The use of a monoclonal-antibody immunoaffinity column for the rapid isolation of 4-aminobutyrate aminotransferases (EC from rabbit brain and liver is described. Homogeneous enzyme protein is eluted from the immunoadsorbent with 100mM-citrate buffer, pH5, and remains stable at 4 degrees C for several days. One such column (bed volume 8 ml) has been used 40 times in a 9-month period to isolate 10-15 units of enzyme activity (specific activity approx. 3.5-7.5 units/mg) per extraction. Kinetic and spectral analysis of the enzymes from the two tissues revealed a close similarity. Sodium dodecyl sulphate/polyacrylamide-gel electrophoresis showed the isolated enzyme to have a monomeric Mr of 52 000, and this was confirmed by h.p.l.c. gel exclusion at pH 5.0. The results of Sephadex G-100 chromatography at different pH values are taken to indicate that the enzyme behaves as a dimer at pH 7.0 and above, but as a monomer at pH 5.0. 4-Aminobutyrate aminotransferase isolated from the brain by the procedure of Fowler & John [(1981) Biochem. J. 197, 149-152] is more stable than the immunoaffinity-purified material, and has been shown to contain a contaminant protein of Mr 84 000 that exhibits succinic semialdehyde dehydrogenase activity.

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