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The neurophysins are a family of cystine-rich proteins found associated with the hormones oxytocin and vasopressin in the neurosecretory granules of the posterior pituitary. Until now, neurophysins have been investigated mainly in the ox and the pig (Rauch, Hollenberg & Hope, 1969; Uttenthal & Hope, 1970), but for experimental purposes knowledge of the properties of the neurophysins of laboratory animals is desirable. On the basis of [35S]cysteine incorporation studies in vivo, two of the main components seen after polyacrylamide-gel electrophoresis of rat neurohypophysial extracts have been tentatively identified as neurophysins (Pickering, Jones & Burford, 1971; Burford & Pickering, 1971). In a parallel study, Norström, Sjöstrand, Livett, Uttenthal & Hope (1971) found a single protein band, in the same region on polyacrylamide gels, which cross-reacted with antibodies to porcine neurophysin. The present communication describes the initial chemical characterization of rat neurohypophysial proteins.

Acetone-dried posterior pituitary powder (136 mg) obtained from

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Division of Pharmacology and Therapeutics, Faculty of Medicine, University of Calgary, Calgary, Alberta, Canada T2N 1N4

(Received 26 February 1976)

The production of highly specific antisera to oxytocin and vasopressin for radioimmunoassay was only recently achieved (Chard, 1973). Two major difficulties encountered in the attempts at production of antisera to these hormones have been: first, that both hormones have low molecular weights (approximately 1000); and secondly, they are usually native to the animals being immunized, although [Arg8]-vasopressin (AVP) has been successfully used for the production of antisera in rabbits. The molecular weights can be augmented by covalently coupling the hormones, before immunization, to large molecular weight ligands such as thyroglobulin (Skowsky & Fisher, 1972; Chard, 1973). [8-Lysine]-vasopressin (LVP), which is found in suiformes, is closely related chemically and biologically to AVP - the vasopressin of most mammals. Lysine-vasopressin has been used successfully for the production of antisera for radioimmunoassays (Skowsky,

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G. D. Burford and I. C. A. F. Robinson

The content of vasopressin, oxytocin and their related neurophysins was measured in the hypothalamus and pituitary gland of mid-trimester human fetuses. Vasopressin was present in both tissues approximately 3–4 weeks before oxytocin. The levels of the hormones in the pituitary gland increased 1000-fold over the next 3–4 months. During this time, the very high vasopressin/oxytocin ratio gradually decreased but did not reach unity in the period studied. In contrast, both the vasopressin-associated neurophysin and the oxytocin-associated neurophysin appeared in the pituitary gland at the same gestational age and showed the same exponential increase with fetal age. Lower levels of the neurophysins and the nonapeptides were found in the hypothalamus and the levels increased more slowly with fetal age. Our results suggest that the high vasopressin/oxytocin ratios observed in fetal life are due to differences in the rate of maturation of the hormone precursor, rather than to differences in the rate of de-novo synthesis.