Asparaginase and asparagine transaminase in soybean leaves and root nodules

Abstract

Asparaginase activity (</=1 mumol/mg protein . hr) was detected in extracts of soybean (Glycine max [L.] Merr.) leaf blades, but, even after efforts to optimize extraction and assay of the enzyme, specific activity was not sufficient to metabolize the estimated amount of asparagine translocated to leaves. Asparagine transaminase activity with glyoxylate or pyruvate was at least 52 and 62 nmol/mg protein . hr, respectively. This estimate of transaminase activity is based on the analysis of the reaction product alpha-ketosuccinamate. Formation of glycine and alanine was confirmed by amino acid analysis. alpha-Ketosuccinamate deamidase had a specific activity of 85 nmol/mg protein . hr in leaf blade extracts.A large amount of asparaginase (300-500 nmol/mg protein . hr) was found in root nodules. The enzyme is stable in 75% ethanol at room temperature, has a Km of 5 mum for asparagine, and was six times more active (protein basis) in bacteroids than cytosol. The relatively high activity, stability, and Km of the enzyme complicate efforts to study asparagine synthesis in the nodule, an organ known to export large amounts of this amino acid.