Abstract
The adenyl cyclase of beef cerebral cortex required both Mg 2+ and Ca 2+ for maximal activity. The requirement for Mg 2+ was absolute, and it was demonstrated that this cation was necessary for the interaction of the substrate with the enzyme. Enzymic activity was dependent upon the ratio of Mg 2+ to ATP, and maximal activity was obtained only when there was an excess of the divalent cation. Under these conditions, the apparent K m for ATP was 1.2–1.3 mM and V was 45 nmoles/mg. The requirement for Ca 2+ was demonstrated by use of the chelating agent 1,2-bis-(2-dicarboxymethylaminoethoxy) ethane (EGTA) which specifically chelates Ca 2+ in the presence of Mg 2+. Adenyl cyclase activity was inhibited non-competitively by this compound. The V and the initial velocity were reduced approx. 60% in the presence of 0.1 mM EGTA. There was no significant effect on the apparent K m for ATP. The velocity of the reaction was returned to normal by the addition of Ca 2+ in concentrations significantly lower than the concentration of the chelating agent.
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