Separate binding sites in rat brain synaptic membranes for l-cysteine sulfinate and for l-glutamate

Binding of l-[(3)H]cysteine sulfinic acid (CSA) and l-[(3)H]glutamate were compared in various subcellular fractions and in the presence of a variety of pharmacological and ionic manipulations in order to test the possibility that the two amino acids possessed separate binding sites. The specific l-[(3)H]cysteine sulfinate binding was found to be enriched maximally in medium and high density synaptic membranes, while the crude mitochondrial synaptosomal fraction displayed the highest l-[(3)H]glutamate binding. The ratio of l-[(3)H]cysteine sulfinate binding/l-[(3)H]glutamate binding was variable across brain regions. Several compounds differentially affected l-[(3)H]cysteine sulfinate and l-[(3)H]glutamate binding. l-cysteine sulfinate was the most potent displacer regardless of the binding considered. Finally, while cations produced qualitatively similar effects on the binding of the two amino acids, quantitative differences were evident. In sum, these data revealed the complexity of l-[(3)H]cysteine sulfinate and l-[(3)H]glutamate binding. They suggest the existence of several binding sites and that some of these are shared by both substances. However, the results also indicate that separate binding sites for the two amino acids exist in synaptic membrane, giving further support to the hypothesis that cysteine sulfinate serves a neurotransmitter role in the central nervous system.