Demonstration by covalent cross-linking of a specific interaction between beta-endorphin and calmodulin

Characterization of chicken gizzard calcyclin and examination of its interaction with caldesmon

Using a procedure developed to purify calcyclin from mouse Ehrlich ascites tumor cells calcyclin was purified from smooth muscle of chicken gizzard. Chicken gizzard calcyclin bound to phenyl-Sepharose in a calcium dependent manner as did mouse EAT cells and rabbit lung calcyclin but appeared to be more acidic than its mammalian counterparts as revealed by ion exchange chromatography on Mono Q. Chicken gizzard calcyclin bound 45Ca2+ on nitrocellulose filters and exhibited a shift in electrophoretic mobility on urea-PAGE depending on Ca2+ concentration. Crosslinking experiments with BS3 showed that chicken gizzard calcyclin was able to form noncovalent dimers. As indicated by a decrease in maximum tryptophan fluorescence emission of caldesmon (about 14% at 1:1 molar ratio) and displacement of calmodulin from its complex with caldesmon, chicken gizzard calcyclin binds caldesmon. This binding was, however, much weaker than that of calmodulin and could not influence the interaction of caldesmon with actin. In consequence, calcyclin was unable to reverse the inhibitory effect of caldesmon on actin-activated Mg(2+)-ATPase activity of myosin in the presence of Ca2+.

Prostaglandin E2 induces up-regulation of murine macrophage beta-endorphin receptors

Cultured murine bone marrow macrophages specifically bound 125I-labeled beta-endorphin. Binding was displaceable by 100 times molar excess of full-length beta-endorphin but was insensitive to the opioid receptor antagonist, naloxone. Binding was inhibited by beta-endorphin's C-terminal tetrapeptide, lys-lys-gly-glu, but not by the truncated N-terminal 27 amino acid fragment, indicating that binding of beta-endorphin to this receptor is dependent on its C-terminus. Macrophages incubated for 24 h with 10(-8)-10(-5) M prostaglandin E2 showed a dose-dependent increase in beta-endorphin binding, implying receptor up-regulation. This was also observed in response to the phosphodiesterase inhibitor, isobutylmethylxanthine, indicating that regulation of these receptors may be mediated through a cAMP-dependent process. This is the first demonstration that beta-endorphin receptor expression can be positively regulated.

Beta-endorphin and the immune system--possible role in autoimmune diseases

The immune system and the neuroendocrine system are closely interconnected having such means of bidirectional communication and regulation. In this review, a hypothesis is put forward regarding the possible role of beta-endorphins in the pathogenesis of autoimmune diseases: It is suggested that the increased cytokine production in immunoinflammatory disorders induces production of beta-endorphins from the pituitary and the lymphocytes; the enhanced level of beta-endorphin causes inhibition of human T helper cell function, which potentially down-regulate the antibody production. Also the beta-endorphin-induced enhancement of the natural killer cell activity may suppress the B cell function. In addition, beta-endorphin also exerts a direct inhibitory effect on the antibody production. Thus, in autoimmune disorders the enhanced cytokine level may via stimulation of the production of beta-endorphins exert a negative feed back on the antibody production and potentially so on the production of autoantibodies.

Cation binding and conformation of tryptic fragments of Nereis sarcoplasmic calcium-binding protein: calcium-induced homo- and heterodimerization

Nereis sarcoplasmic calcium-binding protein (NSCP) is a compact 20-kDa protein that competitively binds three Ca2+ or Mg2+ ions and displays strong positive cooperativity. Its three-dimensional structure is known. It thus constitutes a good model for the study of intramolecular information transduction. Here we probed its domain structure and interaction between domains using fragments obtained by controlled proteolysis. The metal-free form, but not the Ca2+ or Mg2+ form, is sensitive to trypsin proteolysis and is preferentially cleaved at two peptide bonds in the middle of the protein. The N-terminal fragment 1-80 (T1-80) and the C-terminal fragment 90-174 (T90-174) were purified to electrophoretic homogeneity. T1-80, which consists of a paired EF-hand domain, binds one Ca2+ with Ka = 3.1 x 10(5) M-1; entropy increase is the main driving force of complex formation. Circular dichroism indicates that T1-80 is rich in secondary structure, irrespective of the Ca2+ saturation. Ca2+ binding provokes a difference spectrum which is similar to that observed in the intact protein. These data suggest that this N-terminal domain constitutes the stable structural nucleus in NSCP to which the first Ca2+ binds. T90-174 binds two Ca2+ ions with Ka = 3.2 x 10(4) M-1; the enthalpy change contributes predominantly to the binding process. Metal-free T90-174 is mostly in random coil but converts to an alpha-helical-rich conformation upon Ca2+ binding. Ca2+ binding to T1-80 provokes a red-shift and intensity decrease of the Trp fluorescence but a blue-shift and intensity increase in T90-174.(ABSTRACT TRUNCATED AT 250 WORDS)

