The isolation of the native hormone-binding proteins from bovine pituitary posterior lobes. Crystallization of neurophysin-I and-II as complexes with [8-arginine]-vasopressin.

1. The native hormone-binding proteins, neurophysin-I and -II, have been isolated from acetone-desiccated bovine pituitary posterior lobes. 2. Neurophysin-I and -II are present in approximately equal quantities in the tissue and are localized in the neurosecretory granules. 3. The apparent molecular weight, determined by equilibrium sedimentation of neurophysin-I, was 19000 and that of neurophysin-II was 21000; their sedimentation coefficients, S(20,w), were 1.66 and 2.02s respectively. 4. Neurophysin-I and -II are similar in amino acid composition. Neurophysin-II was distinguished from neurophysin-I by the absence of histidine. 5. The proteins form complexes with oxytocin as well as with vasopressin. Complexes of both proteins with [8-arginine]-vasopressin have been crystallized. 6. Bioassay of the pressor and oxytocic activities of the crystals shows that neurophysin-I binds three molecules of either vasopressin or oxytocin whereas neurophysin-II binds only two molecules of each hormone per molecule of protein. Complexes containing two molecules of oxytocin and one molecule of [8-arginine]-vasopressin per molecule of protein are formed by neurophysin-I and -II; both proteins appear to possess three polypeptide-binding sites/molecule.