Stereospecificity and electrogenicity of amino acid transport in Riccia fluitans

Abstract

In the aquatic liverwort Riccia fluitans, membrane depolarization (ΔΨ_m), change in membrane conductance (Δg_m), and current-voltage (I-V) characteristics in the presence of different amino acids as well as the uptake of ¹⁴C-labeled amino acids were measured. L-isomers of the tested amino acids generate larger electrical effects (ΔΨ_m, Δg_m) than D-isomers, and the I-V characteristics show that the positive electrical inward-current of 20 mA m^-2 generated by 0.5 mM D-serine is only about 50% of the current generated by adding 0.5 mM L-serine. Whereas α- and β-amino acids rapidly depolarize the membrane to the same extend, with γ-aminobutyric acid (γ-AB) and dipeptides no significant electrical effects have been measured. The uptake kinetics of ¹⁴C-labeled amino acids display three components: (I) A saturable high-affinity component with K_s-values of 48 μM D-alanine, 12 μM α-aminoisobutyric acid (AIB), 9 μM L-alanine, 8 μM L-proline, and 6 μM L-serine, respectively; (2) an apparently linear low-affinity component, and (3) an also linear but unspecific component at concentrations >20 times the given K_s-value. Uptake of ¹⁴C-labeled AIB can be inhibited competitively by all tested neutral amino acids, the L-isomers being more effective than the D-isomers, as well as by ammonium or methylamine. Vice versa, AIB competitively inhibits uptake of L-serine and L-alanine. It is concluded that an uncharged stereospecific carrier for the investigated amino acids exists in the plasmalemma of Riccia fluitans. Accumulation ratios of about 50 suggest secondary active transport driven by a transmembrane electro-chemical gradient (mainly ΔΨ_m) which is generated by the electrogenic proton pump. It is suggested that this carrier binds to the amino group forming either a charged binary complex with positively charged amines (Felle 1980), or an uncharged complex with γ-AB or dipeptides, whereas electrogenic transport of α- and β-amino acids is mediated by a ternary carrier complex, probably charged by a proton.