ISOLATION OF HYDROPHOBIC PROTEINS BINDING AMINO ACIDS. STEREOSELECTIVITY OF THE BINDING OF L‐[¹⁴C]GLUTAMIC ACID IN CEREBRAL CORTEX

Abstract

Abstract From the total lipid extract of ncrve‐ending membranes or the homogenate of cerebral cortex a hydrophobic protein fraction binding L‐[ 14 C]glutamic acid was separated by chromatography on Sephadex LH20. This protein could only be partially separated from the [ 14 C]GABA‐binding protein and from the lipids that are present in the fraction; however, it was demonstrated that both amino acids bind to different sites. The saturation of the binding showed a high (Kd 1 = 0.3μM), a medium (Kd, = 5 μM) and a low (Kd, = 55 μM) affinity binding site. The high affinity binding site had a binding capacity of 0.53 nmol/mg of protein and was highly stereoselective for the L‐enantiomer. The binding of L‐[ 14 C]glutamic acid was not inhibited by GABA, was slightly inhibited by glycine and glutamine and was strongly inhibited in a progressive order by DL‐a‐methylglutamic acid, L‐nuciferine, L‐aspartic acid and L‐glutamic acid diethyl ester. These results are compared with those previously obtained with the L‐glutamic acid‐binding protein isolated from crustacean muscle. The stereoselectivity of the binding and the possible role of this protein in synaptic transmission are discussed.