Solid-phase syntheis and some pharmacological properties of [8-phenylalanine]-oxytocin

Oxytocin analogs with substitutions in postions 3 and 4

Synthesis and biological properties are reported for some analogs of oxytocin with replacements of the isoleucine residue in position 3, i.e., (3-proline)oxytocin and(3-D-alanine)oxytocin, and the glutamine residue in position 4, i.e., (4-D-alanine)-oxytocin and (4-D-leucin)oxytocin. (3-Proline)oxytocin exhibited smaller than0.02 U/MG oxytocic activity, 0.005 plus or minus smaller than 0.001 U/mg rat pressor activity and 0.003 plus or minus 0.0001 U/mg antidiuretic activity. (3-D-Alanine)oxytocin had no agonistic activity in the bioassays tested except for the rat antidiuretic assay (smaller than 0.0005 U/mg). The 4-D-alanine analog showed 0.05 plus or minus 0.003 U/mg oxytocic activity, 0.07 plus or minus 0.01 U/mg avian vasodepressor activity, and smaller than 0.001 U/mg rat antidiuretic activity. (4-D-Leucine)oxytocin possessed 0.001 plus or minus U/mg rat pressor activity, and showed slight inhibitory properties in the oxytocic and avian vasodepressor assays, inhibiting the oxytocin response in the latter assay by about 60% at a girnibe-to-analog ratio of 1:5000. The activity profiles of the analogs are compared to that of oxytocin and are discussed on the basis of the proposed solution conformation of oxytocin.

Impact of the Merrifield solid phase method on the design and synthesis of selective agonists and antagonists of oxytocin and vasopressin: a historical perspective

This tribute to Bruce Merrifield traces the author's fortuitous path in 1964 from Vincent du Vigneaud's laboratory to the laboratory of D. W. Woolley to learn the solid phase method and then to his first faculty position in the Department of Biochemistry, McGill University, Montreal in 1965. It recalls the key roles played from early 1966 to July 1967 by Bruce Merrifield, John Stewart, Arnold Marglin, Herb Takashima, and Vincent du Vigneaud in providing key advice to the author's efforts to use the solid phase method to synthesize oxytocin; while simultaneously the du Vigneaud and Merrifield laboratories were collaborating on the solid phase synthesis of deamino-oxytocin. Both syntheses were published in the same issue of the Journal of American Chemical Society in 1968. Also described is how this breakthrough impacted the author's scientific career: by leading to highly productive collaborative studies, initially with Wilbur H. Sawyer and subsequently with others, on the design and synthesis of selective agonists, antagonists, and radioiodinated ligands for oxytocin and vasopressin receptors. These syntheses were greatly facilitated by the contributions of highly talented graduate students, research technicians, and visiting peptide chemists from Hungary, England, Poland, Bulgaria, and China. Many of these peptides have become very valuable pharmacological tools in studies on the peripheral and central effects of oxytocin and vasopressin: further attesting to the profound impact of the solid phase method as the cornerstone for all the discoveries, which he and his collaborators and coworkers have made over the past 40 years.

Partial purification and characterization of post-proline cleaving enzyme: enzymatic inactivation of neurohypophyseal hormones by kidney preparations of various species

The inactivation of the neurohypophyseal hormones arginine vasopressin and oxytocin, both 14C-labelled in the C-terminal glycine residue, by enzymes present in kidney homogenates of various species has been investigated, and some of the enzymes responsible have been partially purified and characterized. The Leu-Gly peptide bond of oxytocin is generally most effectively cleaved by kidney homogenates, although with certain species enzymic activity hydrolyzing the Pro-Leu bond is significant. Degradation of arginine vasopressin is slower than oxytocin in all species studied, and appears to occur by a different overall mechanism since cleavage of the Pro-Arg bond is more significant than hydrolysis of the Arg-Gly bond. The enzyme releasing glycinamide from oxytocin and the "Post-Proline Cleaving Enzyme", which releases C-terminal dipeptide from oxytocin and arginine vasopressin, were partially purified from lamb kidney by ammonium sulfate fractionation and column chromatography. The two enzymes are shown to be separate entities with different pH profiles. The prolyl peptidase activity released the C-terminal dipeptides from oxytocin and arginine vasopressin at similar rates and was inhibited by p-chloromercuriphenylsulfonic acid, 1,10-phenanthroline, L-1-tosylamido-2-phenylethylchloromethyl ketone, Co2+, Ca2+, and Zn2+, but significantly enhanced by dithiothreitol. The prolyl peptidase preparation cleaves proline-containing peptide substrates at the Pro-X bond. The rate of cleavage is dependent on the nature of residue X and with the conditions used there is no cleavage when X equals Pro; however, cleavage occurs when X is a D isomer: [Mpr1, D-Arg8] vasopressin is inactivated at a rate similar to [Mpr1, Arg8]- and [Mpr1, Lys8] vasopressin, suggesting that the known prolonged biological action of [Mpr1, D-Arg8] vasopressin is not due to resistance to the prolyl peptidase. In all characteristics tested the lamb kidney prolyl peptidase was identical to the post-proline cleaving enzyme isolated earlier from human uterus. In vivo experiments in the cat suggested that both the glycinamide-releasing enzyme and post-proline cleaving enzyme are present and effective in inactivating neurohypophyseal hormones in the intact animal.

Enzymes as reagents in peptide synthesis: enzymatic removal of amine protecting groups

A model system is described for the enzymatic deprotection of suitably masked amino groups during stepwise peptide synthesis. Nitrophenyl esters of amino acids, N-protected with trypsin-labile benzyloxycarbonylarginyl groups, were prepared as crystalline, analytically pure picrate salts in a standardized procedure. These intermediates were shown to react with amino compounds to form the expected peptide linkages. A pair of diasteriomeric peptides prepared in this way and featuring benzyloxycarbonylarginyl-L-, AND -D-glutaminyl sequences, respectively, were subjected to tryptic digestion. In both cases, a specific cleavage of the arginyl bond was achieved; however, the peptide containing the L-glutaminyl residue was deprotected much more rapidly than its diasteriomer containing the D-glutaminyl residue. The hydrolysis of the former isomer was not noticeably impeded by the presence of the latter. The results of these studies suggest that C-activated amino-acid derivatives, N-protected with trypsin-labile groups, are readily prepared in convenient form and that the peptide derivatives prepared from these intermediates are readily freed of their amino-protecting groups under mild, aqueous conditions with a potentially useful degree of stereospecificity. Theoretical implications of this first enzyme-catalyzed step in the repetitive cycle of peptide elaboration are discussed along with the procedural advantages implicit in the alternation of strongly and weakly basic groups in the protected and unprotected peptide intermediates, respectively.