Neurophysin I is a carrier protein with a size of 10 KDa and containing 90 to 97 aminoacids that is a cleavage product of preprooxyphysin. It is a neurohypophysial hormone that is transported in vessicles with oxytocin, the other cleavage product, along axons, from magnocellular neurons of the hypothalamus to the posterior lobe of the pituitary. Although it is stored in secretory vesicles with oxytocin and released with oxytocin, its biological action is unclear.

Neurophysins are carrier proteins for oxytocin and vasopressin (ADH). Oxytocin and vasopressin are carried by unique neurophysins from their site of production in the cell bodies of the paraventricular and supraoptic nuclei to their site of release in the axon terminals of the posterior pituitary. Point mutations in neurophysin II underline most cases of hereditary hypothalamic diabetes insipidus, a disorder resulting from insufficient ADH release into systemic circulation.

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