beta-Alanine aminotransferase from rabbit liver has been purified 1,700-fold over the initial liver extract. The purified enzyme was shown to be homogeneous by disc electrophoresis and SDS polyacrylamide electrophoresis. The molecular weight of the purified enzyme determined by gel filtration was 95,000 +/- 5,700 and the subunit molecular weight was 48,000 +/- 2,100. The enzyme showed absorption maxima at 282, 330, and 414 nm and contained only 1 mol of pyridoxal 5'-phosphate/mol of dimer. The pH optimum for enzyme activity was 8.8 and the Km values for beta-alanine and 2-oxoglutaric acid were calculated to be 3.9 and 1.4 mM, respectively. The enzyme catalyzed transamination of various omega-amino acids with 2-oxoglutaric acid, which was a favourable amino acceptor. beta-Alanine, gamma-aminobutyric acid, and beta-aminoisobutyric acid, which are naturally occurring substrates, were preferred amino donors, but taurine, alanine, ornithine, spermine, and spermidine were not. 6-Azauracil inhibited the enzyme activity with a Ki of approximately 1.5 mM. From the above properties, beta-alanine aminotransferase from rabbit liver was seen to closely resemble with 4-aminobutyrate aminotransferase from liver and brain.

Among uracil derivatives investigated, 6-azauracil, 6-azathymine, and 5-iodouracil were found to be potent inhibitors of purified rabbit liver 4-aminobutyrate aminotransferase while 6-azauridine and 6-azauridine 5'-phosphate were not. The enzyme inhibited by 6-azauracil was reactivated by dialysis but not by addition of pyridoxal 5'-phosphate. 6-Azauracil acted as a non-competitive inhibitor with respect to beta-alanine as well as 2-oxoglutaric acid, and had a K1 of approximately 0.7 mM at pH 7.3. The kinetic data suggested that 2-oxoglutaric acid acted as an inhibitor as well as an amino acceptor for the enzyme; a catalytic site was associated with an apparent Km of 0.15 mM for 2-oxoglutaric acid and a low affinity site was associated with an I50 of approximately 5 mM for the 2-oxo acid. With inhibitory concentrations of 2-oxoglutaric acid as substrate the inhibitory effect of 6-azauracil was considerably diminished. From these findings, the inhibitory effect of 6-azauracil was revealed to be different from that of structural analogs of 4-aminobutyric acid showing that 6-azauracil is a new type of 4-aminobutyrate aminotransferase inhibitor.