Proton magnetic resonance studies of cholinergic ligand binding to the acetylcholine receptor in its membrane environment

Structure-Activity Relationships of Baicalein and its Analogs as Novel TSLP Inhibitors

Thymic stromal lymphopoietin (TSLP) plays an important role in the differentiation and proliferation of Th2 cells, resulting in eosinophilic inflammation and numerous allergic diseases. Baicalein (1), a major component of Scutellaria baicalensis, was found to be the first small molecule to block TSLP signaling pathways. It inhibited effectively eosinophil infiltration in house dust mite-induced and ovalbumin-challenged mouse models. Structure-activity relationship studies identified compound 11a, a biphenyl flavanone analog, as a novel human TSLP inhibitor for the discovery and development of new anti-allergic drugs.

High resolution magic angle spinning NMR spectroscopy used to investigate the ability of drugs to bind to synthetic melanin

Iodobenzamides are known to possess an affinity for melanoma tissue dependent on tumor pigmentation. In order to investigate the molecular interactions of drugs with melanin in vitro, a synthetic pigment swelled in deuterium buffer at physiological pH was used. The spectra of various mixtures of each Iodobenzamide (BZ) with melanin were studied at 25 degrees C by NMR under MAS conditions. The drug which interacts with the pigment exhibits linewidths greater than those observed for the free drug in solution. Line-broadening of the resonance occurred for the N-methyl group of acetylcholine or N-ethyl and aromatic groups of BZ. However, linewidths associated with methanol or hippuric acid were less altered by the presence of melanin. These observations indicate the specificity of the interaction between some drug moieties and the sites of melanin. From the concentration dependence of line-broadening, the apparent equilibrium dissociation constant (K(d)) of drug interaction with melanin was approached. It seems that the residual concentration-dependent line-broadening is caused by perturbations of ligand exchange between free and bound states and by differences in magnetic susceptibility present in the sample at the pigment-interacting drug moiety interface. Taken together, these results demonstrate the utility of this technique for investigating binding drugs.

In addition to membrane injury, an affinity for melanin might be involved in the high sensitivity of human melanoma cells to hederacolchiside A1

We previously reported that hederacolchiside A1 (Hcol A1), a new oleanene saponin isolated from Hedera colchica Koch (Araliaceae) exhibits a preferential cytotoxicity on a pigmented melanoma cell line. The present study confirms the high susceptibility of melanoma cell lines to this drug and shows concentrations producing a 50% decrease in cell content (IC50 values) inversely proportional to the melanin content of each cell line. At cytotoxic concentrations, Hcol A1 induces membrane-damaging effects within 6 h, cytoplasmic vacuolization within 24 h, and non-apoptotic cell death within 48 h. Using a new high-resolution magic-angle spinning nuclear magnetic resonance method, we have shown for the first time that this hederasaponin specifically interacts with melanin. The dissociation constant (2.7 mM) is comparable to those observed with drugs known to interact with melanin. Taking into consideration that the IC50 values were inversely proportional to the melanin in each cell line, our data suggest that, in addition to the delayed membrane injury induced by this drug, the ability of Hcol A1 to bind melanin could contribute to the higher toxicity of Hcol A1 in pigmented melanoma cells.

Volatile anesthetics compete for common binding sites on bovine serum albumin: a 19F-NMR study

There is controversy as to the molecular nature of volatile anesthetic target sites. One proposal is that volatile anesthetics bind directly to hydrophobic binding sites on certain sensitive target proteins. Consistent with this hypothesis, we have previously shown that a fluorinated volatile anesthetic, isoflurane, binds saturably [Kd (dissociation constant) = 1.4 +/- 0.2 mM, Bmax = 4.2 +/- 0.3 sites] to fatty acid-displaceable domains on serum albumin. In the current study, we used 19F-NMR T2 relaxation to examine whether other volatile anesthetics bind to the same sites on albumin and, if so, whether they vary in their affinity for these sites. We show that three other fluorinated volatile anesthetics bind with varying affinity to fatty acid-displaceable domains on serum albumin: halothane, Kd = 1.3 +/- 0.2 mM; methoxyflurane, Kd = 2.6 +/- 0.3 mM; and sevoflurane, Kd = 4.5 +/- 0.6 mM. These three anesthetics inhibit isoflurane binding in a competitive manner: halothane, K(i) (inhibition constant) = 1.3 +/- 0.2 mM; methoxyflurane, K(i) = 2.5 +/- 0.4 mM; and sevoflurane, K(i) = 5.4 +/- 0.7 mM--similar to each anesthetic's respective Kd of binding to fatty acid displaceable sites. These results illustrate that a variety of volatile anesthetics can compete for binding to specific sites on a protein.

