Presynaptic Binding of Amino Acids: Characterization of the Binding and Dissociation Properties of Taurine, Gaba, Glutamate, Tyrosine and Norleucine

Abstract

Calf brain synaptic plasma membranes (SPM) were saturated under extracellular conditions with [¹⁴C] glutamic acid, [¹⁴C] GABA, [¹⁴C] taurine, [³H] tyrosine or [¹⁴C] norleucine and the resulting labelled membrane complexes fractionated by a differential extraction procedure using 0.9% NaCl, distilled water, n-butanol-water, 0.05 M NaOH and 0.5% Triton X-100 solutions in this order. Free and protein-bound radioactivities were measured in the extracts. Glutamate had the highest and taurine the lowest affinity to be bound to the original membranes, the ratios of bound/free label being 1/40 for glutamate, about 1/200 for GABA and norleucine, 1/673 for tyrosine and 1/1447 for taurine. The molar binding capacities were lowest for tyrosine and highest for glutamate. About two thirds of the proteins of SPM could be solubilized, and one third of them remained insoluble after the extractor series. Triton X-100 was the most effective solubilizer, liberating about 24% of the membrane proteins, but only 4-7% of the label. Distilled water solubilized about 15%, 0.9% NaCl 10% and 0.05 M NaOH also about 10% of the proteins. NaOH was the most potent solubilizer for the protein-bound label (50-70%), even though used in the fourth position in the series. On the other hand, 0.9% NaCl and distilled water extracted the mostly loosely-bound radioactivity from SPM. It appeared that GABA and norleucine had in general the highest affinity to dissociate off, and taurine, tyrosine and glutamate the highest affinity to associate with the protein fractions of SPM.