Purified porcine erythrocyte membrane Ca2+-ATPase and 3′:5′-cyclic nucleotide phosphodiesterase were stimulated in a dose-dependent, saturable manner with the vitamin D-dependent calcium binding protein from rat kidney, calbindin-D28k (CaBP-D28k). The concentration of CaBP-D28k required for half-maximal activation (K 0.5 act.) of the Ca2+-ATPase was 28 nM compared to 2.2 nM for calmodulin (CaM), with maximal activation equivalent upon addition or either excess CaM or CaBP-D28k, 3′:5′-Cyclic nucleotide phosphodiesterase (PDE) also showed equivalent maximum saturable activation by calbindin (K 0.5 act. = 90 nM) or calmodulin (K 0.5 act. = 1.2 nM). CaBP-D28k was shown to effectively compete with CaM-Sepharose for PDE binding. Immunoprecipitation with CaBP-D28k antiserum completely inhibited calbindin-mediated activation of PDE but had no effect on calmodulin's ability to activate PDE. While the physiological significance of these results remains to be established, they do suggest that CaBP-D28k can activate enzymes and may be a regulator of yet to be identified target enzymes in certain tissues.
