Adenohypophyseal Secretory Granules

Pure secretory granules are isolated in high yield from bovine anterior pituitary gland by a method which uses differential and isopycnic centrifugation. The secretory granules are phosphorylated in the presence of [γ-32P]ATP by an endogenous protein kinase. At least 77% of the radioactivity is incorporated into protein components of the secretory granules. A negligible amount of 32P is associated with phospholipids (2%). [32P]Phosphate is incorporated into phosphoserine and phosphothreonine. Only 18% of the incorporated 32P can be removed by washing the granules in a high salt solution (0.50 m KCl). ATP-ase activity is associated with the secretory granules. Adenosine 3',5'-monophosphate does not stimulate the protein kinase activity under endogenous conditions of substrate availability but leads to a 40 to 80% stimulation of enzymatic activity in the presence of histones. The enzyme associated with the secretory granules is Mg2+-dependent and its activity is inhibited by Ca2+.