Increased Intra- and Extracellular Concentrations of γ-Glutamylglutamate and Related Dipeptides in the Ischemic Rat Striatum: Involvement of γ-Glutamyl Transpeptidase

Abstract: The present work relates to the possibility that the ATP-independent enzyme γ-glutamyl transpeptidase (EC 2.3.2.2), which has been postulated to be part of an amino acid uptake system, is active during cerebral ischemia. This was evaluated in the ischemic rat striatum by determination of intra- and extracellular concentrations of γ-glutamyl dipeptides (the products of the transpeptidation) and glutathione (the physiological γ-glutamyl donor). An ischemic period (0–30 and 31–60 min) resulted in prominent increases in the respective concentration of extracellular γ-glutamylglutamate (24- and 67-fold), γ-glutamyltaurine + γ-glutamylglycine (5.8- and 19-fold), and γ-glutamylglutamine (2.6- and 6.8-fold) as revealed using in vivo microdialysis. The changes coincided with increased respective extracellular concentrations of glutamate (83- and 115-fold), taurine (17- and 25-fold), glycine (4.6- and 6.1-fold), and glutamine (1.7- and 2.1-fold). Furthermore, under anoxic conditions in vitro (0–30 and 0–60 min), respective striatal tissue concentrations were increased for γ-glutamylglutamate (20- and 17-fold), γ-glutamyltaurine (6.7- and 11-fold), γ-glutamylglutamine (1.7- and 1.2-fold), and γ-glutamylglycine (14- and 18-fold), whereas glutathione levels were, on an average, decreased by ∼350 µ M . In summary, γ-glutamyl transpeptidase is involved in de novo dipeptide synthesis in the mammalian brain during anoxic conditions, indicating transport of amino acids such as glutamate.