Interactions of Bovine Neurophysin II with Oxytocin and [8-Lysine] Vasopressin

Synthesis of peptide spin-labels that bind to neurophysin and their application to distance measurements within neurophysin complexes

The synthesis of two spin-labels capable of binding to the hormone-binding site(s) of neurophysin is described. The two spin-labels are 4-(glycyl-L-phenylalanylamido)-2,2,6,6-tetramethylpiperidinyl-1-oxy and S-[[[3-(2,2,5,5-tetramethylpyrrolidine-1-oxy)amino]carbonyl]methyl]-L-cysteinyl -L-tyrosine amide; synthesis of the former is achieved by a novel route to circumvent problems associated with nitroxide instability under standard conditions of peptide deblocking. NMR studies of the effects of binding these spin-labels on relaxation rates of individual proton resonances of neurophysin were used to calculate correlation times and distances between the bound nitroxides and the observed protons. The results indicate that residue 3 of peptides bound to the strong site of neurophysin is greater than or equal to 14 A from Tyr-49 and argue against a distance of < 5 A between the ortho ring protons of Tyr-49 and those of residue 2 of peptides bound to the strong site. Alternatively, the data suggest that the previously observed nuclear Overhauser effect between these protons reflects spin diffusion at the strong site and a contribution of uncertain magnitude from a second but very weak binding site; this second site is close to Tyr-49 and is detected by the increased relaxation rate of Tyr-49 ring protons when 4-(glycyl-L-phenylalanylamido)-2,2,6,6-tetramethylpiperidinyl-1-oxy is displaced from the strong site by competing diamagnetic peptide. Additionally, the data indicate that residue 3 of bound peptides at the strong site is distant from His-80 but approximately 12 A from the amino terminus. The extended side chain of residue 1 of peptides at the strong site is calculated as less than or equal to 10 A from Tyr-49.

N.M.R. and equilibrium dialysis studies of the interaction of bovine neurophysin-1 with vasopressin and small peptides

The binding to bovine neurophysin of lysine-vasopressin and of lysine-vasopressin selectively deuterated at the protons ortho to the tyrosine hydroxyl was studied by proton n.m.r. and equilibrium dialysis. The principal object of these studies was to investigate reports that, at standard salt concentrations, neurophysin contained a second site specific for vasopressin. At pH 6, the effects of neurophysin-I on the line-width, longitudinal relaxation rate and nuclear Overhauser properties of the lysine-vasopressin tyrosine ring protons were interpretable in terms of a slow-exchange 1:1 interaction between lysine-vasopressin and neurophysin. Additionally, n.m.r. competition studies between lysine-vasopressin and L-phenylalanyl-L tyrosinamide suggested 1:1 competition for a single binding site on neurophysin. No evidence pointing to a significant second lysine-vasopressin-binding site was obtained from the n.m.r. studies. The lack of a moderately strong second binding site for lysine-vasopressin at neutral pH was also indicated by equilibrium dialysis studies at relatively high free hormone concentrations. These studies demonstrated only a single thermodynamically significant site for either oxytocin or vasopressin and failed to confirm a reported effect of LiCl on the number of sites available to oxytocin. It is suggested that secondary sites for the hormones are probably markedly weaker and less specific than reported elsewhere.

Application of nuclear magnetic resonance spectroscopy to proteins

The active sites of enzymes can be studied in great detail using nuclear magnetic resonance spectroscopy. The determination of pKa values of active site histidine residues in bovine pancreatic ribonuclease and the characterization of the binding of peptide hormones to carrier proteins are two such examples. The study of the active site of staphylococcal nuclease is another example and is presented in detail in this paper. The structure of 3'5'-thymidine diphosphate bound in the active site of staphylococcal nuclease has been studied by measuring the relaxation rate enhancement of substrate analog nuclei by a paramagnetic metal ion. The lanthanide ion, Gd(III), was substituted for Ca(II) in the formation of the ternary complex of nuclease: Gd(III) : 3'5'-thymidine diphosphate. Measurements were made of the transverse relaxation rates of protons and the longitudinal and transverse relaxation rates of the phosphorus nuclei of bound nucleotide. Internuclear distances between the metal ion and atoms of the 3'5'-thymidine diphosphate nucleotide were determined from these data by using the Solomon-Bloembergen equation. In general, these distances corresponded closely to those determined by previous X-ray crystallography of the thymidine diphosphate complex. These internuclear distances were also used with a computer program and graphics display to solve for metal : nucleotide geometries which were consistent with the experimental data. A geometry similar to the structure of the metal : nucleotide complex bound to nuclease determined by X-ray analysis was one of the solutions to this computer modeling process. For staphylococcal nuclease the NMR and X-ray methods yield compatible high resolution information about the structure of the active site.