Gas chromatographic—mass spectrometric determination of glutamic acid decarboxylase activity in subregions of rat brain

A quantitative gas chromatographic—mass spectrometric method has been developed for the determination of glutamic acid decarboxylase (GAD) activity in subregions of rat brain. The five subregions analyzed, weighing approximately 2.51 mg each, were globus pallidus, entopeduncular nucleus, ventromedial thalamus, and substantia nigra medial and lateral. The activity of the GAD enzyme has been determined indirectly by measurement of γ-aminobutyric acid (GABA) using γ-[2,2-²H₂]aminobutyric acid as the internal standard. Both compounds were quantitatively converted to trimethylsilyl-GABA and trimethylsilyl-[²H₂]GABA in 90 min with hexamethylchlorosilane, trimethylchlorosilane, pyridine and N,O-bis(trimethylsilyl)trifluoroacetamide silylating agents. Using selective ion monitoring and electron impact ionization at 70 eV, the limit of detection was 15 ng GABA per mg tissue. This method is compared with a fluorimetric procedure.