ACTIVATION OF CALMODULIN BY VARIOUS METAL-CATIONS AS A FUNCTION OF IONIC RADIUS

  • 1 January 1984
    • journal article
    • research article
    • Vol. 26  (1) , 75-82
Abstract
The active form of calmodulin is a Ca2+ .cntdot. calmodulin complex. Whether other metal cations [some of which occur as environmental pollutant] substitute for Ca2+ to activate calmodulin was determined. Binding of Ca2+ resulted in an altered conformation of calmodulin with an increased quantum yield in its Try fluorescence. Qualitatively similar results were obtained with Zn2+, Mn2+, Cd2+, Hg2+, Sr2+, Pb2+, Tb3+, Sm3+ and La3+. The relative extents of fluorescence enhancement by these cations were related to their ionic radii; all cations with ionic radii close to Ca2+ (0.99 .ANG.) increased Try fluorescence, whereas those with different ionic radii were not effective, or much less so. The change in calmodulin conformation by the cations was confirmed by its altered electrophoretic mobility on polyacrylamide gels. Cations that change the conformation of calmodulin allow it to stimulate bovine brain phosphodiesterase. The relative extents of stimulation of phosphodiesterase by cations were also related to their ionic radii. The ability of metal cations to inhibit Ca2+ binding was similarly related to their ionic radii. The closer the radius of a metal cation was to that of Ca2+, the more effective was the cation to substitute for Ca2+. The range of effective ionic radii was approximately 1 .+-. 0.2 .ANG.. Calmodulin-stimulated phosphodiesterase activity by the cations was reversed by trifluoperazine, an antagonist of calmodulin.