Model for the interaction of amphiphilic helices with troponin C and calmodulin

Crystals of troponin C are stabilized by an intermolecular interaction that involves the packing of helix A from the N-terminal domain of one molecule onto the exposed hydrophobic cleft of the C-terminal domain of a symmetry related molecule. Analysis of this molecular recognition interaction in troponin C suggests a possible mode for the binding of amphiphilic helical molecules to troponin C and to calmodulin. From the template provided by this troponin C packing, it has been possible to build a model of the contact region of mastoporan as it might be bound to the two Ca2+ binding proteins. A possible binding mode of melittin to calmodulin is also proposed. Although some of the characteristics of binding are similar for the two amphiphilic peptides, the increased length of melittin requires a significant bend in the calmodulin central helix similar to that suggested recently for the myosin light chain kinase calmodulin binding peptide (Persechini and Kretsinger: Journal of Cardiovascular Pharmacology 12:501-512, 1988). Not only are the hydrophobic interactions important in this model, but there are several favorable electrostatic interactions that are predicted as a result of the molecular modeling. The regions of troponin-C and calmodulin to which amphiphilic helices bind are similar to the regions to which the neuroleptic drugs such as trifluoperazine have been predicted to bind (Strynadka and James: Proteins 3:1-17, 1988).

Diffuse reflectance infrared Fourier transform spectroscopic characterization of a silica-immobilized N-hydroxysuccinimide active ester crosslinking agent and its precursors

Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy was used to characterize the product of each step in the preparation of a silica-immobilized N-hydroxysuccinimide (NHS) active ester. The preparation of this NHS active ester linkage was based on a literature procedure for the immobilization of proteins. The DRIFT method was used to guide modification of this literature procedure. The DRIFT method also was used to indicate an impurity entrapped in the 60-A diameter pores of the silica support during the formation of the immobilized active ester. Degradation of the immobilized NHS active ester, stored under either argon or dioxane, can be followed by the DRIFT method. Myoglobin and glycine were allowed to react with the active ester, and the result for this silica support was evaluated by the DRIFT method. Elemental analysis was used to provide information on the loading of the silica-immobilized moieties that were presented for DRIFT analysis.

Analysis of calmodulin acceptor proteins and the influence of calmodulin antagonists on human spermatozoa

The possible role of calmodulin in regulating a number of calcium-dependent functions exhibited by human spermatozoa was investigated by using the antagonists trifluoperazine and calmidazolium. At high doses both antagonists inhibited the motility of human spermatozoa and induced a concomitant rise in [Ca2+]i and a decline in cAMP. Lower doses of these antagonists, particularly calmidazolium, suppressed the ability of human spermatozoa to generate reactive oxygen species and exhibit sperm-oocyte fusion, without influencing [Ca2+]i, cAMP, or motility. This inhibition of sperm-oocyte fusion was effective even if the spermatozoa were subsequently exposed to A23187, suggesting that calmodulin may regulate this aspect of human sperm function at a point downstream from calcium influx. Both radiolabelling and affinity chromatography techniques were used to detect a number of calcium-dependent and calcium-independent calmodulin acceptor proteins in the human spermatozoon. The major calcium-dependent acceptor proteins exhibited Mr values of 32,000 and 22,000-27,000, respectively, and did not appear to be associated with the sperm plasma membrane.

