Formation of extracellular glutamate from glutamine: exclusion of pyroglutamate as an intermediate.
A 4.6-fold increase in interstitial glutamate was observed following the reverse microdialysis of 5 mM glutamine into the rat hippocampus. Two possible mechanisms of glutamine hydrolysis were examined: (a) an enzymatic glutaminase activity and (b) a non-enzymatic mechanism. Injection of 14C-glutamine at the site of microdialysis followed by microdialysis with artificial cerebral spinal fluid allowed isolation of 14C-glutamine (63%), 14C-glutamate (14%), and a compound tentatively identified as pyroglutamate (22%). In this study, we determined if non-enzymatic pyroglutamate formation from glutamine contributed to the synthesis of glutamate. Pyroglutamate is in chemical equilibrium with glutamate, although under physiological conditions, the chemical equilibrium is strongly in the direction of pyroglutamate. In vitro stability studies indicated that 14C-glutamine and 14C-pyroglutamate are not subject to significant non-enzymatic breakdown at pH 6.5-7.5 at 37 degrees C for up to 8 h. Reverse microdialysis with 1 mM pyroglutamate did not increase interstitial glutamate levels. Following injection of 14C-pyroglutamate and microdialysis, radioactivity was recovered in 14C-pyroglutamate (88%) and 14C-glutamine (11%). Less than 1% of the radioactivity was recovered as glutamate. Our data do not support a role of pyroglutamate as an intermediate in the formation of extracellular glutamate following the infusion of glutamine. However, it confirms that pyroglutamate, a known constituent in brain, is actively metabolized in brain cells and contributes to glutamine in the interstitial space.
Mena
Fernando V
FV
Department of Pediatrics, University of Maryland School of Medicine, Bressler Research Building, Room 10-035, 655 West Baltimore Street, Baltimore, MD 21201-1559, USA.
Baab
Peter J
PJ
Zielke
Carol L
CL
Huang
Yinyin
Y
Zielke
H Ronald
HR
D000818:Animals; D002247:Carbon Isotopes / Q000378:metabolism; D002851:Chromatography, High Pressure Liquid / Q000379:methods; D003956:Dialysis / Q000379:methods; D005110:Extracellular Space / Q000378:metabolism*; D018698:Glutamic Acid / Q000378:metabolism*; D005972:Glutaminase / Q000378:metabolism; D005973:Glutamine / Q000378:metabolism*; D006624:Hippocampus / Q000187:drug effects*; D008297:Male; D011761:Pyrrolidonecarboxylic Acid / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013997:Time Factors
D002247:Carbon Isotopes; D005973:Glutamine; D018698:Glutamic Acid; D005972:Glutaminase; D011761:Pyrrolidonecarboxylic Acid