The dependence of non-quantal acetylcholine release on the choline-uptake system in the mouse diaphragm

The time course of local end-plate hyperpolarization after d-tubocurarine application measured by an intracellular microelectrode was followed in vitro in anticholinesterase-treated mouse diaphragm pinned to the bottom of the perfusion chamber. The d-tubocurarine-induced hyperpolarization, which served as an indicator of non-quantal acetylcholine release, started to decline from 6 mV after 1 h and was negligible after 3 h in continuously perfused preparations. This decline was slowed down by 10 mumol l-1 choline and almost completely prevented by long-term nerve stimulation with a frequency of 3 Hz. The rapid decrease of the d-tubocurarine-induced hyperpolarization was observed within 10-15 min after the application of 1 mumol l-1 hemicholinium-3 and substitution of lithium for sodium. Both these procedures inhibit the fast choline uptake into nerve terminals. Our data suggest that the amount of available acetylcholine for non-quantal release is proportional to the rate of its synthesis and to the number of available carriers in the nerve terminals. Some of our observations might also be explained by postulating that the choline-uptake system as such is responsible for the non-quantal release.

Investigation by NMR spectroscopy of the interaction between synthetic soluble (-)-dopa melanin and drugs

In order to understand the molecular interactions of drugs with melanin, synthetic soluble (-)-dopa-melanin was prepared in deuterium buffer. The spectra of various drug moieties with the pigment at 30 degrees C were studied employing the line width measurements obtained with a pulse NMR (AF270) instrument. As compared to drug effects in fresh melanins (48 h), the aged melanins (greater than or equal to 168 h) gave consistent spectral measurements, even in dilute solutions of pigment. NMR signals of aromatic and N-methyl protons of drugs were relatively easy to quantify and, in the presence of melanin, line broadening of various drug moieties occurred. The line widths of the N-methyl groups of acetylcholine (3.02 ppm), the N-methyl group of atropine (2.52 ppm), N-isopropyl of isoprenaline bitartrate (1.14 ppm) and N-ter-butyl of timolol maleate (1.22 ppm) in the presence of the pigment were increased. Line widths associated with acetate, bitartrate, maleate or tropic acid, however, were not altered by the melanin. This indicates the specificity of the interaction between drug moieties and the site(s) of melanin. Based on the line width measurements of N-methyl protons of ephedrine, two dissociation constants were obtained (Kd1 2.08 mM and Kd2 greater than 20 mM). The constants for atropine melanin complex were Kd1 0.79 mM and Kd2 greater than 6 mM. Furthermore, based on N-methyl resonances, it appears that atropine and ephedrine compete for at least one common interacting site of the melanin polymer.

Measuring relative acetylcholine receptor agonist binding by selective proton nuclear magnetic resonance relaxation experiments

A method is presented that uses selective proton Nuclear Magnetic Resonance (NMR) relaxation measurements of nicotine in the presence of the acetylcholine receptor to obtain relative binding constants for acetylcholine, carbamylcholine, and muscarine. For receptors from Torpedo californica the results show that (a) the binding constants are in the order acetylcholine greater than nicotine greater than carbamylcholine greater than muscarine; (b) selective NMR measurements provide a rapid and direct method for monitoring both the specific and nonspecific binding of agonists to these receptors and to the lipid; (c) alpha-bungarotoxin can be used to distinguish between specific and nonspecific binding to the receptor; (d) the receptor--substrate interaction causes a large change in the selective relaxation time of the agonists even at concentrations 100x greater than that of the receptor. This last observation means that these measurements provide a rapid method to monitor drug binding when only small amounts of receptor are available. Furthermore, the binding strategies presented here may be useful for the NMR determination of the conformation of the ligand in its bound state.