Binding of [3H]muscimol to calf cerebrocortical synaptic membranes and the effects of sulphur-containing convulsant and non-convulsant compounds.

Endogenous and xenobiotic sulphur-containing convulsant and non-convulsant compounds containing structural moieties of, or bearing a structural resemblance to, GABA and homocysteine were tested in binding studies for their potency in displacing the GABA-mimetic [3H]muscimol from specific, high-affinity sites (Kd = 3.6 nM; Bmax = 3.94 pmol/mg protein) on freeze-thawed, Triton-treated calf-brain synaptic membranes. The xenobiotic convulsants, 4-mercaptobutyric acid (MBA), 3-mercaptopropionic acid (3-MPA) and 2-mercaptopropionic acid (2-MPA) were found to be two-site competitive inhibitors exhibiting apparent inhibition affinity constants (Kiapp) of 5000 microM, 3750 microM, and 4800 microM, respectively; while homocysteic acid (Kiapp = 4800 microM) was shown to be a one-site partial competitive inhibitor. Intermediary metabolites of methionine: S-adenosyl-L-homocysteine, L-cysteine, the convulsant L-homocysteine, and its non-convulsant disulphide oxidation product, homocystine, were found to be one-site partial competitive inhibitors exhibiting Kiapp values of 5750 microM, 8350 microM, 5000 microM, and 510 microM, respectively. The endogenous anticonvulsant neuroeffector, taurine, and the tripeptide, reduced glutathione (GSH) were shown to be, respectively, one-site (Ki = 20 microM) and two-site (Kiapp = 4300 microM) competitive inhibitors of [3H]muscimol binding. These findings are discussed with regard to a previously proposed mechanism for the convulsant action of homocysteine.