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Some general properties of the enzyme, ATPlombricine phosphotransferase (lombricine kinase), which catalyses the reaction: forward ATP + lombricine I-=_ ADP reverse + phospholombricine were described by Gaffney, Rosenberg & Ennor (1964). In common with other ATP-guanidino phosphotransferases (Ennor & Morrison, 1958) this enzyme is activated by Mg2+, Mn2+, Ca2+ and Co2+ ions. The role of the activating metal ion in enzymic reactions concerned with the transfer ofthe terminal phosphoryl group of ATP to an acceptor molecule has been difficult to elucidate. Evidence has been presented that for ATP-creatine phosphotransferase (EC 2.7.3.2) the function of the metal ion is to combine with the nucleotide (Kuby, Noda & Lardy, 1954). On the other hand Griffiths, Morrison & Ennor (1957) have suggested that the activity of ATP-arginine phosphotransferase (EC 2.7.3.3) depends on the formation of an active enzyme-metal ion complex. More recent studies on ATP-creatine phosphotransferase indicate that it is unlikely that the mechanism is simple and that it is necessary to consider the possible interaction of all of the species, free metal ion, free nucleotide and metal ion-nucleotide complex, with the enzyme (Kuby & Noltmann, 1962). In the present paper, some observations on the activation of lombricine kinase by Mg2+ ions are presented. The approach used was that outlined by Morrison, O'Sullivan & Ogston (1961) whereby all possible pathways leading to the formation of an active enzyme-metal ion-substrate complex were considered. The results are consistent with the reaction of both free nucleotide and metal ionnucleotide complex with the enzyme and also with a possible, but much weaker, interaction between Mg2+ ion and the enzyme.

Kinetic studies of the activation of adenosine triphosphate-lombricine phosphotransferase by magnesium ions