General and kinetic properties of pig heart mitochondrial adenylate kinase

The precise localization of adenylate kinase (ATP:AMP phosphotransferase, EC 2.7.4.3) has been studied in pig heart mitochondria. This enzyme, which was distinct from the cytoplasmic enzyme, was insensitive towards SH reagents and exhibited a relatively weak inhibition by the specific inhibitor P1, P5-diadenosine-5'-pentaphosphate. The enzyme has been partially purified from isolated mitochondria. In the forward reaction adenylate kinase was very specific for AMP and less specific for the ATP site. Kinetic studies showed that in the forward direction, KMgATP and KAMP the dissociation constants of the substrates from the binary complexes were lower than the dissociation constants from the ternary complexes. In the reverse direction KMgADP was higher than KADP, but these values were not modified by the binding of the other substrate. In the forward direction, the enzyme was inhibited by excess of substrate when AMP concentrations were greater than 1 mM. This inhibition could prevent the phosphorylation of AMP to ADP and thus decrease the amount of adenine nucleotides available for oxidative phosphorylations.