The measurement of glutamate decarboxylase activity in brain tissues by a simple microradiometric method

A simple method was developed for the assay of l-glutamate decarboxylase (GAD) in brain homogenate and subcellular fractions. In a dual tube assay system, the incubation mixture (0.1 ml) was placed in an inner disposable culture tube (6 × 50 mm) which was then placed in an outer reusable culture tube (10 × 75 mm) and sealed with a serum cap. The procedure utilizes absorption of ¹⁴CO₂ (released from [1-¹⁴C]l-glutamate) on a Hyamine hydroxide spotted Whatman 3 MM chromatographic paper strip (5 × 40 mm) which is then counted with a MiniVial containing 5 ml of toluene scintillation solution. Only very small tissue samples (0.05-0.4 mg of protein) and minimum manipulations are necessary. The assay method reported here could be used for the measurement of any decarboxylase-catalyzed reaction, provided that suitable carboxyl labeled substrates are employed.