Synaptosomal high affinity transport systems for essential amino acids in rat brain cortex.

High affinity uptake systems have been identified for the transport of leucine, isoleucine and methionine in synaptosomes but not in the mitochondria of rat cerebral cortex. These systems were found to be different from the conventional low affinity uptake systems in terms of their affinity, sodium dependency and the rate of transport. As these amino acids have no neurotransmitter function, it is suggested that high affinity uptake systems might be involved in the transport of essential amino acids and maintain a minimal level in brain when the concentrations of these amino acids are low in blood (as in starvation, malnutrition). As some of these amino acids serve as precursors for neurotransmitters, such as glutamate (leucine, isoleucine), taurine (methionine), it is also suggested that high affinity uptake systems for the essential amino acids might also replenish the precursor pools of neurotransmitter amino acids.