The "binding" activities of 3H-γ-aminobutyric acid (3H-GABA) and 14C-glycine in synaptosome-enriched factions of rat cerebral cortex and spinal cord have been compared in an isosmotic sucrose medium (containing 32 mM NaCl) over a wide range of concentrations using differential centrifugation and a double-isotope method. It was shown that GABA was bound preferentially to particles of cerebral cortex whereas glycine "binding" occurred preferentially to those of spinal cord. At equimolar concentrations of 3H-GABA and 14C-glycine in the medium (0.8 × 10−7 M – 0.8 × 10−4 M) the order of potency of "binding" was as follows: GABA, cortex > GABA, cord [Formula: see text] glycine, cord > glycine, cortex. Ratios of GABA/glycine were always much greater than 1.0 for cerebral cortex when these amino acids were present in the medium at 10 −7–10−4 M, whereas these ratios were about 1.0 for spinal cord at all concentrations of the amino acids that were used. The "maximal binding capacity" (Bmax) for GABA, at 0 °C, was estimated to be about 65 nmol of GABA/g cortex, and its "binding" constant (KB) was about 1.8 × 10−5M. For the "binding" of glycine to fractions of spinal cord at 0 °C, the respective values for Bmax and KB were 43 nmol of glycine/g, cord, and 3.3 × 10−5 M. It is suggested that these energy-independent "binding" mechanisms may be involved in the inactivation and receptor interactions of these "active" amino acids.
Determination of amino acids by double isotope-dilution technique
