Partial conversion of vasopressinyl-Gly-Lys-Arg into pharmacologically active vasopressin through secretory granule carboxypeptidase E and alpha-amidating processing enzymes.

Vasopressinyl-Gly-Lys-Arg, the first intermediate derived from vasopressin protein precursor, has been converted into mature vasopressin by an "in vitro" two-step reaction through neurohypophysial secretory granule enzymes. Whereas the conversion into vasopressinyl-Gly is virtually complete at pH 5.5 as judged by HPLC, the conversion of vasopressinyl-Gly into vasopressin is weak at pHs 6.0 or 8.0 as judged by HPLC and measure of generated pressor activity. It is suggested that the high conversion yield usually seen in mammalian neurohypophysis, where no intermediate is detected, might be due to additional "in vivo" factors such as particular membrane-association or binding of the intermediate onto a neurophysin carrier.