Kinetic Studies of the Activation of Mitochondrial Adenosine Triphosphatase by Mg++.

The activation of mitochondrial adcnosine triphosphatase (EC 3.6.1.4) by Mg++ has been studied extensively, particularly in connection with the mechanism of oxidative phosphorylation (1). However, virtually nothing is known about the interaction of ATPase with 5’-triphosphate ribonucleotides and the role of Mg++ in activating the enzyme. One of the key problems in this field appears to be whether Mg++ ions bring the nucleotide and enzyme close enough for catalysis by forming a bridge between them, or whether by forming a chelate with the nucleotide, the configuration of the latter is so altered that the enzyme is able to attack the substrate. These two hypothetical functions of Mg+f need not, of course, be mutually exclusive. The recent finding that alkali metal ions inhibit Mg++-activated mitochondrial ATPase (2) also raises the question whether divalent cations can interact with ATPase directly, and thereby affect its activity without first having to form a chelate with the substrate. The following study was undertaken to obtain some insight into these problems. The results suggest that MgATP’ is the “true” or “active” substrate for mitochondrial ATPase, and that both free ATP and free Mg+f react with the enzyme and inhibit its activity. A preliminary account of part of this study has been presented (3).