Investigating hydrophobic ligand-receptor interactions in parathyroid hormone receptor using peptide probes.

With an increasing number of new chemical entities entering clinical studies, and an increasing share of the market, peptides and peptidomimetics constitute one of the most promising classes of therapeutics. The success of synthetic peptides as therapeutics relies on the lead optimization step in which the lead candidates are modified to improve drug-like properties of peptides related to potency, pharmacokinetics, solubility, and stability, among others. Peptidomimetics based on the N-terminal stretch of the first 11 amino acids of the PTH have been investigated as potential lead compounds for the treatment of osteoporosis. On the basis of a peptide reported in the literature, referred to here as the Parent Peptide (H-Aib-Val-Aib-Glu-Ile-Gln-Leu-Nle-His-Gln-Har-NH2), we conducted systematic SAR analyses to investigate the effects of altering peptide hydrophobicity on PTH receptor functional potency as measured by the cAMP (cyclic adenosine monophosphate) accumulation and ß-arrestin recruitment assays. Among hydrophobic residues, we found that the Val2 position shows the least flexibility in terms of the SAR studies, whereas the Leu7 position appeared to be most flexible. Through circular dichroism and nuclear magnetic resonance spectroscopy studies, we were able to establish that changes in hydrophobic residues significantly change the extent of peptide helicity and that the helical character correlates well with receptor agonist activity. Here, we report several novel PTH 1-11 peptidomimetics that show comparable or enhanced potency to stimulate Gs-signaling over ß-arrestin recruitment as compared with such properties of PTH 1-34 and the Parent Peptide.

Molecular approaches to receptors as targets for drug discovery.

The cloning of a great number of receptors and channels has revealed that many of these targets for drug discovery can be grouped into superfamilies based on sequence and structural similarities. This review presents an overview of how molecular biological approaches have revealed a plethora of receptor subtypes, led to new definitions of subtypes and isoforms, and played a role in the development of high selective drugs. Moreover, the diversity of subtypes has molded current views of the structure and function of receptor families. Practical difficulties and limitations inherent in the characterization of the ligand binding and signaling properties of expressed recombinant receptors are discussed. The importance of evaluating drug-receptor interactions that differ with temporally transient and distinct receptor conformational states is emphasized. Structural motifs and signal transduction features are presented for the following major receptor superfamilies: ligand-gated ion channel, voltage-dependent ion channel, G-protein coupled, receptor tyrosine-kinase, receptor protein tyrosine-phosphatase, cytokine and nuclear hormone. In addition, a prototypic receptor is analyzed to illustrate functional properties of a given family. The review concludes with a discussion of future directions in receptor research that will impact drug discovery, with a specific focus on orphan receptors as targets for drug discovery. Methods for classifying orphan receptors based upon homologies with members of existing superfamilies are presented together with molecular approaches to the greater challenge of defining their physiological roles. Besides revealing new orphan receptors, the human genome sequencing project will result in the identification of an abundance of novel receptors that will be molecular targets for the development of highly selective drugs. These findings will spur the discovery and development of an exciting new generation of receptor-subtype specific drugs with enhanced therapeutic specificity.

Reconstitution of high-affinity binding of a beta-scorpion toxin to neurotoxin receptor site 4 on purified sodium channels.

Reconstitution of purified sodium channels into phospholipid vesicles restores many aspects of sodium channel function including high-affinity neurotoxin binding and action at neurotoxin receptor sites 1-3 and 5, but neurotoxin binding and action at receptor site 4 has not previously been demonstrated in purified and reconstituted preparations. Toxin IV from the venom of the American scorpion Centruroides suffusus suffusus (Css IV), a beta-scorpion toxin, shifts the voltage dependence of sodium channel activation by binding with high affinity to neurotoxin receptor site 4. Sodium channels were purified from rat brain and reconstituted into phospholipid vesicles composed of phosphatidylcholine and phosphatidylethanolamine (65:35). 125I-Css IV, purified by reversed-phase HPLC, bound rapidly and specifically to reconstituted sodium channels. Dissociation of the bound toxin was biphasic with half-times of 0.22 min-1 and 0.015 min-1. At equilibrium, the toxin bound to two classes of specific high-affinity sites, a variable minor class with KD of approximately 0.1 nM and a major class with a KD of approximately 5 nM. Approximately 0.8 mol 125I-Css IV was bound per mole of reconstituted, right-side-out sodium channels, as assessed from comparison of binding of saxitoxin and Css IV. Binding of Css IV was unaffected by membrane potential or by neurotoxins that bind at sites 1-3 or 5, consistent with the characteristics of binding of beta-scorpion toxins to sodium channels in cells and membrane preparations.(ABSTRACT TRUNCATED AT 250 WORDS)

Specific binding of the novel Na+ channel blocker PD85,639 to the alpha subunit of rat brain Na+ channels.

