Peroxisomal localization and properties of tryptophan aminotransferase in plant leaves

In spinach leaves, both tryptophan:glyoxylate aminotransferase and tryptophan:hydroxypyruvate aminotransferase activities were located only in the peroxisomes and in the soluble fraction. The two enzymes co-purified to homogeneity from both peroxisomal and soluble fractions of spinach leaves. The evidence indicates that the two activities are associated with the same protein. The peroxisomal and soluble enzyme preparations had nearly identical properties, suggesting that the soluble enzyme is from broken peroxisomes. The two enzyme preparations utilized various L-amino acids as amino donors in the following order of activity with glyoxylate as amino acceptor; serine greater than alanine greater than tryptophan greater than asparagine greater than 5-hydroxytryptophan. Other amino acids tested were all much less active. With L-tryptophan as amino donor, the effective amino acceptors were glyoxylate and hydroxypyruvate; other 2-oxo acids tested were all inactive. They had molecular weights of approximately 185,000 with four identical subunits, isoelectric points of 5.9, and pH optima between 8.0 and 8.5.