Cysteine Sulfinic Acid in the Central Nervous System: Specific Binding of [35S]Cysteic Acid to Cortical Synaptic Membranes‐An Investigation of Possible Binding Sites for Cysteine Sulfinic Acid

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Abstract
Specific binding sites for cysteinesulfinic acid, an excitatory amino acid, in crude synaptic membrane fractions of rat cerebral cortex were examined, using L-[35S]cysteic acid as a ligand. Two specific binding systems of [35S]cysteic acid were found, one Na+-dependent and the other Na+-independent. The Na+-independent specific binding of [35S]cysteic acid was saturable, with a Kd of 474 nM and Bmax [maximum binding sites] of 3.29 pmol/mg protein. The binding was optimal at pH 7.4 and at 37.degree. C. Treatment of the membranes with proteases, concanavalin A or Triton X-100 markedly reduced the binding. Of various compounds related to cysteic acid, L-cysteinesulfinic acid was the most effective competitor of this binding. The existence of an Na+-independent specific binding site for cysteic acid was indicated in the synaptic membrane of rat cerebral cortex, which may be different from that for glutamate. Possible involvement of cysteinesulfinic acid as an endogenous ligand for this binding site is discussed.