Determination of alpha-aminoadipic acid in brain, peripheral tissues, and body fluids using GC/MS with negative chemical ionization.

alpha-Aminoadipic acid (alphaAA) is a structural homolog of the excitatory amino acid glutamate and a natural product of lysine metabolism in mammalian cells. Under experimental conditions, alphaAA can influence various elements of glutamatergic neurotransmission. Moreover, as a selective inhibitor of kynurenine aminotransferase II, alphaAA is capable of decreasing the levels of the neuroinhibitory metabolite kynurenic acid in the brain. We now describe the identification of this potential endogenous neuromodulator in tissues and body fluids by gas chromatography/mass spectrometry (GC/MS) analysis of its pentafluorobenzyl (PFB) derivative. alphaAA was recovered from the GC column with a retention time of approximately 7 min. Subsequent MS analysis using electron capture with negative ionization revealed two separate ions for alphaAA (m/z 520, approximately 45% and m/z 322, approximately 55%). Both of these ions were positively identified with two different GC methodologies. In the rat, alphaAA levels ranged from 5 to 30 microM in various brain areas and from 8 to 40 microM in peripheral organs, whereas serum and urine contained only 1-2 microM alphaAA. Levels in the human brain were 18.7+/-2.4 microM (cortex) and 18.0+/-1.7 microM (striatum) alphaAA (n=9 each), and the mouse forebrain contained 8.3+/-1.9 microM alphaAA (n=6). Neuronal depletion, caused in rats by an intrastriatal injection of NMDA (300 nmol/2.5 microl), did not alter the striatal content of alphaAA, indicating that brain alphaAA resides at least in part in glial cells. alphaAA may therefore function as a glia-derived modulator of excitatory neurotransmission.