Purification and characterization of a high-molecular-weight endogenous glutamate-binding inhibitor in porcine brain

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Abstract
A high-molecular-weight glutamate-binding inhibitor (HGBI) from porcine brain extract was purified to homogeneity. The results of this purification process show that glutamate receptor activity can be regulated by a high-molecular-weight protein, which inhibits [3H]L-glutamate binding to excitatory amino acid (EAA) receptors. The purified HGBI appears to be a protein with a molecular weight of approximately 70 kD. The purified HGBI is negatively charged, suggesting that it may contain acidic amino acids, and most likely,L-glutamate- andL-aspartate-enriched regions, responsible for its surface charge as well as for its binding to glutamate receptors. Inhibition of [3H]L-glutamate binding by the purified HGBI is reversible, and appears to change the binding kinetics. This endogenous ligand for glutamate receptors has unique characteristics separating it from all the other ligands found so far in the EAA receptor system. This HGBI represents a new class of modulator for the EAA receptor, thus further investigation of the function and structure of the HGBI should provide new understanding of the mechanisms of EAA-mediated neurotransmission.