Maize Leaf Adenylate Kinase

Abstract

Adenylate kinase (EC 2.7.4.3) from leaves of maize (Zea mays) was purified to homogeneity using (NH₄)₂SO₄ fractionation, followed by chromatography on DEAE-cellulose, hydroxyapatite, Sephadex G-75SF, and Green A dye-ligand columns. The purified enzyme had specific activity of about 1,550 micromoles ADP produced per minute per milligram protein, and the ratio of velocities of the reverse (utilization of ATP) to forward (formation of ATP) reaction was about 1.5. The M_r value of adenylate kinase, determined by electrophoresis in dissociating conditions and by gel filtration, was 29,000 and 31,000 respectively, suggesting monomeric nature of the enzyme. Purified preparations were stable for at least 1 month at 0 to 4°C. Magnesium ions were essential for activity of adenylate kinase in both directions of the reaction. Optimal rates in the forward direction were observed at the magnesium to ADP ratio of about 0.6 to 0.8. For the reverse reaction, ATP served as a substrate only when complexed with magnesium, while AMP reacted as a free species. The enzyme preferentially utilized adenine ribonucleotides in both directions of the reaction. The nucleoside triphosphate-binding site of adenylate kinase was fairly nonspecific with regard to nucleotide species. On the other hand, the primary amino group of either adenine and cytosine moieties was essential for effective binding to the nucleoside monophosphate site of the enzyme.