The local anesthetic-like Na+ channel-blocking drug [3H]PD85639 [alpha-([4-3H]phenyl)-N-[3-(2,6-dimethyl-1-piperizinyl)-alpha-prop yl] [4-3H]benzeneacetamide] binds specifically to receptor sites on Na+ channels in intact synaptosomes and synaptosomal membranes, purified and reconstituted Na+ channels, and type IIA Na+ channel alpha subunits expressed in the transfected Chinese hamster ovary cell line CNaIIA-1. No specific binding was observed in nontransfected CHO-K1 cells, confirming the specificity of binding to Na+ channels. Two classes of binding sites that differed in affinity and dissociation rate were observed in all three preparations. In synaptosomes, the high affinity sites had Kd values of 3-20 nM and a Bmax of approximately 0.2 pmol/mg, whereas the low affinity sites had Kd values of 0.4-20 microM and a Bmax of approximately 5 pmol/mg. Binding of PD85,639 was inhibited by the local anesthetics tetracaine, bupivacaine, and mepivacaine at concentrations in the same range as those that inhibit Na+ channels. Tetracaine did not affect the dissociation rate of PD85,639, consistent with competitive binding of these two drugs at the same receptor site. In contrast, binding of PD85,639 was unaffected by the anticonvulsants phenytoin and carbamazepine, which also inhibit Na+ channels. Veratridine and batrachotoxin, which bind at neurotoxin receptor site 2 on Na+ channels, inhibited specific PD85,639 binding completely. PD85,639 accelerated dissociation of specifically bound batrachotoxin, consistent with an indirect allosteric interaction between these two compounds. Thus, like local anesthetics, PD85,639 inhibits binding of batrachotoxin by an allosteric mechanism. The results indicate that PD85,639 binds specifically to a local anesthetic receptor site on the Na+ channel alpha subunit that is allosterically linked to neurotoxin receptor site 2. PD85,639 may be a useful molecular probe of this important drug receptor site on the Na+ channel.

Photoaffinity labeling of the brevetoxin receptor on sodium channels in rat brain synaptosomes.

Brevetoxin, a neurotoxin isolated from the marine dinoflagellate Ptychodiscus brevis, has been derivatized into a photoaffinity probe by carbodiimide linkage to p-azidobenzoic acid. Rosenthal analysis of a tritiated p-azidobenzoate brevetoxin derivative indicates that specific binding of the toxin occurs at two distinct and separate sites, with Kd and Bmax values of 0.21 nM and 2.12 pmol/mg of protein for the high affinity site and 50.7 nM and 91.5 pmol/mg of protein for the low affinity site, respectively. Binding of tritiated photoaffinity probe to the high affinity/low capacity site can be displaced in a competitive manner by native brevetoxin (Kd = 1.9 nM), demonstrating a specific competitive interaction with the receptor site. Rat brain synaptosomes, covalently labeled with the brevetoxin photoaffinity probe, were subjected to detergent solubilization. The covalently labeled membrane protein was estimated to have a Stokes radius of 55 +/- 3 A. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed specific labeling of a 260-kDa protein. Treatment with 2-mercaptoethanol and neuraminidase resulted in retention of brevetoxin binding to this high molecular weight protein. The affinity-purified membrane protein-brevetoxin photoaffinity probe complex was specifically recognized by a sodium channel antibody directed against the intracellular side of transmembrane segment IS6. The sodium channel alpha subunit is implicated as the specific site of brevetoxin interaction.

Localization of the receptor site for alpha-scorpion toxins by antibody mapping: implications for sodium channel topology.

