Amine ion porter inChara australis: Effects of alkyl substitution and external pH

Summary The rate of transport of amine ions intoChara australis internodes is studied by measuring changes in membrane current when amine solutions are presented to voltage-clamped cells. The dependence of this rate on ion concentration is investigated for a series of alkyl-amine ions: methyl-, ethyl-, isopropyl-, dimethyl-, trimethyl- and tetramethylammonium. A Michaelis-Menten relationship is displayed by all except tri- and tetramethylammonium, where currents are irregular and difficult to reproduce. Evidence suggests that the different ions cross the plasmalemma via a common uniport.K_M values for this porter increase as the amine ion becomes more highly substituted. TheV_m values are similar for all amines and lie within the range 10 to 100 mA m^–2 (for cell potential at –200 mV). The changes inK_M indicate that hydrogen bonding may be involved in the binding interaction.V_m varies with external pH in a way which suggests that an ionizable group on the transport protein with pK_a5.8 directly affects the transport rate.K_M is independent of external pH over the range 4.5 to 10.5