Tryptophan Transport into Plasma Membrane Vesicles Derived from Rat Brain Synaptosomes

Tryptophan uptake by membrane vesicles derived from rat brain was investigated. The uptake is dependent on the Na+ gradient [Na+] outside greater than [Na+] inside, and is maximal when both Na+ and Cl- are present. The uptake represents transport into an osmotically active space and not a binding artifact, as indicated by the effect of increasing the medium osmolarity. The uptake of tryptophan is stimulated by a membrane potential (interior negative) as demonstrated by the effects of the ionophores valinomycin and carbonyl cyanide m-chlorophenylhydrazone and anions with different permeabilities. Kinetic data show that tryptophan is accumulated by two systems with different affinities. Ouabain, an inhibitor of Na+,K+-activated ATPase, does not affect tryptophan transport. The uptake of tryptophan is inhibited by high concentrations of phenylalanine, tyrosine, leucine, and 3,4-dihydroxyphenylalanine.