Neurophysin complex

Radicals normally do not occur in peptides but they can be generated by irradiation at low temperature with UV, electrons or X-rays, usually at the a-carbon atom, and then can be recognized by ESR. There are some radicals that are stable at ambient temperature, for instance N-oxides with the structural element C-NO-C. In biochemistry, e.g. derivatives of 2,2,5,5-tetramethyl-pyrrolidine-l-oxide are being used as spin labels, conjugated to bioactive compounds. In this way ESR can localize the position of possible receptor sites. For instance, in hormone research the distances in the neurophysin complex between spin-labeled small peptides, models of oxytocin, have been determined [27]. 

S.T. Lord, E. Breslow, Synthesis of peptide spinlabels that bind to neurophysin and their application to distance measurements within neurophysin complexes. Biochemistry 19 5593-5602 (1980)... 

Formation of the oxytocin-neurophysin complex results in unob-servably broad resonances. Slow exchange does not occur because in this case two states should be observed, the bound (with a very broad resonance) and the free (with a narrow resonance). The N-terminal tripeptide does not bind to neurophysin, and its spectrum was unaffected by the presence of neurophysin. The line broadening of the deuterium label can be a sensitive monitor of hormone-protein interaction. However, in many cases only qualitative information will be obtainable, i.e., when the breadth of the lines renders them practically unmeasurable. Few other direct studies on peptides using quadrupolar nuclei have been reported, but with increasing instrumental sophistication it can be anticipated that greater use of these nuclei will be made in the future. 

VP (also called antidiuretic hormone [ADH]) and oxytocin travel through nerve axons to the posterior pituitary where they are stored, each complexed with a neurophysin. 

Chen, L., Rose, J. R, Breslow, E., Yang, D., Chang, W. R., Furey, W. R, Sax, M., and Wang, B.C. (1991). Crystal structure of a bovine neurophysin II dipeptide complex at 2.8 A determined from the single-wavelength anomalous scattering signal of an incorporated iodine atom., Proc. Natl. Acad. Sci. U.S.A., 88, 4240-4244. 

Oxytocin is synthesized in the cell bodies of supraoptic and paraventricular neurons and, then, transported (complexed with neurophysin) in membrane-bound vesicles to the posterior lobe of the pituitary gland, where it may be released by a reflex mechanism or mechanisms, initiated or amplified by genital stimulation, coitus, parturition, or suckling of the infant. Suckling also releases prolactin. The action of oxytocin on the uterus (muscular contraction and parturition) and mammary glands (contraction of myoepithelial cells and milk secretion) is a direct one and is not influenced by the autonomic nervous system. 

The 93- to 95-residue neurophysins act as carriers for vasopressin and ocytocin, forming specific complexes with them. Neurophysins contain 14 cysteine residues, which form seven disulfide bonds. There is a striking similarity in sequence between the neurophysins, snake venom toxins, a wheat germ lectin (agglutinin), a ragweed pollen allergen, and a small plant protein called hevein. On the basis of the alignment of cysteine residues, Drenth proposed that all of these proteins have a disulfide-linked core whose structure is shown in Fig. 30-16. 

Griffin JH, Cohen JS, Cohen P and Gamier M, Drug-biomolecule interactions Proton magnetic resonance studies of complex formation between bovine neurophysins and oxytocin at molecular level, J. Pharm. Sci, 64,507-511 (1975). 

Proton magnetic resonance spectroscopy was used to monitor individual amino acid residues in bovine neurophysin, in the nonapeptide hormone oxytocin, and in the complex formed between them. For neurophysin I alone, a normal titration curve... 

Neurophysins (NP), precursor proteins and additionally transport molecules of the hormones oxytocin (OT) and vasopressin (VP). The NP (95 aa Mr 10 kDa) contain the sequence of OT and VP, respectively, at the N-terminal end. OT is associated with NP I, and VP with NP II. NP II contains additionally a C-terminal 39-glycopeptide with unknown function. The NP are synthesized in the hypothalamus and transported in neurosecretory vesicles down the axon to the posterior pituitary, which acts as a storage and release facility for VP and OT. The crystal structure of the neurophysin-oxytocin complex was described in 1996 [H. Land et al.. Nature 1983, 302, 342 R. Acher, J. Chauvet, Biochemie 1988, 70, 1197 J. P. Rose et al.. Nature Struct. Biol. 1996, 116, 258]. 

Biochemical Aspects of Neurosecretion Neurophysin-Neurohjrpophyseal Hormone Complexes Paul Cohen, Pierre Nicholas, and Maryse Camier... 

CRYSTALLINE COMPLEXES OF NEUROPHYSIN-I AND -II WITH 8-ARGININE VASOPRESSIN... 

The complexes formed by neurophysin-M contained 3 Moles of vasopressin, or 2 Moles of oxytocin or 2 Moles of vasopressin and 1 Mole of oxytocin per Mole of protein It appears that oxytocin competes with vasopressin for one of the binding sites. Neurophysin-II, a constituent of neurophysin-M, forms crystalline complexes containing 2 Moles of vasopressin or 2 Moles of... 

Complex formation between bovine neurophysin II and oxytocin was followed using NMR of C-enriched amino acids incorporated into the peptide hormone. [ C-Leu ] oxytocin, [ C-Gly ] oxytocin, and [ C-Ile, C-Gly ] oxytocin were bound to bovine neurophysin II in D2O, 0.1 M NaCl, pH 6.8 (Griffin et al, 1975a,b). Complex formation results in changes in both the chemical shifts and relaxation times of carbons of the residue in position 3, but led to no effect on the glycyl residue in position 9. This indicates that segmental motion of the C-terminal tripeptide is unaffected by complex formation. 

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