Role of cations in the regulation of baker's yeast AMP deaminase.

The effect of polyamines and divalent cations including alkaline earth metals and transition metals on the AMP deaminase (AMP aminohydrolase EC 3.5.4.6) purified from baker's yeast was investigated. (1) Polyamines and alkaline earth metals activated the enzyme in the absence of ATP: these cations largely enhanced the maximal velocity without alteration of S0.5 and nH (Hill coefficient) values. However, transition metals acted as potent inhibitors, which decreased the maximal velocity of the enzyme in the absence of ATP. (2) All of the divalent cations showed an activation of the enzyme in the presence of ATP, followed by a progressive decrease in activity as the concentrations of transition metals increased. (3) The increase in the concentrations of polyamines or alkaline earth metals showed no more activating effect when the enzyme was fully activated by the addition of excess alkali metals in the absence of ATP, but divalent cation-activation was observed in the presence of ATP even if alkali metals were saturating. These results suggest the presence of two types of binding sites for cations: 1, the sites for free cations and 2, those for ATP-metal complexes. The former sites include the activating sites for alkali metals, polyamines and free alkaline earth metals, and the inhibitory sites for free transition metals. The latter sites are the activating sites for ATP-metal complexes, which are suggested to be commonly occupied by alkaline earth metals and transition metals and to form an ATP bridge (E-ATP-M) complex.