Purification and studies on some properties of the 4-aminobutyrate : 2-oxoglutarate transaminase from rat brain.

Using various chromatographic procedures, 4-aminobutyrate : 2-oxoglutarate transaminase from rat brain has been purified 2400 times with respect to the initial brain homogenate. The purified protein, which has a specific activity of 10 mumol times min -1, x mg-1 gave a single band by acrylamide gel electrophoresis and isoelectric focusing. It has a molecular weight of 105000 +/- 5000 and an isoelectric point of 6.8. In the presence of 0.1% sodium dodecylsulphate, a single protein band is seen on polyacrylamide gel, corresponding to a molecular weight of 57000 +/- 5000. N-terminal analysis reveals two chains with the same N-terminal amino acid, thus the enzyme may be considered as a dimer consisting of two identical subunits. The pH optimum for enzyme activity is 8.5. Studies of the enzymic reaction show that the general mechanism is of the ping-pong bi-bi model. The Km for 2-oxoglutarate at saturating 4-aminobutyrate extrapolated to saturating 2-oxoglutarate concentration is 4 mM. 2-Oxoglutarate competitively inhibits the enzyme with respect to 4-aminobutyrate, with a Ki of 1.8 times 10(-4) M. The same phenomenon is seen for the reverse reaction where the Ki is 6.6 times 10(-4) M for succinic semi-aldehyde.