Development of a Stable-Isotope Dilution Assay for g-Aminobutyric Acid (GABA) Transaminase in Isolated Leukocytes and Evidence That GABA and b-Alanine Transaminases Are Identical

Background: Several methods have been published for measuring g-aminobutyric acid transaminase (GABA-T) activity, but these methods are either impracticable because of the use of radioisotopes or insufficiently sensitive to determine small enzyme activities in leuko-cyte extracts. We developed a direct and sensitive en-zyme method. Methods: We developed a stable-isotope dilution method for the measurement of [15N]glutamic acid de-rived from [15N]GABA and a-ketoglutaric acid, cata-lyzed by GABA-T. The method for analysis of [15N]glu-tamic acid comprised a solid-phase extraction procedure to isolate this analyte from incubation samples. After derivatization, [15N]glutamic acid was quantified by gas chromatography–mass spectrometry relative to its 2H5