Biosynthesis and Release of Neurophysin

In vivo labeling of protein of the hypothalamo-neurohypophysial tract was accomplished by slow infusion of 35S-cysteine into the third ventricle of dehydrated dogs. In shortterm experiments hypothalamo-median eminence tissue was found to contain labeled protein which possessed chemical and serological properties very similar to those of bovine neurophysin. The subcellular distribution of this material paralleled the distribution of vasopressin. When the animals were permitted to survive 10–21 days with free access to water, labeled neurophysin was found in their posterior pituitary glands. When hemilobes of these glands were incubated in vitro and challenged with elevated K+, there occurred a simultaneous increase in the release of radioactive vasopressin and neurophysin into the incubation medium compared with the amounts released during incubation in media containing either low K+ or high K+ in the presence of Mg++. Some evidence was also obtained for simultaneous release in vivo of hormone and carrier protein into the blood stream by comparing the content of radioactive protein in plasma immediately before and after hemorrhage of 2 dogs which had previously received intraventricular infusion of 35S-cysteine. The results presented in this paper, together with the findings of other investigators concerning changes in the levels of neurophysin-like proteins in the posterior lobe, are highly suggestive of simultaneous biosynthesis and release of the hormones of the neurohypophysis and their carrier proteins. The high molecular weight protein fraction which accompanies beef neurophysin prepared by salt fractionation was found to be immunologically unrelated to the hormone-binding proteins. (Endocrinology83: 1299, 1968)