Influence of the peptide-chain length on disulphide-bond formation in neurohypophysial hormones and analogues

Abstract

[8-Arginine]vasopressin, [8-arginine]vasotocin, oxytocin and oxypressin, the 'ring' derivatives pressinamide and tocinamide, and the extended-chain analogues Pro-Arg-Val- [8-arginine]vasopressin and [8-arginine]vasopressinoyl-Ala-Met-Ala-NH2, were synthesized by the solid-phase method and purified by sequential gel filtration on Sephadex G-15 in 50% acetic acid and 0.2M-acetic acid. Controlled oxidation of the thiol groups of the reduced peptides obtained after deprotection with sodium in liquid ammonia gave rise to products that depended on the length of the peptide chain: (i) nonapeptides gave monomer and dimer species, (ii) hexapeptides produced mixtures containing higher polymers, and (iii) dodecapeptides gave predominantly monomer with some dimerized material. The evidence suggests that the presence of the acyclic tail tripeptide in the nonapeptide hormones induces a conformation in the preceding hexapeptide that favours the formation of an intramolecular disulphide bond. For [8-argininelvasopressin, intramolecular disulphide-bond formation is enhanced by extension of the peptide chain from either the N-or the C-terminus. The possible significance of these studies to neurohypophysial hormone-prohormone relationships is discussed.The formation of an intramolecular disulphide bond occurs at some stage during the biosynthesis of the neurohypophysial hormones, and presumably takes place in an environment that ensures intramolecular alignment of the thiol groups so that intermolecular disulphide bonds are not formed. During the chemical synthesis of neurohypophysial hormones, both intra-and inter-molecular disulphidebond formation are observed after oxidation of the reduced peptides, suggesting that the hormones Abbreviations used: abbreviations for amino acids and protecting groups are those recommended by the IUPAC-ILJB Commission on Biochemical Nomenclature (1972); Boc, t-butyloxycarbonyl. Synthetic analogues of neurohypophysial hormones: themselves do not represent an 'efficient' environment for intramolecular disulphide-bond formation. The possibility exists that these hormones are biosynthesized in the form of prohormones, which, when released from the ribosomes, are conformationally predisposed to spontaneous and efficient intramolecular disulphide-bond formation.An index of the precyclic conformation of a given neurohypophysial-hormone analogue may be obtained by determining the degree of difficulty with which its intramolecular disulphide bond is formed, as reflected in the ratio of monomer to polymer(s) obtained after controlled.oxidation of the reduced peptide. By using the appropriate analogues, this approach can give information on the relative [Arg8]vasopressinoyl-Ala-Met-Ala-NH2, Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-Gly-Ala-Met-Ala-NH2-Vol. 173

trans-Dichlorotetracyanoplatinate(IV) as a Reagent for the Rapid and Quantitative Formation of Intramolecular Disulfide Bonds in Peptides

“…Potassium ferricyanide has been found to be useful in the formation of single intramolecular disulfide bonds in small peptides, in particular peptides in the oxytocin and somatostatin families. , However, methionine and tryptophan residues in the peptides can be oxidized, giving rise to side products . Further, even in the oxidation of relatively small peptides in the oxytocin family, significant amounts of side products, including dimers and polymers, were found . These side reactions can be interpreted in terms of an oxidation mechanism that involves free radicals .…”

Discovery of a Highly Selective and Efficient Reagent for Formation of Intramolecular Disulfide Bonds in Peptides

“…This, however, was not the case for the related hexapeptides, tocinamide and pressinamide. 19 Oxidation of the dithiol forms of tocinamide and pressinamide by ferricyanide results mainly in the formation of dimers and higher polymers. 19 In contrast, reaction with [Pt(en) 2 Cl 2 ] 2+ results in the quantitative formation of intramolecular disulfide bonds, regardless of the peptide sequence.…”

“…In previous syntheses of oxytocin and arginine-vasopressin and their derivatives, − it was reported that oxidation of the dithiol groups by ferricyanide results predominantly, but not exclusively, in formation of intramolecular disulfide bonds. This, however, was not the case for the related hexapeptides, tocinamide and pressinamide . Oxidation of the dithiol forms of tocinamide and pressinamide by ferricyanide results mainly in the formation of dimers and higher polymers .…”

Rapid, green disulphide bond formation in water using the corrin dicyanocobinamide

“…[2][3][4] The oxidation of a pair of cysteine residues to form a cystine produces a covalent bond and decreases the configurational entropy of the folded peptide. This process can be achieved via air oxidation, 5 use of dimethyl sulfoxide (DMSO), 6 hydrogen peroxide, 7 iodine, 8 or ferricyanide, 9 to name a few. 2,10,11 Many of the methods, however, require long reaction times, produce dimers, involve environmentally harmful organic solvents (which also limit hydrophilic peptide solubility), or result in modification of sensitive amino acid side chains (Tyr, Met, or Trp).…”