Calmodulin binding to secretory granules isolated from bovine neurohypophyses

Secretory granules, isolated from bovine neurohypophyses on isoosmolar Percoll-sucrose-EGTA gradients had a calmodulin content of 0.09 +/- 0.01 micrograms/mg protein (SE, n = 6). The distribution of calmodulin on the gradient showed that it did not copurify with the granules. Specific binding sites for calmodulin with a high affinity (Kd = 2.43 +/- 0.27 X 10(-9) M (SE, n = 5] and a maximum binding capacity of 1.3 +/- 0.4 pmol/mg protein (SE, n = 5) could be demonstrated when such secretory granules were incubated with 125I-calmodulin.