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 [gamma-32P]ATP in the presence of cyclic AMP or Ca2+ plus calmodulin and a range of Mg2+ concentrations. Incorporation of 32P 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.