Stimulation and inhibition by magnesium ions of intrinsic protein phosphorylating systems in synaptosomal membrane fragments from rat brain

Synaptosomal membrane fragments from rat brain were incubated with -³²P]ATP in the presence of cyclic AMP or Ca²⁺ plus calmodulin and a range of Mg²⁺ concentrations. Incorporation of³²P into membrane polypeptides was examined by electrophoresis and radioautography. Cyclic AMP-stimulated reactions were stimulated by low concentrations and inhibited to varying degrees by high concentrations of Mg²⁺ in the range 1–50 mM. In general the Ca²⁺ plus calmodulin-stimulated reactions were maximally active in the range 30–50 mM Mg²⁺, but the Ca²⁺ plus calmodulin dependent phosphorylation of Protein I was progressively inhibited by concentrations of Mg²⁺ above 5 mM. These results emphasize the importance of establishing optimum Mg²⁺ concentrations in the study of specific membrane protein phosphorylating systems.