Mg2+ counteracts the inhibitory effect of spermine on liver phosphorylase kinase

When the effect of polyamines on pig liver phosphorylase kinase was examined, spermine was found to be the most inhibitory. Although putrescine stimulated the reaction slightly, the spermidine effect was dependent on the phosphorylase b concentration. The inhibitory effect of spermine was counteracted by increasing the Mg2+ concentration. At 0.3 mM Mg2+, the apparent Km for phosphorylase b was increased 9-fold by the addition of 5 mM spermine. However, increasing Mg2+ to 3 mM decreased the value to the initial level obtained at 0.3 mM Mg2+ alone. These results suggest that a possible role of Mg2+ in the regulation of liver phosphorylase kinase is to protect the enzyme from the inhibitory action of a polyamine such as spermine.