Binding of amphiphilic peptides to a carboxy-terminal tryptic fragment of calmodulin

Calmodulin (CaM) fragments 1-77 (CaM 1-77) and 78-148 (CaM 78-148) were prepared by tryptic cleavage of CaM. CaM 78-148 exhibited Ca2+-dependent binding to mastoparan X, Polistes mastoparan, and melittin with apparent dissociation constants less than 0.2 microM as judged from changes in the fluorescence spectrum and anisotropy of the single tryptophan residue of each of these cationic, amphiphilic peptides. This interaction was accompanied by a large spectral blue shift of the peptide fluorescence spectrum. These findings are consistent with earlier results [Malencik, D.A., & Anderson, S.R. (1984) Biochemistry 23, 2420-2428] on the binding of mastoparan X to CaM fragment 72-148. The binding of the peptide to CaM 78-148 also caused a significant loss of the accessibility of the peptide tryptophan to the fluorescence quencher acrylamide. The CaM 78-148 induced effects on the fluorescence spectra and tryptophan accessibility of the peptides were most pronounced for mastoparan X, a peptide with tryptophan on the apolar face of the putative amphiphilic helix. The data were comparable with results from parallel experiments on the Ca2+-dependent interaction of these peptides with intact CaM. Difference circular dichroic spectra suggested that binding to CaM 78-148 was associated with the induction of considerable degrees of helicity in the amphiphilic peptides, which by themselves have predominantly random coil structures in aqueous solution. This finding is also reminiscent of the interaction of these peptides with intact CaM.(ABSTRACT TRUNCATED AT 250 WORDS)

Association of melittin with the isolated myosin light chains

Melittin is a 26-residue peptide which undergoes high-affinity calcium-dependent binding by calmodulin [Barnette, M.S., Daly, R., & Weiss, B. (1983) Biochem. Pharmacol. 32, 2929; Comte, M., Maulet, Y., & Cox, J.A. (1983) Biochem. J. 209, 269; Anderson, S.R., & Malencik, D.A. (1986) Calcium Cell Funct. 6, 1]. The results in this paper show that three different types of myosin light chain--the smooth muscle regulatory light chain, the smooth muscle essential light chain, and the skeletal muscle regulatory 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) light chain--also associate with melittin. The resulting complexes have dissociation constants ranging from 1.1 to 2.5 microM in the presence of 0.10 M NaCl and from approximately 50 to approximately 130 nM in solutions of 20 mM 3-(N-morpholino)propanesulfonic acid alone. The regulatory smooth muscle myosin light chain exhibits two equivalent melittin binding sites while each of the others displays only one. The myosin light chains evidently contain elements of structure related to the macromolecular interaction sites present in calmodulin and troponin C but not in parvalbumin. The association of melittin and other peptides with the light chains requires consideration whenever assays of the calmodulin-dependent activity of myosin light chain kinase are used to determine peptide binding by calmodulin. The binding measurements performed on the DTNB light chain and melittin necessitated derivation of the equation relating complex formation to the observed fluorescence anisotropy of a solution containing three fluorescent components. This analysis is generally applicable to equilibria involving the association of two fluorescent molecules emitting in the same wavelength range.

Interaction of alpha-N-Acetyl-beta-endorphin and calmodulin

Acetylation at the alpha-amino terminal is a common post-translational modification of many peptides and proteins. In the case of the potent opiate peptide beta-endorphin, alpha-N-acetylation is a known physiological modification that abolishes opiate activity. Since there are no known receptors for alpha-N-acetyl-beta-endorphin, we have studied the association of this peptide with calmodulin, a calcium-dependent protein that binds a variety of peptides, phenothiazines, and enzymes, as a model system for studying acetylated endorphin-protein interactions. Association of the acetylated peptide with calmodulin was demonstrated by cross-linking with bis(sulfosuccinimidyl)suberate; like beta-endorphin, adducts containing 1 mol and 2 mol of acetylated peptide per mole calmodulin were formed. Some of the bound peptides are evidently in relatively close proximity to each other since, in the presence of amidated (i.e., lysine-blocked) calmodulin, cross-linking yielded peptide dimers. The acetylated peptide exhibited no appreciable helicity in aqueous solution, but in trifluoroethanol (TFE) considerable helicity was formed. Also, a mixture of acetylated peptide and calmodulin was characterized by a circular dichroic spectrum indicative of induced helicity. Empirical prediction rules, applied earlier to beta-endorphin, suggest that residues 14-24 exhibit alpha-helix potential. This segment has the potential of forming an amphipathic helix; this structural unit is believed to be important in calmodulin binding. The acetylated peptide was capable of inhibiting the calmodulin-mediated stimulation of cyclic nucleotide phosphodiesterase (EC 3.1.4.17) activity with an effective dose for 50% inhibition of about 3 microM; this inhibitory effect was demonstrated using both an enzyme-enriched preparation as well as highly purified enzyme. Thus, acetylation at the alpha-amino terminal of beta-endorphin, although abolishing opiate activity, does not interfere with the binding to calmodulin. Indeed, beta-endorphin and the alpha-N-acetylated peptide behave very similarly with respect to calmodulin association.

