AMP deaminase from porcine blood platelets, regulation by adenine nucleotides, phosphate and by Na+ and K+ ions.

Porcine blood platelets contain a relatively high amount of AMP deaminase (14.4 U per 10 11 cells). The enzyme showed sigmoidal behaviour as a function of AMP concentration with a S0.5 value (substrate concentration required for half-maximum velocity) of 3.6 and 4.0 mM in the presence of Na+ and K+ respectively. MgATP and MgADP at micromolar concentration activated the enzyme. Activation by saturating MgATP and MgADP in the presence of Na+ or K+ converted the rate versus substrate plots to hyperbolic with a dramatic decrease of S0.5. Phosphate at milimolar concentrations inhibited the enzyme and this inhibitory effect was totally reversed as the concentrations of MgATP and MgADP rised to physiologically high levels. Na+ and K+ activated the enzyme in the absence of MgATP and MgADP. Both cations largely enhanced the Vmax with Na+ being more potent. A comparison of the kinetic behaviour of the enzyme in vitro with the metabolite concentrations in vivo suggest that a substantial regulation can occur through changes in AMP and Na+ concentrations.