Purification and characterization of alanine aminotransferase from Panicum miliaceum leaves

Three alanine aminotransferases, two minor (AlaAT-1, AlaAT-3) and one major (AlaAT-2), were detected by native gel electrophoresis of leaf extracts from Panicum miliaceum L. AlaAT-2 was purified to homogeneity and a specific polyclonal antibody was raised against it which did not react with the other two forms of the enzyme. The enzyme, with an apparent molecular size of 102 kDa, appeared to be a dimer of a single 50-kDa polypeptide. The enzyme has a relatively broad pH optima with similar curves for the forward and reverse directions, ranging between 6.5 and 7.5. The Km values of this enzyme were 6.67, 0.15, 5.00, and 0.33 mM for alanine, 2-oxoglutarate, glutamate, and pyruvate, respectively. The activity of AlaAT-2 was found to increase markedly during leaf greening in parallel with the increase of immunochemically titrated protein, and it is suggested to function in the C4 photosynthetic cycle.