A radiochemical microassay for soluble and membrane-bound glutamate decarboxylase (GAD) is described. Up to 180 samples can be determined per day with a variation coefficient of 2%. The method detects newly synthesized gamma-amino-n-butyric acid in the picomole range and can easily be applied to other enzymes whose substrate and product differ by charge. In an aqueous homogenate of brain (1 + 10; w/v) about 15% of the total GAD activity are spun down by centrifugation (1 h, 100,000g) increasing to 35% of the total GAD activity in solutions with 8 mM calcium chloride or 100 mM potassium acetate. There is similar dependence on the cation concentration when GAD binds to phospholipid vesicles (liposomes) as well as dependence on lipid concentration and lipid composition. The coenzyme pyridoxal 5'-phosphate has no influence on GAD binding to liposomes.
