PARTIAL-PURIFICATION AND CHARACTERIZATION OF AN AROMATIC AMINO-ACID AMINOTRANSFERASE FROM MUNG BEAN (VIGNA-RADIATA L WILCZEK)

An aromatic amino acid aminotransferase (aromAT) was purified over 33 000-fold from the shoots and primary leaves of mung beans (Vigna radia ta L. Wilczek). The enzyme was purified by ammonium sulfate precipitat ion, gel filtration and anion exchange followed by fast protein liquid chromatography using Mono Q and Phenylsuperose. The relative aminotra nsferase activities using the most active amino acid substrates were: tryptophan 100, tyrosine 83 and phenylalanine 75, with K-m values of 0 .095, 0.08 and 0.07 mM, respectively. The enzyme was able to use 2-oxo glutarate, oxaloacetate and pyruvate as oxo acid substrates at relativ e activities of 100, 128 and 116 and K-m values of 0.65, 0.25 and 0.24 mM, respectively. In addition to the aromatic amino acids the enzyme was able to transaminate alanine, arginine, aspartate, leucine and lys ine to a lesser extent. The reverse reactions between glutamate and th e oxo acids indolepyruvate and hydroxyphenylpyruvate occurred at 30 an d 40% of the forward reactions of tryptophan and tyrosine, with K-m va lues of 0.1 and 0.8 mM, respectively. The enzyme was not inhibited by indoleacetic acid, although alpha-naphthaleneacetic acid did inhibit s lightly. Addition of the cofactor pyridoxal phosphate only slightly in creased the activity of the purified enzyme. The aromAT had a molecula r weight of 55-59 kDa. The possible role of the aromAT in the biosynth esis of indoleacetic acid is discussed.