SUMMARY: An -aromatic-amino-acid aminotransferase (AT-IA) and an -aspartate aminotransferase (AT-IB) were partially purified from 47 grown on -phenylalanine as sole nitrogen source and some properties were examined. Both enzymes were active with -aromatic amino acids and -aspartate. AT-IA showed 6 to 12 times higher affinity and slightly higher 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 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 (-value, 7·9°C) as the temperature increased than AT-IA (-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 -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 catabolic role. AT-IA is probably the key enzyme for the utilization of the aromatic amino acids as sole ritrogen sources in 47.


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