The regulatory role of Mg2+ on rabbit skeletal muscle phosphorylase kinase

Abstract The stimulation of phosphorylase kinase by Mg 2+ was studied. Both the nonactivated and activated kinases are stimulated by Mg 2+ at concentrations that are 100- to 200-fold greater than ATP. This stimulation is observed at both pH 6.8 and 8.2 and results in a 10-fold increase in the activity of the nonactivated kinase. Mg 2+ stimulation is additive with that observed by calmodulin. Both the Ca 2+ -dependent and -independent activities of the kinase are stimulated by high [Mg 2+ ]. Kinetically this stimulation can be explained by a decrease in the K m for both phosphorylase b and ATP or an increase in V . The pH 6.8 8.2 ratio (0.06) is unaffected by [Mg 2+ ] between 5 and 20 m m , but increases when [Mg 2+ ] is less than 5 m m or greater than 20 m m . The stimulation by high [Mg 2+ ] is explained by a direct effect of this cation on the kinase molecule rather than on its protein substrate, phosphorylase. This activating effect of high [Mg 2+ ] does not result in any permanent change in the kinase molecule and can be readily reversed by diluting [Mg 2+ ] to a low value.