Amino Acid Aminotransferases in an Amino Acid-producing Bacterium, Brevibacterium flavum

抄録

Five aminotransferases, TA-A, -B, -T, -P_I, -P_II, which might participate in the biosyntheses of aspartate, branched-chain amino acids, tyrosine, and phenylalanine, respectively, were separated from one another in Brevibacterium flavum, and examined for their keto-acid substrate specificities and the apparent Michaelis constants (Km') for their main substrates. TA-A specifically reacted with oxalacetate, among the keto-acids tested, and accounted for 90% of the total activity in the cellular extracts. TA-B was found to be the only aminotransferase that reacted with branchedchain keto-acids, and also showed some activity with prephenate and phenylpyruvate but having a much larger Km' than the other enzymes. TA-T specifically reacted with prephenate and oxalacetate, among the keto-acids tested, and accounted for 80 and 2% of the total activities of the cellular extracts, respectively, but showed a much larger Km' for oxalacetate than did TA-A. TA-P_I and -P_II showed similar substrate-specificities and Km's to each other. They were active with phenylpyruvate, p-hydroxylphenylpyruvate, prephenate, and oxalacetate. Their activity with phenylpyruvate accounted for only 36% of the total activity, but showed a markedly smaller Km' than did TA-B having 64% of the total activity. An aspartate auxotroph, No. 102-7, was found to lack TA-A and TA-P_I, and to have a reduced amount of TA-P_II. The relative activity of the mutant TA-P_II with oxalacetate decreased to about 1/10 that of the wild-type TA-P_II. Two groups (I and II) of prototrophic revertants were derived from No. 102-7: Group I restored TA-A but neither TA-P_I nor TA-P_II, while group II restored none of the three enzymes but had a new type of aminotransferase which was active with aspartate, but showed different chromatographic properties from TA-A.