Inhibition byl‐phenylalanine of tryptophan transport by synaptosomal plasma membrane vesicles: Implications in the pathogenesis of phenylketonuria

Phenylalanine is accumulated in the genetically linked deficiency phenylketonuria. The effect of L-phenylalanine on the transport of tryptophan was studied using membrane vesicles from rat-brain synaptosomes. Phenylalanine at similar concentrations to those found in phenylketonuric patients competitively inhibits tryptophan uptake, with a Ki of the same order as the Km for tryptophan. This inhibition could be responsible for the depletion of serotonin found in phenylketonuria.