Preferential "Binding" of γ-Aminobutyric Acid and Glycine to Synaptosome-Enriched Fractions of Rat Cerebral Cortex and Spinal Cord

Abstract

The "binding" activities of 3 H-γ-aminobutyric acid ( 3 H-GABA) and 14 C-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 3 H-GABA and 14 C-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" (B max ) for GABA, at 0 °C, was estimated to be about 65 nmol of GABA/g cortex, and its "binding" constant (K B ) was about 1.8 × 10 −5 M. For the "binding" of glycine to fractions of spinal cord at 0 °C, the respective values for B max and K B 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.