Subcellular organization of neurophysins, oxytocin, [8−lysine]-vasopressin and adenosine triphosphatase in porcine posterior pituitary lobes

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Abstract
Posterior pituitary lobes from young pigs were fractionated by differential and sucrose-density-gradient centrifugation. The distributions of oxytocin and [8-lysine]-vasopressin were measured by bioassay and the distributions of neurophysin-I and -II by radioimmunoassays specific for each of these two proteins. Most of the hormone and neurophysin applied to the density gradient was localized in particles with the density expected of neurosecretory granules. However, the neurosecretory granules were separated into two bands (D and E). A close statistical correlation between the distributions of [8-lysine]-vasopressin and neurophysin-I, and of oxytocin and neurophysin-II on the gradients, suggested that in vivo porcine neurophysin-I binds [8-lysine]-vasopressin within one population of granules and porcine neurophysin-II binds oxytocin within another type of granule. However, there was no significant separation of oxytocin and vasopressin in fractions D and E. The molar ratios of hormones and neurophysins indicated that there was insufficient of either neurophysin to bind the [8-lysine]-vasopressin in the granule fractions or in the whole gland. Polyacrylamide-gel electrophoresis showed that only bands corresponding in mobility to porcine neurophysins-I, -II and -III were present in large amounts in the whole gland and in the granule fractions. The component with the mobility of neurophysin-III was, however, relatively enriched in whole young glands and granule fractions compared with adult gland extracts. It is suggested that the vasopressin that cannot be assigned to neurophysin-I may occur in (a) vesicles containing vasopressin but no neurophysin, (b) vesicles containing vasopressin and a protein that cannot be quantified by the radioimmunoassays used, such as porcine neurophysin-III, or (c) normal vasopressin–neurophysin granules which have accumulated extra vasopressin. Band E of the gradient was rich in adenosine triphosphatase activity, whereas band D possessed very little of this enzyme.