Characterization of an amphiphilic helical structure in beta-endorphin through the design, synthesis, and study of model peptides

Can amphipathic helices influence the CNS antinociceptive activity of glycopeptides related to β-endorphin?

Glycosylated β-endorphin analogues of various amphipathicity were studied in vitro and in vivo in mice. Opioid binding affinities of the O-linked glycopeptides (mono- or disaccharides) and unglycosylated peptide controls were measured in human receptors expressed in CHO cells. All were pan-agonists, binding to μ-, δ-, or κ-opioid receptors in the low nanomolar range (2.2-35 nM K(i)'s). The glycoside moiety was required for intravenous (i.v.) but not for intracerebroventricular (i.c.v.) activity. Circular dichroism and NMR indicated the degree of helicity in H2O, aqueous trifluoroethanol, or micelles. Glycosylation was essential for activity after i.v. administration. It was possible to manipulate the degree of helicity by the alteration of only two amino acid residues in the helical address region of the β-endorphin analogues without destroying μ-, δ-, or κ-agonism, but the antinociceptive activity after i.v. administration could not be directly correlated to the degree of helicity in micelles.

Glycosylated neuropeptides: a new vista for neuropsychopharmacology?

The application of endogenous neuropeptides (e.g., enkephalins) as analgesics has been retarded by their poor stability in vivo and by their inability to effectively penetrate the blood-brain barrier (BBB). Effective BBB transport of glycosylated enkephalins has been demonstrated in several labs now. Analgesia (antinociception) levels greater than morphine, and with reduced side effects have been observed for several glycopeptides related to enkephalin. Somewhat paradoxically, enhanced BBB transport across this lipophilic barrier is achieved by attaching water-soluble carbohydrate groups to the peptide moieties to produce biousian glycopeptides that can be either water-soluble or membrane bound. Transport is believed to rely on an endocytotic mechanism (transcytosis), and allows for systemic delivery and transport of the water-soluble glycopeptides. Much larger endorphin/dynorphin glycopeptide analogs bearing amphipathic helix address regions also have been shown to penetrate the BBB in mice. This holds forth the possibility of transporting much larger neuropeptides across the BBB, which may encompass a wide variety of receptors beyond the opioid receptors.

The rational design of highly stable, amphiphilic helical peptides

A computer algorithm was devised for the evaluation of helical stability of potentially amphiphilic peptide sequences of specified length containing a set number of leucines in the hydrophobic region. All possible combinations of Glu, Lys and Gln in the hydrophilic region are rated using a set of empirical rules for salt bridge formation in alpha-helices, and the sequences which rate the highest are displayed. The rules for salt bridge formation were largely derived from published studies on the effects of salt bridges on helical stability. The algorithm was tested by redesigning a known amphiphilic alpha-helical peptide, alpha 1B or 1, which has been shown to aggregate into four-helix bundles. Comparison of the circular dichroism spectra of two peptides, 2 and 3, to 1 demonstrated that the redesigned peptide with the highest priority score from the algorithm, 2, was more helical when aggregated and slightly more helical as a monomer, whereas the peptide with the low priority score, 3, was somewhat less helical when aggregated and much less helical when monomeric. These results support the design of the algorithm, although conclusions based on aggregation data are complicated by the importance of interhelix contacts in the bundle. Further studies are underway to examine the reliability of the algorithm's predictions regarding the design of other helical peptides.

The posttranslational modification of beta-endorphin in the intermediate pituitary of the toad, Bufo marinus, includes processing at a monobasic cleavage site

Fractionation of an acid extract of 15 B. marinus intermediate pituitaries by a combination of gel filtration chromatography and cation exchange chromatography revealed one major and five minor forms of beta-endorphin in this tissue. Based on reversed-phase HPLC and immunological properties, as well as amino acid composition and primary sequence analysis, it was deduced that the sequence of the major form of B. marinus beta-endorphin is N-acetyl-YGGFMTPE. Overall, the steady-state analyses of the minor forms of beta-endorphin indicated that the posttranslational processing of beta-endorphin in the toad intermediate pituitary includes endoproteolytic cleavage at both paired basic and monobasic cleavage sites.

