The stability constants of some metal ion complexes of 3',5'-cyclic AMP

The stability constants of complexes of 3', 5'-cyclic AMP with Mg2+, Ca2+, Mn2+, Ni2+ and Co2+ were estimated at 30 degrees C in solutions of ionic strength about 0.15 containing about 130 mM K+ or tetramethylammonium ions. Values between 13 and 22 M-1 were obtained, indicating that only about 2% of cyclic AMP is present as metal complexes in vivo, but that at commonly used in vitro concentrations of 10 mM bivalent metal ions, 10--20% of cyclic AMP is present as metal complexes. The possible significance of these metal complexes, for example as competitive inhibitors, is discussed.