼20 μm NH4+, in agreement with the apparent KM for NH4+ binding to its uniporter site. This suggests that NH4+ and CH3NH3+ enter by the same uniporter, competing for binding to its binding site. © 1979 Springer-Verlag New York Inc.