Neurophysins arecarrier proteins which transport thehormonesoxytocin andvasopressin to theposterior pituitary from theparaventricular andsupraoptic nucleus of thehypothalamus, respectively. Inside theneurosecretory granules, the analogous neurophysin I and II form stabilizing complexes viacovalent interactions.[1] Stabilizing neurophysin-hormone complexes that are formed within neurosecretory granules located in theposterior pituitary gland aid in intra-axonal transport.[2] During intra-axonal transport, the neurophysin's are believed to prevent the bound hormone from leaking into the cytoplasmic space andproteolytic digestion via enzymes.[3] However, due to the low concentration of neurophysin in the blood, it is likely the protein-hormone complex dissociates, indicating the neurophysin does not aid in transporting the hormone through thecirculatory system.[2]
Neurophysins are also secreted out of the posterior pituitary hypothalamus, each carrying their respective associated passenger hormone. When the posterior pituitary hypothalamus secretes vasopressin and its neurophysin carrier, it also secretes aglycopeptide.
There are two types:
These proteins are synthesized in the cell bodies of thesupraoptic andparaventricular regions of thehypothalamus.
Thedisulfide-rich neurophysin protein is suggested to be congruent with the synthesis ofinsulin in which a precursor molecule of higher molecular weight is proteolytically cleaved and forms disulfide linkages.[2]
Although not enough data has been obtained, it is hypothesized that there is a common precursor molecule between neurophysin and the two hormones it stabilizes.[2]
Neurophysins are acidic proteins with a molecular weight of approximately 10,000 Da that are rich incysteine,glycine, andproline residues . The protein is double domain with apolypeptide chain of 93-95 residues with 14 cysteine residues forming 7 disulfide bridges . Domain I contains a COOH terminal with a disulfide loop; domain II lacks this COOH terminal disulfide loop . Based on the resemblance of the disulfide loop present on vasopressin and oxytocin, it is suggested that the hormones form covalent linkages to this disulfide loop present on the COOH terminal of domain I.[4]
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