Muscle 5′-Adenylic Acid Aminohydrolase

The treatment of purified rat muscle AMP deaminase with pryidoxal-5'-P produces a decrease of the enzyme activity which is time- and pyridoxal-5'-P concentration-dependent. The reaction is reversible and can be changed to an irreversible reaction by reduction with NaBH4. The reduced pyridoxal-5'-P-AMP deaminase complex shows an absorption spectrum which is typical for pyridoxamine derivatives. Chromatographic analyses confirm the formation of a Schiff base between pyridoxal-5'-P and the e-amino group of lysine residues in the enzyme. The loss of activity is not a simple inactivation process due to modification of the catalytic site of the enzyme. GTP and ATP, which are allosteric inhibitors of AMP deaminase, show a protective effect against pyridoxal-5'-P inactivation whereas KCl at high concentration exerts a little protection and Pi and P-creatine are ineffective. The reduced pyridoxal-5'-P-AMP deaminase derivative with 30 to 35% residual activity and 6 to 7 moles of pyridoxal-5'-P bound per mole of protein has different kinetic and regulatory properties from the native enzyme: Vmax is the same but the Km value is increased; the enzyme shows a homotropic cooperativity also in the presence of activators, K+ and ADP, and is no longer sensitive to GTP, ATP, and high salt concentration inhibitions. Pi and P-creatine still exert an inhibitory effect. These kinetic properties are similar to those of the native enzyme in the presence of nucleoside triphosphates. The reduced pyridoxal-5'-P-AMP deaminase derivative prepared in the presence of GTP is still sensitive to inhibition by nucleoside triphosphates. It is likely that pyridoxal-5'-P binds at or near the binding sites for these compounds.