Examination of the role of the amphipathic alpha-helix in the interaction of neuropeptide Y and active cyclic analogues with cell membrane receptors and dimyristoylphosphatidylcholine

To test the potential importance of the putative C-terminal amphipathic alpha-helical region of neuropeptide Y (NPY) in receptor binding, the interactions of porcine NPY and several peptide analogues with lipid and cell membrane receptors were compared. Cyclic analogues were designed to constrain the N- and C-terminal regions of the peptide and to retain the folded conformation of NPY predicted from its sequence analogy with pancreatic polypeptide and its similar spectral behavior. The three cyclic peptides were [Cys2, 8-aminooctanoic acid5-24, D-Cys27]-NPY (C2-NPY), [Cys5, 8-aminooctanoic acid7-20, D-Cys24]-NPY (C5-NPY), and [D-Cys7, 8-aminooctanoic acid8-17, Cys20]-NPY (C7-NPY). All of the peptides bind with high affinity to pig spleen membranes, but only NPY and [Glu16, Ser18, Ala22, Leu28,31]-NPY (ESALL-NPY) bind quantitatively to dimyristoylphosphatidylcholine (DMPC) liposomes. C7-NPY and NPY20-36 bind with moderate affinity to liposomes, but only NPY and C7-NPY bind with high affinity to mouse brain receptors. Thus, lipid binding and receptor binding are not correlated in this series of peptides, and binding to the pig spleen receptor appears to require only the C-terminal region of the peptide. Simple lipid binding, as in NPY20-36, is insufficient for binding to the mouse brain receptor, suggesting that the N-terminal region of the peptide is required for high-affinity binding to this receptor. Data from fluorescence, differential scanning calorimetry, and liposome clearing experiments suggest that, although the interaction of NPY with lipid is consistent with formation of an amphipathic alpha-helix, a simple amphipathic alpha-helical model for the interaction with the high-affinity NPY receptor is insufficient to explain the data.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis of analogues of peptide YY with modified N-terminal regions: relationships of amphiphilic secondary structures and activity in rat vas deferens

Peptide YY (PYY) not only has distinct sequence homology with neuropeptide Y (NPY) and avian pancreatic polypeptide (APP), but it also exhibits both NPY- and APP-like biological activities. We synthesized two analogues of PYY, A1 and A2, with modified N-terminal regions, and compared their chemical and biological properties to those of PYY and the C-terminal fragment of PYY, (13-36)PYY. This study shows that there is a good correlation between the stability of amphiphilic alpha-helical structure and the biological activity of these peptides. A CD study of (13-36)PYY in mixed H2O and trifluoroethanol (TFE) solutions indicated a significant increase in alpha-helical segments (26-79%) with increasing TFE proportions. Since the fragment (13-36)PYY had potent activity in the rat vas deferens (RVD) assay, the secondary structure is possibly required on the RVD cell surface receptors. The analogues, A1 and A2, were designed to increase the stability of the alpha-helical structures by incorporation of modified N-terminal regions. The CD studies and the RVD assays of A1 and A2, suggest that the amphiphilic alpha-helical structures are stabilized by intramolecular hydrophobic interactions with the N-terminal regions and/or by intermolecular hydrophobic interactions in the self-association process, and subsequently potentiate the activities of the peptides compared to those of PYY and (13-36)PYY.

Secondary structure determination of human beta-endorphin by 1H NMR spectroscopy

The 1H NMR spectra of human beta-endorphin indicate that the peptide exists in random-coil form in aqueous solution but becomes helical in mixed solvent. Thermal denaturation NMR experiments show that in water there is no transition between 24 and 75 degrees C, while a slow noncooperative thermal unfolding is observed in a 60% methanol-40% water mixed solvent in the same temperature range. These findings are consistent with circular dichroism studies by other workers concluding that beta-endorphin is a random coil in water but that it forms 50% alpha-helix or more in mixed solvents. The peptide in the mixed water-methanol solvent was further studied by correlated spectroscopy (COSY) and nuclear Overhauser effect spectroscopy (NOESY) experiments. These allow a complete set of assignments to be made and establish two distinct stretches over which the solvent induces formation of alpha-helices: the first occurs between Tyr-1 and Thr-12 and the second between Leu-14 and extending to Lys-28. There is evidence that the latter is capped by a turn occurring between Lys-28 and Glu-31. These helices form at the enkephalin receptor binding site, which is at the amino terminus, and at the morphine receptor binding site, located at the carboxyl terminus [Li, C. H. (1982) Cell (Cambridge, Mass.) 31, 504-505]. Our findings suggest that these two receptors may specifically recognize alpha-helices.

