Tryptophan Transport into Plasma Membrane Vesicles Derived from Rat Brain Synaptosomes

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Abstract
Tryptophan uptake by membrane vesicles derived from rat brain was investigated. The uptake is dependent on the Na+gradient [Na+] outside > [Na+] inside and is maximal when both Na+and Cl−are present. The uptake represents transport into an os‐motically active space and not a binding artifact, as indicated by the effect of increasing the medium osmo‐larity. 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.