Amine uniport at the plasmalemma of charophyte cells II. Ratio of matter to charge transported and permeability of free base

Summary We have previously reported that inward positive current flows across the plasmalemma of voltage-clampedChara 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 NH ₄ ⁺ are found to inhibit entry of CH₃NH ₃ ⁺ when membrane PD is constant. Half maximum inhibition is found at 20 m NH ₄ ⁺ , in agreement with the apparentK _M for NH ₄ ⁺ binding to its uniporter site. This suggests that NH ₄ ⁺ and CH₃NH ₃ ⁺ enter by the same uniporter, competing for binding to its binding site.