Quantitation of γ-Aminobutyric Acid in Cerebrospinal Fluid Using Liquid Chromatography-Electrospray-Tandem Mass...

We describe a simple stable isotope dilution method for accurate and precise measurement of γ-aminobutyric acid (GABA), a major inhibitory neurotransmitter in human cerebrospinal fluid (CSF) as a clinical diagnostic test. Determination of CSF GABA has clinical utility in diagnosing inborn errors of GABA metabolism, specifically for deficiencies of GABA-transaminase and succinic semialdehyde dehydrogenase. Quantitation of CSF GABA is performed utilizing high-performance liquid chromatography coupled with electrospray positive ionization tandem mass spectrometry (HPLC-ESI-MS/MS). Analysis of free and total GABA requires two individual sample preparations and mass spectrometry analyses. Free GABA in CSF is determined by a 1:2 dilution with internal standard (GABA-D₂) and injected directly onto the HPLC-ESI-MS/MS system. Quantitation of total GABA in CSF requires additional sample preparation in order to hydrolyze all the conjugated GABA in the sample to free GABA. Complete hydrolysis is performed incubating sample at >100 °C in acidic conditions (hydrochloric acid) for 4 h. The sample is then further diluted 1:10 with a 90% acetonitrile/0.1% formic acid solution and injected into the HPLC-ESI-MS/MS system. Each assay is quantified using a five-point standard curve and is linear from 6 to 1000 nM and 0.63 to 80 μM for free and total GABA, respectively.