Site-directed and monoclonal antibodies recognizing different extracellular regions of the RII sodium channel alpha subunit have been used to determine the sequences that comprise the receptor for alpha-scorpion toxins by evaluating the effect of antibody on voltage-dependent binding of radio-labeled toxin isolated from Leiurus quinquestriatus to both reconstituted rat brain sodium channel and rat brain synaptosomes. Of six antibodies tested, two recognizing amino acid residues 355-371 and 382-400 located on an extracellular loop between transmembrane segments S5 and S6 of domain I and one recognizing residues 1686-1703 of a similar loop of domain IV inhibit binding by 30-55%. Inhibition is concentration-(EC50 = 0.4-2 microM) and time- (t1/2 = 40-80 min) dependent. Five different monoclonal antibodies recognizing the same extracellular loop in domain I inhibit binding completely with similar EC50 values as observed for site-directed antibodies. Kinetic studies of the antibody effect are consistent with a slowly reversible competition for the toxin receptor site. Our results suggest that the extracellular loops between segments S5 and S6 of domains I and IV comprise at least part of the alpha-scorpion toxin receptor site and support the membrane topology models in which domains I and IV are adjacent in the tertiary structure of the channel protein and six transmembrane sequences are contained in each of the four homologous domains.

How does a key fit a flexible lock? Structure and dynamics in receptor function.

The preceding five years have brought remarkable advances in our understanding of the primary structure of drug receptors. The roles of certain amino acid residues in binding drugs and effecting receptor function have been proposed. As even more detailed structures become available, the goal of rational design of drug molecules based on predicted fits between the drug and its receptor will be near at hand. Although none of the classical receptors has yet yielded to X-ray crystallographic analysis, the methods of molecular biology facilitate the production of the large amounts of these rare proteins necessary for crystallization. Receptor proteins share one fundamental characteristic with allosterically regulated enzymes. Both have the structural flexibility that allows information to be transmitted to distant parts of the molecule. We will discuss recent observations about receptor structure and the dynamic nature of drug receptors, and pose questions about the significance of receptor dynamics for drug design.

Rapid kinetics of alpha 2-adrenergic inhibition of adenylate cyclase. Evidence for a distal rate-limiting step.

Activation and inhibition of adenylate cyclase in the presence of GTP, the natural guanine nucleotide regulator, are too fast to study by standard biochemical methods. In order to identify the rate-limiting steps in adenylate cyclase regulation, we measured the kinetics of stimulation and inhibition of the enzyme on a subsecond to second time scale using a novel rapid-mix quench technique. Even using our rapid-mix quench method, activation by PGE1 and forskolin was instantaneous (cAMP accumulation was linear between 0.5 and 30 s). In contrast, we found a lag period of 1.2-10 s for epinephrine-mediated inhibition. The length of the lag depended on the concentration of GTP and monovalent cations present. In the absence of NaCl, the rate constant for the onset of inhibition (kinh) increased only slightly with GTP concentration saturating at a value of 0.16 s-1 (t1/2 4.3 s) at 1 microM GTP. In the presence of 100 mM NaCl, kinh was strongly dependent on GTP concentration, reaching a maximum value of 0.57 s-1 (t1/2 1.2 s) at 100 microM GTP. Thus, activation of both Gi and Gs in intact platelet membranes is much faster (t1/2 less than 5 s) than previously reported for reconstituted systems. Also, the strong dependence of the rate of adenylate cyclase inhibition on GTP concentration implies that the rate-limiting step in inhibition is distal to GTP binding. The effect of NaCl to increase the maximal rate of inhibition is specific for sodium since KCl has no effect on kinh.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of adenylate cyclase is mediated by the high affinity conformation of the alpha 2-adrenergic receptor.

The functional significance of high affinity agonist binding to receptors that interact with guanine nucleotide regulatory proteins has remained controversial. Preincubation of human platelet membranes with the full alpha 2-agonist UK 14,304 in the absence of GTP increases the potency of the agonist to inhibit adenylate cyclase in a pre-steady state (15-sec) assay. The EC50 after preincubation (6 +/- 1 nM) is within a factor of 2 of the high affinity Kd for [3H]UK 14,304 binding determined under identical conditions (2.7 +/- 0.1 nM). In contrast, in the usual steady state measurements (15 min) or in pre-steady state measurements without agonist preincubation, the EC50 values (74 +/- 1 and 207 +/- 8 nM, respectively) are near the low affinity Kd for [3H]UK 14,304 binding. Reduction of the GTP concentration in steady state adenylate cyclase assays also decreases the EC50 for UK 14,304 from 40 +/- 5 nM at 10 microM GTP to 14 +/- 5 nM with no added GTP. Both sets of experimental observations are accommodated by a complete kinetic model of inhibition in which the high affinity ternary complex of drug, receptor, and G protein leads to the response. Explicit rate parameters are included for agonist binding, receptor-G protein interactions, GTP binding, and hydrolysis. Despite the functional role of the high affinity state of the alpha 2-receptor in this model, the steady state EC50 for agonist-mediated inhibition correlates best with the Kd of low affinity agonist binding in the presence of high levels of GTP. Under conditions in which formation of the high affinity ternary complex is favored, the EC50 for responses approaches the high affinity Kd.

