Human Adenylosuccinate Synthetase

Abstract

Abstract Human adenylosuccinate synthetase (IMP:aspartate ligase (GDP) EC 6.3.4.4) has been purified 110-fold from placenta. The partially purified preparation was stable and free of nucleotidase activity, permitting study of substrate kinetics and regulatory properties. The Michaelis constants for the substrates, IMP, GTP, and aspartate were 37 µm, 31 µm, and 0.95 mm, respectively. Based on studies of initial velocity and product or end product inhibition, the enzyme kinetic mechanism was compatible with a sequential rapid equilibrium fully random mechanism. Human adenylosuccinate synthetase was inhibited by a variety of purine nucleotides; the corresponding nucleosides and bases were ineffective. Inhibitory effectiveness decreased with the series mono g di g triphosphates. The mechanism of inhibition by specific purine nucleotides revealed that adenylosuccinate, AMP, and XMP were competitive inhibitors with respect to IMP with Ki values of 57 µm, 170 µm, and 140 µm, respectively. GMP and GDP were competitive with respect to GTP with Ki values of 10 µm and 45 µm, respectively. These findings suggest that human adenylosuccinate synthetase activity, despite its importance in a branched pathway, is not regulated by a highly specific molecular control mechanism.