Calmodulin binding to secretory granules isolated from bovine neurohypophyses

Abstract

Secretory granules, isolated from bovine neurohypophyses on isoosmolar Percoll-sucrose-EGTA [ethyleneglycol bis(.beta.-aminoethyl ether)N,N,N'',N''-tetraacetic acid] gradients had a calmodulin content of 0.09 .+-. 0.01 .mu.g/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 .times. 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.