BINDING OF VASOPRESSIN AND OXYTOCIN TO PROTEIN IN EXTRACTS OF BOVINE AND RABBIT NEUROHYPOPHYSES

Abstract

SUMMARY Association of oxytocin and arginine vasopressin with protein in bovine and rabbit neurohypophysial extracts has been studied by co-precipitation of the hormones with protein on addition of NaCl, by gel filtration and by dialysis. Although precipitates formed by addition of NaCl (2·5–20 g./100 ml.) to bovine neurohypophysial extracts at pH 3·1 contain both hormones, oxytocin and arginine vasopressin were partially separated at 5·0 g. NaCl/100 ml. when the proportion of oxytocin precipitated was approximately double that of arginine vasopressin. No precipitate formed on addition of NaCl (15 g./100 ml.) to bovine neurohypophysial extract at pH 5.8. Experiments by gel filtration and dialysis showed that the binding of oxytocin and vasopressin to protein in neurohypophysial extracts is pH dependent and is maximal in the range pH 5·2–5·8. Dilution of a solution containing neurohypophysial hormones and protein results in dissociation of the complexes and this could account for differences observed in experiments with bovine and rabbit neurohypophysial extracts. It is suggested that the mode of binding between the hormones and protein is ionic association between the cationic free terminal NH2 of the cystine residue in the hormones and free carboxyl groups in protein.