Reactions of N-hydroxysulfosuccinimide active esters

N-Hydroxysulfosuccinimide esters are reactive functional groups employed in a variety of protein modification reagents, especially cross-linking reagents. For these compounds, hydrolysis is the most important reaction competing for reaction of the esters with nucleophilic groups in proteins. We have employed model compounds to investigate the rates of hydrolysis of N-hydroxysulfosuccinimide esters and their reactions with the alpha-amino group and the side chains of naturally occurring amino acids, under conditions comparable to those used for protein modification studies. The rats of hydrolysis observed were found to be very low, as compared with their rates of reaction with nitrogen nucleophiles found in proteins. Further, within the ranges investigated, the rate of aminolysis was observed to increase more rapidly than the rate of hydrolysis with increasing pH or with increasing temperature. Four amino acid side chains and the alpha-amino group were found to react measurably with N-hydroxysulfosuccinimide esters. At pH 7.4 and room temperature, the order of reactivity was found to be N alpha-Cbz-histidine greater than N alpha-Cbz-lysine approximately phenylalanine (alpha-amino group) much greater than N-acetylcysteine approximately N-acetyltyrosine; however, the acylimidazole adduct formed with the side chain of histidine was found to be a transient product, subject to hydrolysis or reaction with another nucleophile.

Calmodulin-drug interaction. A fluorescence and flow dialysis study

Various Ca2+-antagonists and related compounds were probed for possible anti-calmodulin properties. Some of them efficiently inhibit calmodulin dependent activity (the plasma membrane Ca2+-ATPase and the cyclic nucleotide phosphodiesterase). The I50-values for the most potent inhibitors varied between 15 and 30 uM. Using fluorescence spectroscopy and flow dialysis methods the stoichiometry of the binding of some of the drugs to calmodulin has been investigated. The number of Ca2+-dependent high affinity binding sites has been studied on trypsin fragments of calmodulin. Compound 12-114 was bound with high affinity in a Ca2+-dependent way to both halves of calmodulin, compound 200-737 recognized one high affinity binding site only in the C-terminal half of the molecule, whereas compound 36-079 demanded the intact protein to be able to interact with high affinity in a Ca2+-dependent manner.

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.

Mastoparan binding induces a structural change affecting both the N-terminal and C-terminal domains of calmodulin. A 113Cd-NMR study

113Cd-NMR studies of solutions of cadmium-loaded calmodulin (Cd4CaM) and the tetradecapeptide mastoparan in different ratios show that mastoparan binds to Cd4CaM with high affinity. The off-rate of protein- bound mastoparan is found to be 40 s-1 or less. The binding of one molecule of mastoparan to Cd4CaM is observed to affect all four metal-binding sites, indicating that both the N-terminal and C-terminal globular domains of the protein undergo conformational changes.

