Characterization of porcine neurophysin. III. Its resemblance and possible relationship to porcine neurophysin I.

Homogeneous porcine neurophysin III has been obtained from slightly contaminated neurophysin material by rechromatography on diethylaminoethyl-cellulose. The purified protein binds both oxytocin and lysine vasopressin. Gel filtration on a calibrated column of Sephadex G-75 gives an estimate of the molecular weight of 10,000. Amino acid analyses establish the composition Lyla8, 1/2Cys14, Val2, Met1, Ile2, Leu7, Tyr1, Phe3. The total number of amino acid residues is 95. This composition exceeds that of porcine neurophysin-I by 1 alanine and 2 arginine residues. It has an NH2-terminal alanine and the COOH-terminal sequence- Arg-Arg-Ala. Results of peptide maps, the amino acid composition of tryptic peptides, and the sequences of two small tryptic peptides suggest that porcine neurophysin III contains the entire molecule of porcine neurophysin I plus a tripeptide -Arg-Arg-Ala connected the COOH terminus. It is threfore possible that porcine neurophysin I may have been derived from porcine neurophysin III by the proteolytic removal of the last 3 or 4 amino acid residues from the COOH terminus, and that the porcine hypothalamic tissue synthesizes only two neurophysins, II and III.