1. The subcellular distributions of glutamate decarboxylase and aspartate transaminase were studied in rat and guinea-pig brain. 2. Glutamate decarboxylase is localized in the synaptosome fraction. The mean density of the particles containing the enzyme is slightly greater than those derived from cholinergic neurones, though overlap is substantial. 3. The enzyme is readily released from synaptosomes by hypo-osmotic treatment, but in the presence of Ca2+, Na+ and K+ it sediments with particulate material. 4. The release and binding of the enzyme to membrane fractions by Ca2+ were investigated. 5. Aspartate transaminase is present in brain as two isoenzymes with different kinetic properties. One isoenzyme is associated with the cytoplasm and the other with mitochondria.