Reversible chemical cross-linking of the light-harvesting polypeptides of Rhodopseudomonas viridis

The topography of the light-harvesting polypeptides of Rhodopseudomonas viridis was investigated using cleavable chemical cross-linkers. To this end a set of succinimidyl esters and surface-specific sulfosuccinimidyl esters of different span widths were synthesized. The cross-linking reagents have been characterized using NMR and infrared spectroscopy and thin-layer chromatography. The cross-linking reaction was carried out under physiological conditions and the aggregates were analyzed by the methods of one- and two-dimensional polyacrylamide gel electrophoresis and by immunoblot analysis. We found cross-linkage between B1015-alpha and B1015-alpha, between B1015-alpha and B1015-beta and B1015-beta and B1015-beta. Aggregates of higher molecular mass were hetero-oligomers of B1015-alpha and B1015-beta containing three and four polypeptides, respectively. The results obtained in this work indicate a very tight contact among the light-harvesting polypeptides. We assume that the light-harvesting polypeptides are localized alternately as dimers of B1015-alpha and B1015-beta around the reaction centre core.

Inhibition of calmodulin-stimulated phosphodiesterase activity by vasoactive intestinal peptide

The effects of certain peptides of the glucagon family on calmodulin activity were determined from their capacity to inhibit a calmodulin-dependent form of phosphodiesterase. Vasoactive intestinal peptide and secretin were potent inhibitors of calmodulin activity, having IC50 values of 0.5 microM and 2 microM, respectively. By contrast, glucagon failed to inhibit calmodulin activity even at concentrations of 100 microM. None of these compounds significantly inhibited the basal activity of phosphodiesterase at concentrations up to 100 microM. These findings support the suggestion that important structural features of peptides for anticalmodulin activity include a net positive charge and a hydrophobic surface.

Fructose-1,6-bisphosphatase from Synechococcus leopoliensis. Substrate-dependent dimer-tetramer interconversion

Extracts of Synechococcus leopoliensis (Anacystis nidulans) contain two forms of D-fructose-1,6-bisphosphatase (EC 3.1.3.11) previously designated as forms A and B [Gerbling, K.-P., Steup, M., and Latzko, E. (1984) Arch. Microbiol. 137, 109-114]. Form B, which probably represents the major part of the total extractable fructose-1,6-bisphosphatase activity, has been purified to apparent homogeneity. Gel filtration, non-denaturing polyacrylamide gel electrophoresis, and cross-linking with bis(sulfosuccinimidyl)suberate revealed that the fructose-1,6-bisphosphatase B exists in either a dimeric or in a tetrameric subform, depending upon the absence or presence of fructose-1,6-bisphosphate and Mg2+. The dimer--tetramer interconversion was readily reversible. The results provide evidence for a two-step activation of fructose-1,6-bisphosphatase B involving the reduction of the dimeric subform and the subsequent substrate-dependent conversion of the reduced dimer to a reduced tetramer, which is the only catalytically active state. In contrast to form B, no substrate-dependent interconversion was detected with form A from S. leopoliensis.

Evolutionary relationship between nonmammalian and mammalian peptides

An hypothesis has been developed to rationalize the evolution of regulatory peptides. In order to account for critical relationships involving peptide regulators, their receptors, and peptide processing enzymes, the following generalizations will be supported: (1) peptides arose from protein precursors as proteolytic digestion by-products and acquired hormonal status during the course of natural selection; (2) initially, peptides served primarily nutritional roles, thereby permitting increased growth rates and reproductive advantages for recipient cells; (3) specific peptide sequences were conserved during evolution and were associated with biological activities which were essential for survival of species as divergent as unicellular organisms, amphibians, and mammals; and (4) regulatory peptides probably arose simultaneously with their membrane-oriented, macromolecular receptor sites. In support of the conservation of sequence information or function, or both, during evolutionary development, evidence has been obtained to indicate that peptide sequences which occur in two classes of amphibian peptides appear to be extensively conserved in mammals. Studies with an antiserum directed against the N-terminal sequence of amphibian physalaemin have permitted the recognition of a mammalian octapeptide which exhibits 80% homology with residues 1-5 in that region. Another study with an antiserum directed against the midregion (sequence 5-8) of amphibian bombesin has indicated the existence of milk peptides which mimic bombesin in several pharmacological bioassays. These studies indicate that radioimmunoassays can be powerful tools in facilitating recognition of peptide sequences conserved throughout evolution.

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.

