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 x 50 mm) which was then placed in an outer reusable culture tube (10 x 75 mm) and sealed with a serum cap. The procedure utilizes absorption of 14CO2 (released from [l-14C]L-glutamate) on a Hyamine hydroxide spotted Whatman 3MM chromatographic paper strip (5 x 40 mm) which is then counted with a MiniVial containing 5 ml of toluene scintillation solution. Only very small tissue samples (0.05 to 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. (U.S.)