ISOLATION OF HYDROPHOBIC PROTEINS BINDING AMINO ACIDS: <scp>l</scp>‐ASPARTIC ACID‐BINDING PROTEIN FROM THE RAT CEREBRAL CORTEX

Abstract

Abstract— Lyophilized rat cerebral cortex was treated with chloroform‐methanol (2:1, v/v), and the extracted hydrophobic proteins (i.e. proteolipids) were separated by column chromatography on Sephadex LH‐20. The first peak of protein, eluting with chloroform in the void volume, had high affinity binding for l ‐[ 14 C]aspartic acid. The saturation of the binding showed three saturable sites with apparent dissociation constants of 0.2 μ m , 10 μ m and 50 μ m . The binding capacities of the three sites were 2.8, 132 and 617 nmol/mg of protein, respectively. There were 8.0 nmol of high affinity binding sites for l ‐aspartic acid and 1.53 nmol for l ‐glutamic acid per g of fresh tissue in the cerebral cortex of the rat. Differentiation between binding of l ‐aspartic and l ‐glutamic acid was clearly established by cross‐binding and competition experiments with agonists and antagonists. It is suggested that the isolated protein fraction may correspond to a synaptic receptor and not to the transport system. It is concluded that in the cerebral cortex there is a separate receptor for l ‐aspartic acid. This is further support to the possible role of this amino acid as a central excitatory transmitter.