Partial Purification and Some Properties of an Aromatic-amino-acid and an Aspartate Aminotransferase in Brevibacterium linens 47

An l-aromatic-amino-acid aminotransferase (AT-IA) and an l-aspartate aminotransferase (AT-IB) were partially purified from Brevibacterium linens 47 grown on l-phenylalanine as sole nitrogen source and some properties were examined. Both enzymes were active with l-aromatic amino acids and l-aspartate. AT-IA showed 6 to 12 times higher affinity and slightly higher V max for the aromatic amino acids than for aspartate. AT-IB had higher affinity for tryptophan and tyrosine than for aspartate. However, this enzyme showed 6 to 10 times higher V max for the latter than for the aromatic amino acid substrates. Both enzymes had similar pH (8·5-9·0) and temperature (37–40 °C) optima, but they differed in their molecular weights (126000 for IA and 81000 for IB) and markedly in their thermostability. At 50 °C, AT-IB was 14 times more rapidly inactivated and its inactivation rate also increased more rapidly (z-value, 7·9 °C) as the temperature increased than AT-IA (z-value, 15·5 °C). The cofactor pyridoxal-5′-phosphate was tightly bound to both enzymes. Two enzymes co-eluted on DEAE-Trisacryl M column chromatography at pH 7·5 and could be separated by chromatography on a hydroxyapatite (HA-Ultrogel) column at the same pH. Chromatography on hydroxyapatite of AT-I from cells grown on l-phenylalanine and on ammonium sulphate revealed that AT-IA was present only in phenylalanine grown cells. AT-IB was present in both extracts at similar levels, suggesting that it is constitutive. The inducibility of IA suggested that it has an in vivo catabolic role. AT-IA is probably the key enzyme for the utilization of the aromatic amino acids as sole ritrogen sources in B. linens 47.