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 analysi s of both the tissue and the surrounding medium of incubated rat hippo campal slices. High potassium concentrations (50 mM; 2 x 4 min) increa sed the medium concentration of gamma-glutamylglutamate (maximal net e fflux 0.07+/-0.06 pmol/mg protein/min; n = 8+/-SD) with a relative tim e delay compared to the increase in glutamate (maximal net efflux 264/-88 pmol/mg protein/min). Release of gamma-glutamylcysteine, the glut athione precursor, demonstrated an immediate response and gradually ap proached prestimulus levels (maximal net efflux 0.36+/-0.13 pmol/mg pr otein/min). Addition of acivicin (0.2 mM), a gamma-glulamyl transpepti dase (EC 2.3.2.2.) blocker, during preincubation for 45 min reduced th e tissue concentrations (pmol/mg protein) of gamma-glutamylglutamate ( 19.4+/-8.2 (control) vs. 5.8+/-3.6 (+acivicin)), gamma-glutamylglutami ne (40.3+/-6.7 vs. 25.7+/-4.2 pmol/mg protein), glutamine (9.9+/-2.0 v s. 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 ef flux of gamma-glutamylglutamine (0.79+/-0.19 vs. 0.21+/-0.07 pmol/mg p rotein/min) whereas that of the glutathione was increased (4.7+/-1.0 v s. 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 blocke d. The results demonstrate differential efflux patterns of gamma-gluta myl dipeptides from brain slices and show that in vitro the activity o f gamma-glutamyl transpeptidase regulates extracellular concentrations of glutathione, gamma-glutamylglutamine and gamma-glutamylglutamate. Copyright (C) 1996 Elsevier Science Ltd.