A CONTINUOUS SPECTROPHOTOMETRIC ASSAY FOR THE ACTIVATION OF PLANT NAD KINASE BY CALMODULIN, CALCIUM(II), AND EUROPIUM(III) IONS

A continuous spectrophotometric assay has been developed to quantify the calmodulin, calcium(II) ion, and europium(III) ion dependence of the activation of NAD kinase from pea seedlings. Experimental enzyme activation data are compared with the theoretical curves for the binding of calcium(II) ions to the individual calcium binding sites of calmodulin. These results indicate that the binding of three calcium(II) ions is necessary for activation of plant NAD kinase. Further studies demonstrate that europium(III) ions-can replace calcium(II) ions in calmodulin with retention of its ability to activate NAD kinase.