gamma-glutamyl peptides and related amino acids in rat hippocampus in vitro: Effect of depolarization and gamma-glutamyl transpeptidase inhibition

The concentrations of gamma-glutamylglutamate, gamma-glutamylglutamine, gamma-glutamylcysteine, glutamate, aspartate, glutamine, cyst(e)ine and glutathione (including disulfides) were determined by HPLC analysis of both the tissue and the surrounding medium of incubated rat hippocampal slices. High potassium concentrations (50 mM; 2 x 4 min) increased the medium concentration of gamma-glutamylglutamate (maximal net efflux 0.07+/-0.06 pmol/mg protein/min; n = 8+/-SD) with a relative time delay compared to the increase in glutamate (maximal net efflux 264+/-88 pmol/mg protein/min). Release of gamma-glutamylcysteine, the glutathione precursor, demonstrated an immediate response and gradually approached prestimulus levels (maximal net efflux 0.36+/-0.13 pmol/mg protein/min). Addition of acivicin (0.2 mM), a gamma-glulamyl transpeptidase (EC 2.3.2.2.) blocker, during preincubation for 45 min reduced the tissue concentrations (pmol/mg protein) of gamma-glutamylglutamate (19.4+/-8.2 (control) vs. 5.8+/-3.6 (+acivicin)), gamma-glutamylglutamine (40.3+/-6.7 vs. 25.7+/-4.2 pmol/mg protein), glutamine (9.9+/-2.0 vs. 4.6+/-1.2 nmol/mg protein) and cysteine (1.0+/-0.2 vs. 0.56+/-0.18 nmol/mg protein). Incubation with acivicin (0.2 mM) reduced the net efflux of gamma-glutamylglutamine (0.79+/-0.19 vs. 0.21+/-0.07 pmol/mg protein/min) whereas that of the glutathione was increased (4.7+/-1.0 vs. 20+/-3 pmol/mg protein/min). The medium concentrations of glutamate in both low and high potassium were unaffected by acivicin, while the high potassium induced increase in gamma-glutamylglutamate was blocked. The results demonstrate differential efflux patterns of gamma-glutamyl dipeptides from brain slices and show that in vitro the activity of gamma-glutamyl transpeptidase regulates extracellular concentrations of glutathione, gamma-glutamylglutamine and gamma-glutamylglutamate. Copyright (C) 1996 Elsevier Science Ltd.