Lanthanide ions and Cd2+ are able to substitute for Ca2+ in regulating phosphorylase kinase.

Trivalent lanthanide ions and Cd2+ were found to mimic effectively the stimulatory action of Ca2+ on rabbit muscle phosphorylase kinase. In the range of concentrations tested, Cd2+ and lanthanides (Tb3+, Gd3+, Pr3+, Ce3+) could substitute for Ca2+ in activating the enzyme to about 60% and 70% respectively of the maximal level seen with Ca2+, at pH 8.2. The effect induced by Cd2+ was biphasic (stimulation followed by inhibition with increasing metal cation concentration). Similar results were obtained at pH 6.8. Cd2+ and Tb3+ were also able to replace Ca2+ required for the stimulation of phosphorylase kinase activity at pH 8.2 by exogenous calmodulin. Maximal stimulation induced by calmodulin in presence of Cd2+ was significantly higher than that in presence of Ca2+ or Tb3+.