Distribution and tissue specificity of glutamate decarboxylase (EC 4.1.1.15).

Abstract— The activity of L–glutamate decarboxylase (EC 4.1.1.15) (GAD) in various mouse tissues was determined by five different methods, namely, the radiometric CO₂ method, column separation, electro–phoretic separation, the filtration method, and amino acid analysis. Results from the latter four methods agreed well, showing that brain had the highest activity, 4.27 nmol/min/mg protein (100%), followed by heart (7.4%), kidney (6.3%) and liver (1.5%). Measurement of brain GAD using the radiometric CO₂ assay method agreed with the other techniques. However, in heart, kidney, and liver, the GAD activities measured by the CO₂ method were about 3–4 times higher than those obtained by the GABA method, suggesting that the CO₂ method does not give a valid measurement of GAD activity in a crude non–neural tissue preparation. GAD activity also was detected in adrenal gland but not in pituitary, stomach, testis, muscle, uterus, lung, salivary gland, or spleen. GAD from brain, spinal cord, heart, kidney and liver were further compared by double immunodiffusion, enzyme inhibition by antibody, and microcomplement fixation using antibody against GAD purified from mouse brain. GAD from brain and spinal cord appear to be identical as judged from the following results: the immunoprecipitin bands fused together without a spur; the enzyme activity was inhibited by anti–GAD to the same extent; and the microcomplement fixation curves were similar in both the shape of the curve and the extent of fixation. No crossreactivity was observed between GAD from heart, kidney or liver and antibody against brain GAD in all the immunochemical tests described above, suggesting that GAD in non–neural tissues is different from that in brain and spinal cord.