Binding of trifluoperazine and fluorene-containing compounds to calmodulin and adducts

Calmodulin can be specifically acylated with a fluorene-containing hydrophobic spin-labeling reagent at just Lys 75 or at Lys 75 and Lys 148. The binding of trifluoperazine to calmodulin and the two adducts was determined using a Hummel-Dreyer procedure, and binding of the phenothiazine was found to be characterized by apparent positive cooperativity and an apparent limiting stoichiometry of about seven binding sites per protein molecule. Two non-reactive fluorene-containing compounds were synthesized, and both reagents exhibited far less binding to calmodulin than did trifluoperazine. One of these was also assayed for binding to the monolabeled adduct, and this binding was about half that observed with calmodulin and was non-cooperative. Thus, the qualitative and quantitative binding parameters of hydrophobic groups to calmodulin can be quite different.

Affinity of nonhomologous amphiphilic peptides toward a monoclonal antibody raised against apolipoprotein A-I

Monoclonal antibodies against human apolipoprotein A-I (apoA-I) were generated by the hybridoma technique. Clone G-10 was selected on the basis of its highest titer. The affinity of this antibody toward a series of synthetic peptides differing in length, amino acid composition, and amphiphilicity was tested by using both the indirect and the competitive enzyme-linked immunosorbent techniques (ELISA). From these measurements we calculated dissociation constants of the complexes of the antibody with apoA-I bound to the surface of the microtiter plate, apoA-I in solution, and any of the several peptides in solution. The dissociation constant (Kd) of the immobilized apoA-I/anti-apoA-I-complex, Kd = 2 x 10(-9) M, was significantly lower than that of the complex resulting from the interaction between anti-apoA-I and either apoA-I in solution or any of the several amphiphilic helical peptides in solution. Peptides devoid of amphiphilic secondary structure were inert. These data are consistent with the proposal that monoclonal G-10 recognizes in antigenic peptides an alpha-helical secondary structure of defined hydrophilic-lipophilic balance and comparatively less the specific amino acid side chains. We propose that the highest contribution to the free energy of binding (8 Kcal/mole) is derived from the docking of the helix to the antibody. It follows that in probing the specificity of a monoclonal antibody the conformation and the physical environment of the interacting antigen must be taken into account.

Amphiphilic growth hormone releasing factor (GRF) analogs: peptide design and biological activity in vivo

The first twenty-nine amino acids of human Growth Hormone Releasing Factor (hGRF) possess a distinct amphiphilic character. This is seen as twisted hydrophobic and hydrophilic bands in the helical net projection. Four amidated analogs were designed by optimizing amphiphilic and helical potentials of the native sequence. These designed analogs, with up to eight-amino acid changes, were tested in sheep via intravenous injection. The growth hormone-stimulating activities of the analogs were significantly higher when compared to bovine Growth Hormone Releasing Factor (bGRF44-NH2). This suggests that the amphiphilic conformation of GRF(1-29) is important to the receptor.

Synthesis of sequences 1-16 and 63-95 of Cerebratulus lacteus toxin AIII. Hemolytic activity in a toxin fragment

Sequences 1-16 and 63-95 of C. lacteus toxin AIII were synthesized on benzhydrylamine and PAM resins respectively; the crude peptide products released by HF contained 90 and 33% of the target peptides. Fragment 63-95 was nearly a full agonist, producing greater than 90% of the hemolytic activity of the intact toxin molecule at 10(3)-fold higher concentrations. Fragment 1-16 had no activity.

Formation of water-soluble complex between the 1-34 fragment of parathyroid hormone and dimyristoylphosphatidylcholine

Two biologically active, 34 amino acid fragments of parathyroid hormone interact with dimyristoylphosphatidylcholine to form lipoprotein particles. In the lipid-bound form these parathyroid hormone peptides exhibit an increased amount of folded secondary structure and the tryptophan residue of [Nle8, Nle18, Tyr34] b PTH (1-34) amide appears to become buried in a more hydrophobic environment. The lipoprotein particle which is formed has dimensions of approximately 65 X 7 nm but aggregates to larger structures with increasing temperature. Above the phase transition of the phospholipid the peptides no longer affect the morphology of the lipid and the spectral properties of the peptide are not perturbed by the lipid. This is similar to the behavior of glucagon with dimyristoylphatidylcholine. The results indicate that several nonhomologous peptide hormones have common features which allow them to fold into an amphipathic helix and solubilize phospholipid.

