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Metabolism and Enzymology

Specificity of Aspartate Aminotransferases from Leguminous Plants for 4-Substituted Glutamic Acids ¹

Department of Biological Chemistry, The University of Michigan, Ann Arbor, Michigan 48109-0606

Aspartate aminotransferase (glutamate-oxalacetate transaminase) was partially purified from extracts of germinating seeds of peanut (Arachis hypogaea), honey locust (Gleditsia triacanthos), soybean (Glycine max), and Sophora japonica. The ability of these enzyme preparations, as well as aspartate aminotransferase purified from pig heart cytosol, to use 4-substituted glutamic acids as amino group donors and their corresponding 2-oxo acids as amino group acceptors in the aminotransferase reaction was measured. All 4-substituted glutamic acid analogs tested were poorer substrates than was glutamate or 2-oxoglutarate. 2-Oxo-4-methyleneglutarate was least effective (lowest relative V_m/K_m) as a substrate for the enzyme from peanuts and honey locust, which are the two species studied that accumulate 4-methyleneglutamic acid and 4-methyleneglutamine. Of the different aminotransferases tested, the enzyme from honey locust was the least active with 2-oxo-4-hydroxy-4-methylglutarate, the corresponding amino acid of which also accumulates in that species. These results suggest that transamination of 2-oxo-4-substituted glutaric acids is not involved in the biosynthesis of the corresponding 4-substituted glutamic acids in these species. Rather, accumulation of certain 4-substituted glutamic acids in these instances may be, in part, the result of the inefficacy of their transamination by aspartate aminotransferase.