Abstract
Pyridoxal 5′-phosphate (PLP) inhibited the uptake of [ 3H]γ-aminobutyric acid into mouse brain synaptosomes in a competitive manner. If, however, the synaptosomes were preincubated with PLP, treated with sodium borohydride and washed, an irreversible inhibition of subsequent γ-aminobutyric acid (GABA) uptake was noted. Moreover, this time- and concentration-dependent inhibition was markedly reduced if GABA was present during the preincubation with the inhibitor. PLP had a similar effect on the binding of [ 3H]GABA to its carrier. These results suggest that the reversible and irreversible inhibition of GABA uptake is the result of an interference with the initial binding of the amino acid to its carrier. Since it is known that PLP can form Schiff bases with lysine residues of proteins, the data presented here point to the idea that essential lysine residues are present at the GABA binding site of its transporter.
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