Endorphins stimulate normal human peripheral blood lymphocyte natural killer activity

Opioid peptides are present in peripheral blood, and may bind to human lymphocytes. In order to determine their influence on human lymphocytes we studied the effect of endogenous opioid peptides on human lymphocyte natural killer function. Beta-endorphin and several analogues (i.e., gamma-endorphin) are shown to enhance human peripheral blood natural killer function. The enhancement of natural killing by these opioid peptides was dose-dependent and naloxone (an opiate antagonist) reversible. In studying various analogues of beta-endorphin, beta-lipotropin and gamma-endorphin were approximately 3-5 times more effective at enhancing peripheral blood NK function than Leu-enkephalin and -endorphin. In addition, we observed that naloxone reversed human fibroblast interferon mediated enhancement of human blood lymphocyte natural killer function. These observations suggest that circulating endogenous opioid peptides may have a physiologic role in regulating human blood lymphocyte natural killing.

Peptide binding by calmodulin and its proteolytic fragments and by troponin C

Calmodulin and troponin C exhibit calcium-dependent binding of 1 mol/mol of dynorphin. The dissociation constants of the complexes, determined in 0.20 N KC1-1.0 mM CaCI2, pH 7.3, are 0.6 microM for calmodulin, 2.4 microM for rabbit fast skeletal muscle troponin C, and 9 microM for bovine heart troponin C. Experiments with deletion peptides of dynorphin show that peptide chain length and especially charge affect the binding of the peptides by calmodulin. Dynorphin, but not mastoparan or melittin, inhibits adenosinetriphosphatase activity in a reconstituted rabbit skeletal muscle actomyosin assay. The inhibition is partially reversed by the addition of calmodulin or troponin C in the presence of calcium. Calmodulin also exhibits calcium-dependent binding of a synthetic peptide corresponding to positions 104-115 of rabbit fast skeletal muscle troponin I. Mastoparan is a tetradecapeptide from the vespid wasp having exceptional affinity for calmodulin, with Kd approximately 0.3 nM [Malencik, D.A., & Anderson, S.R. (1983) Biochem. Biophys. Res. Commun. 114, 50]. The addition of 1 mol/mol of mastoparan to the complex of calmodulin with dynorphin results in complete dissociation of dynorphin. Similar titrations of the skeletal muscle troponin C-dynorphin complex produce a gradual dissociation consistent with a dissociation constant of 0.2 microM for the troponin C-mastoparan complex. Fluorescence anisotropy measurements using the intrinsic tryptophan fluorescence of mastoparan X show strongly calcium-dependent binding by proteolytic fragments of calmodulin. binding by proteolytic fragments of calmodulin.(ABSTRACT TRUNCATED AT 250 WORDS)

High affinity binding of the mastoparans by calmodulin

Calmodulin exhibits high affinity, calcium-dependent binding of the mastoparans--a group of cytoactive tetradecapeptides. The dissociation constants for the peptide-calmodulin complexes determined in 0.20 N KCl, 1.0 mM CaCl2, pH 7.3 are approximately 0.3 nM for mastoparan, approximately 0.9 nM for mastoparan X, and approximately 3.5 nM for Polistes mastoparan. The dissociation constant for the mastoparan-calmodulin complex is the smallest known for any calmodulin binding protein or peptide, suggesting that some type of peptide-calmodulin interaction could be physiologically significant.

des-(1-13) human beta-endorphin interacts with calmodulin

It is known that the 31-residue neuropeptide beta-endorphin inhibits the calcium-dependent, calmodulin-mediated stimulation of cyclic nucleotide phosphodiesterase activity. The results of this study demonstrate that a non-opiate, synthetic amino terminal deletion peptide, des-(1-13), of human beta-endorphin is also capable of inhibiting the stimulated enzymic activity, but not the basal activity. This inhibition occurs with the same efficacy as the intact 31-residue peptide. Thus, the amino terminal region of beta-endorphin, which is responsible for opiate activity, does not appear to contribute to the calmodulin interaction. Circular dichroic spectroscopy of des-(1-13) beta-endorphin, calmodulin, and mixtures of the two shows that the ellipticity at 221 nm was more negative in the peptide-protein mixture than could be accounted for on the basis of simple additivity of the peptide and calmodulin. This spectral change implies enhanced alpha-helicity concomitant with the peptide-protein association. Helix formation may occur in the peptide since this sequence has the potential to form an amphipathic helix.