Abstract
Transport of γ-aminobutyric acid (GABA) into synaptic plasma membrane vesicles exhibits an absolute dependency on both sodium and chloride. The requirement for chloride is not due to its ability to serve as a permeant anion. Chloride ion does not merely fulfill a need for a permeant ion since GABA accumulation still requires external chloride when a K+ diffusion potential (interior negative) is imposed across the vesicle membrane with valinomycin. Km is lowered and Vmax is raised by either sodium or chloride ions. A plot of the logarithm of the concentration ratio of GABA (internal/external) at steady state vs. the logarithm of the concentration ratio of sodium or chloride ions (both external/internal) yields straight lines with slopes of 1.50 ± 0.20 or 0.47 ± 0.02, respectively. Both GABA and tetraphenylphosphonium ion transport are affected to a similar extent by either valinomycin (enhanced) or carbonyl cyanide m-chlorophenylhydrazone (inhibited). In the presence of K+/valinomycin a plot of the logarithm of the concentration ratio of GABA (internal/external) at steady state vs. the logarithm of the concentration ratio of potassium (internal/external) yields a straight line with a slope of 0.90 ± 0.08. The simplest stoichiometry for the translocation cycle catalyzed by the GABA transporter is the influx of two sodium ions and one chloride ion per GABA zwitterion.
Original language | English |
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Pages (from-to) | 1236-1241 |
Number of pages | 6 |
Journal | Biochemistry |
Volume | 22 |
Issue number | 5 |
DOIs | |
State | Published - 1983 |