Specific [(3)H]tryptophan binding sites in rat brain

[(3)H]Tryptophan binds to a single population of sites in the rat cortical synaptosomal membranes. The binding is reversible and follows the law of mass action. By saturation studies using increasing concentration of [(3)H]tryptophan with decreasing specific radioactivity, the apparent K(d) obtained was approx. 0.8 ?M and the B(max) 110 pmol/mg protein. However, the IC(50) obtained for unlabelled tryptophan in displacing [(3)H]tryptophan binding (3.5 nM) was 0.26 ?M. All six naturally occurring aromatic amino acids studied displaced [(3)H]tryptophan binding with tryptophan and phenylalanine showing higher apparent affinity than histidine, tyrosine, dihydroxyphenylalanine and 5-hydroxytryptophan. These binding sites are proteins in nature as they are sensitive to trypsin and ?-chymotrypsin. It is observed that about 37% of the sites seem to be protected from the proteolytic enzymes by the membrane structure. Furthermore, they are extremely sensitive to phospholipase A(2) presumably because altered membrane phospholipids conduce a conformational change in the binding protein. A considerable degree of stereospecificity was demonstrated with the affinity for l-tryptophan about 90 times higher than that for d-tryptophan. The affinity for l-phenylalanine was 8 times higher than that for d-phenylalanine. Ligand specificity for the aromatic amino acids is remarkable as hydrocinnamic acid, 2-phenylethylamine, 5-hydroxytryptamine, histamine, dopamine, ?-aminobutyric acid, glutamic acid and taurine did not displace [(3)H]tryptophan binding. Therefore, these sites are termed aromatic amino acid binding sites (AABS). Whether or not AABS are involved in neuromodulation at the synapse awaits clarification. If so, the endogenous ligand for the AABS may well be tryptophan.