Identification of neurophysin-related proteins in bovine neurosecretory granules

A highly purified fraction of neurosecretory granules isolated from the bovine posterior hypophysis was obtained using a continuous isotonic Percoll gradient. The analysis of their protein content indicates that the main products consist of 10000-Mr neurophysin (95%) and about 5% of high-molecular-weight forms of this protein. Species ranging from Mr 80000-14000 were detected by radioimmunoassay and immunoprecipitated by anti-neurophysin antibodies after 125I labeling of the neurosecretory granule lysates. The chemical relationship with neurophysin, of one of the immunoreactive species (Mr 19000), was ascertained by tryptic map analysis of the radioiodinated material. It is concluded that species both immunologically and chemically related to neurophysin represent a large majority of the protein content of the granules. They compare with the higher molecular weight forms of these neurosecretory components detected in the hypothalamo-neurohypophyseal tract and proposed as putative precursors of both neurophysin and vasopressin.