TRYPSIN LIBERATES AN ARGININE VASOPRESSIN-LIKE PEPTIDE AND NEUROPHYSIN FROM A MR 20,000 PUTATIVE COMMON PRECURSOR

Although the hypothesis that vasopressin and its associated neurophysin are synthesized together in one macromolecular common precursor was put forward more than a decade ago, direct confirmation of this hypothesis has been lacking. A [35S]cysteine-labeled putative precursor for vasopressin-related neurophysin (M(r) 20,000 pI 6.1) has been isolated from the supraoptic nuclei of rats. This precursor was subjected to limited proteolysis with trypsin which produced a M(r) 10,000 protein and peptide products. The former was identified as neurophysin on the basis of its pH-dependent affinity for vasopressin and its behavior in isoelectric focusing systems (pI 4.6-4.8). The tryptic peptides proved to be vasopressin-like because they: were rich in cysteine, comigrated with vasopressin on gel filtration columns in 6 M guanidine HCl, bound to a neurophysin-Sepharose affinity column at pH 5.7, and were recognized by antibodies against vasopressin. These data on the M(r) 20,000, pI 6.1 protein represent direct experimental evidence for a candidate for the common precursor of vasopressin and neurophysin. We propose that this common precursor be called 'propressophysin'.