Structure and action of heteronemertine polypeptide toxins: importance of amphipathic helix for activity of Cerebratulus lacteus toxin A-III

The marine heteronemertine Cerebratulus lacteus produces a family of protein cytolysins designated as A-toxins. Limited subtilisin digests of the most abundant homolog, toxin A-III, yield two major products which may be purified by high-performance liquid chromatography. One product is shown to represent residues 1-86 and the other contains the entire toxin sequence (1-95). Both polypeptides are shown to lack internal protease nicks. The 1-95 polypeptide retains full cytolytic activity in comparison to native toxin, whereas 1-86 has an activity that is approximately four times lower. Extensive treatment of A-III with carboxypeptidase Y yields a polypeptide containing residues 1-75 which is totally devoid of hemolytic activity. Residues 63-95 of native A-III have been predicted to form a relatively hydrophobic alpha-helix which is potentially important for activity. The circular dichroism spectrum of 1-95 is in excellent agreement with both experimental and Chou-Fasman-predicted secondary structures of native A-III, while the spectra of 1-86 and 1-75 indicate a loss of helicity quantitatively consistent with the removal of residues 87-95 and 76-95, respectively. Combined with our earlier data on bilayer penetration by N-terminal sequences (K. M. Blumenthal (1982) Biochemistry 21, 4229-4233], the current results indicate a direct involvement of both ends of A-III in lytic activity. The C-terminal region may function by contributing a membrane binding site in the form of an amphipathic helix.

Presence of an amphipathic helical segment and its relationship to biological potency of calcitonin analogs

The conformational properties of a number of calcitonin analogs were studied by circular dichroism. The ability of dimyristoylphosphatidylglycerol, lysophosphatidylcholine or sodium dodecyl sulfate to induce the formation of more highly ordered structures in these peptides was also assessed by circular dichroism. In all cases sodium dodecyl sulfate induced the largest change in the circular dichroism spectra of the peptides. Salmon calcitonin and its analogs were slightly more helical in the presence of the anionic phospholipid than in the presence of the zwitterionic detergent lysophosphatidylcholine while the reverse is true for human calcitonin and its analogs. Some of the calcitonin analogs convert turbid suspensions of phosphatidylglycerol to a clear solution from which the phospholipid is no longer readily sedimentable by centrifugation. Several of the physical properties of these peptides could be correlated with their biological activity. Generally peptides which showed no hypocalcemic activity had the least negative mean residue ellipticities at 222 nm. Only biologically active analogs were able quantitatively to solubilize dimyristoyl-phosphatidylglycerol and in this solubilized form the peptides have a higher helical content. More active derivatives exhibit larger increases in helix content in the presence of this phospholipid. Inactive analogs had the least negative mean residue ellipticities at 222 nm in the presence of lysophosphatidylcholine or sodium dodecyl sulfate. Thus, the ability of a calcitonin analog to form structures of higher helical content in the presence of amphiphiles is a requirement for the analog to exhibit high potency in assays of biological activity.

The design, synthesis, and characterization of tight-binding inhibitors of calmodulin

Based on a consideration of the probable structure of calmodulin and some natural peptides known to interact with it, two calmodulin-binding peptides were designed. These peptides bind to calmodulin in helical conformations and are capable of forming electrostatic and hydrophobic interactions with calmodulin. Their dissociation constants for binding (less than or equal to 210 and 400 pM) place them as the tightest-binding inhibitors of calmodulin thus far reported. The study of the interactions of these and similar peptides with calmodulin will provide valuable insights into the mechanisms whereby calmodulin binds to target enzymes, and it also serves as an excellent model system for exploring the physical chemistry of protein-protein interaction.

Amphiphilic secondary structure: design of peptide hormones

Peptide synthesis can be used for elucidating the roles of secondary structures in the specificity of hormones, antigens, and toxins. Intermediate sized peptides with these activities assume amphiphilic secondary structures in the presence of membranes. When models are designed to optimize the amphiphilicity of the secondary structure, stronger interactions can be observed with the synthetic peptides than with the naturally occurring analogs.