Synthesis of [3H]2-amino-4-phosphonobutyric acid and characterization of its binding to rat brain membranes: a selective ligand for the chloride/calcium-dependent class of L-glutamate binding sites.

[3H]2-amino-4-phosphonobutyric acid was synthesized by the conjugate addition of 1-lithio-2-trimethylsilyethyne to diethyl ethynylphosphate followed by catalytic tritiation and hydrolysis. Radiolabelled 2-amino-4-phosphonobutyric acid binds to a distinct class of L-glutamate binding sites and does not exhibit appreciable binding to sites not displaced by L-glutamate. The binding affinity (Kd = 5.1 +/- 0.4 microM) and pharmacological profile correspond to those values obtained from physiological studies of 2-amino-4-phosphonobutyric acid inhibition of synaptic transmission, and to those values obtained in [3H]L-glutamate binding assays. [3H]2-amino-4-phosphonobutyric acid does not exhibit significant binding to the Cl-/Ca2+-independent L-glutamate binding site(s), nor to the Na+-dependent L-glutamate binding site (up to 50 mM Na+). These data provide further evidence that the physiological action of 2-amino-4-phosphonobutyric acid is mediated by the previously described Cl-/Ca2+-dependent L-glutamate binding sites, and provides an assay system which is optimal for the study of these sites.