Mechanism of agonist and antagonist binding to alpha 2 adrenergic receptors: evidence for a precoupled receptor-guanine nucleotide protein complex.

The alpha 2 adrenergic receptor (AR) inhibits adenylate cyclase via an interaction with Ni, a guanine nucleotide binding protein. The early steps involved in the activation of the alpha 2 AR by agonists and the subsequent interaction with Ni are poorly understood. In order to better characterize these processes, we have studied the kinetics of ligand binding to the alpha 2 AR in human platelet membranes on the second time scale. Binding of the alpha 2 antagonist [3H]yohimbine was formally consistent with a simple bimolecular reaction mechanism with an association rate constant of 2.5 X 10(5) M-1 s-1 and a dissociation rate constant of 1.11 X 10(-3) s-1. The low association rate constant suggests that this is not a diffusion-limited reaction. Equilibrium binding of the alpha 2 adrenergic full agonist [3H]UK 14,304 was characterized by two binding affinities: Kd1 = 0.3-0.6 nM and Kd2 = 10 nM. The high-affinity binding corresponds to approximately 65% and the low-affinity binding to 35% of the total binding. The kinetics of binding of [3H]UK 14,304 were complex and not consistent with a mass action interaction at one or more independent binding sites. The dependence of the kinetics on [3H]UK 14,304 concentration revealed a fast phase with an apparent bimolecular reaction constant kappa + of 5 X 10(6) M-1 s-1. The rate constants and amplitudes of the slow phase of agonist binding were relatively independent of ligand concentration. These results were analyzed quantitatively according to several variants of the "ternary complex" binding mechanism. In the model which best accounted for the data, (1) approximately one-third of the alpha 2 adrenergic receptor binds agonist with low affinity and is unable to couple with a guanine nucleotide binding protein (N protein), (2) approximately one-third is coupled to the N protein prior to agonist binding, and (3) the remainder interacts by a diffusional coupling of the alpha 2 AR with the N protein or a slow, ligand-independent conformational change of the alpha 2 AR-N protein complex. The rates of interaction of liganded and unliganded receptor with N protein are estimated.

Effects of neuromuscular blockers on carnosine levels in rat skeletal muscle.

A 30-min treatment with neuromuscular blocking doses of either physostigmine or d-tubocurarine was associated with a 44% or 36% (respectively) reduction in rat skeletal muscle carnosine levels in vivo.

Binding of leukotrienes C4 and D4 to membranes from guinea pig lung: regulation by ions and guanine nucleotides.

Tritium-labeled leukotrienes C4 and D4 (LTC4 and LTD4) bind to membranes from guinea pig lung. Binding properties of the two ligands are almost identical. More than 80% of 3H-LTC4 and 3H-LTD4 binding can be blocked by unlabeled LTC4 (IC50 8 nM versus 3H-LTC4 and 8 nM versus 3H-LTD4), LTD4 (12 nM, 16 nM), LTE4 (40 nM, 98 nM), and the leukotriene antagonist FPL 55712 (14 microM, 11 microM). Binding is reversible (50% dissociation at 65 min for both ligands at 25 degrees). Binding of 3H-LTC4 and 3H-LTD4 is enhanced by divalent cations and inhibited by sodium ions, guanine nucleotides, and EDTA. 3H-LTD4 binds in unaltered form, but 3H-LTC4 appears to bind mostly after conversion to 3H-LTD4. The high affinity, reversibility, and regulation by ions and guanine nucleotides of 3H-LTC4 and 3H-LTD4 binding strongly imply that these binding sites are physiological LTD4 receptors.