Urea Receptor Research Articles

Metal Ions as Allosteric Inhibitors in Hydrogen-Bonding Receptors

The presence of copper(I) ions turns off the recognition of uracil and carboxylate derivatives (the “guest”) by bis(bipyridine)triazine and bis(bipyridine)urea receptors (shown schematically). These negative allosteric processes are driven by the complexation of the metal to the chelating ligands, which distorts the hydrogen-bonding surfaces.

Metal Ions as Allosteric Inhibitors in Hydrogen-Bonding Receptors

The presence of copper(I) ions turns off the recognition of uracil and carboxylate derivatives (the “guest”) by bis(bipyridine)triazine and bis(bipyridine)urea receptors (shown schematically). These negative allosteric processes are driven by the complexation of the metal to the chelating ligands, which distorts the hydrogen-bonding surfaces.

The first-nucleotide binding domain of the cystic-fibrosis transmembrane conductance regulator is important for inhibition of the epithelial Na+ channel.

The cystic-fibrosis transmembrane conductance regulator (CFTR) functions as a cAMP-regulated Cl- channel and as a regulator of other membrane conductances. cAMP-dependent activation of CFTR inhibits epithelial Na+ channels (ENaC). The specificity of interaction between CFTR and ENaC was examined by coexpression of ENaC and ATP-binding cassette (ABC) proteins other than CFTR. In addition, we identified domains within CFTR that are of particular importance for the inhibition of ENaC. To that end, two-electrode voltage-clamp experiments were performed on Xenopus oocytes coexpressing ENaC together with CFTR, the multidrug resistance protein MDR1, the sulfonyl urea receptor SUR1, or the cadmium permease YCF1. Except for CFTR, none of the other ABC proteins were able to inhibit ENaC. Several truncated versions of CFTR were examined for their inhibitory effects on ENaC. In fact, it is shown that C-terminal truncated CFTR is able to inhibit ENaC on activation by intracellular cAMP. Moreover, the data also show that an intact first-nucleotide binding domain (NBF-1) is important for inhibition of ENaC. We conclude that NBF-1 of CFTR contains a CFTR-specific regulatory site that down-regulates ENaC. It is speculated that this regulatory site also is needed for CFTR-mediated interactions with other membrane proteins and that it is not present in NBF-1 of other ABC proteins.

Impaired expression of transcription factor IUF1 in a pancreatic β-cell line derived from a patient with persistent hyperinsulinaemic hypoglycaemia of infancy (nesidioblastosis)

Persistent hyperinsulinaemic hypoglycaemia of infancy (PHHI), or nesidioblastosis, is a rare disorder which may be familial or sporadic, and which is characterized by unregulated secretion of insulin and profound hypoglycaemia in the neonate. The defect has been linked in some patients to mutations in the sulphonyl urea receptor gene (SUR). The present study investigated potential defects in the regulation of the insulin gene by glucose in a β-cell line (NES 2Y) derived from a patient with PHHI. The results show that the insulin promoter is unresponsive to glucose in PHHI, and that this defect can be attributed to impaired expression of the transcription factor IUF1. Because IUF1 is involved not only in linking glucose metabolism to the control of the insulin, but is also a major regulator of β-cell differentiation during embryogenesis, we propose that impaired expression of IUF1 contributes to β-cell dysfunction in PHHI by leading to abnormal β-cell differentiation.

Enhanced Carboxylate Binding Using Urea and Amide-Based Receptors with Internal Lewis Acid Coordination: A Cooperative Polarization Effect

A structural design strategy is described that greatly improves the acetate binding ability of neutral urea and amide-based receptors. The enhanced binding is due to a cooperative polarization effect which is induced by intramolecular coordination of the urea or amide carbonyl to a Lewis acidic boronate group. A series of boronate-ureas, 3, and a related bis(boronate-amide), 23, were prepared in two steps from 2-(aminophenyl)boronic acid and their structures elucidated using X-ray crystallography and other spectrometric methods. The abilities of the receptors to associate with tetrabutylammonium acetate in dimethyl sulfoxide solution were determined by (1)H NMR titration experiments. Association constants were calculated using nonlinear curve-fitting methods. The boronate-ureas 3 strongly bind to acetate in dimethyl sulfoxide solution with association constants as high as 6 x 10(4) M(-)(1). This is more than 150 times greater than the association constants for control urea receptors that lacked an appropriate boron substituent. Thermodynamic studies indicate that the enhanced association is due to a favorable enthalpic change. Additional NMR studies eliminated the possibility of proton transfer to the acetate during complex formation. Molecular modeling indicates that the boronate-ureas exhibit improved acetate binding because the intramolecular coordination (i) induces a larger host dipole moment which strengthens the guest/host ion-dipole interaction, and (ii) increases the positive surface potential at the urea NH residues which strengthens short range Coulombic interactions with the anionic acetate. The observed association constants correlate better with calculated host dipole moments, suggesting that for the boronate-ureas described here this is the more influential factor controlling association.

Analogues of platelet activating factor. 7. Bis-aryl amide and bis-aryl urea receptor antagonists of PAF.

A series of bis-aryl amide (13-57 and 66-81) and bis-aryl urea (58 and 85) antagonists of platelet-activating factor (PAF) was prepared that contain, separating the two aromatic rings, linear amide linkages of the form -(CH2)nCONH- (n = 0-2), -OCH2CONH-, and -(CH2)nNHCO- (n = 0-1), branched amide linkages of the form -(CH2)nN(COR)- (n = 1-3, R = CH3 or n-C3H7), and -N(COCH3)CH2-, and urea linkages of the form -NHCONH- and -CH2N(CONHCH3)-. These compounds were examined for their ability to inhibit PAF-induced platelet aggregation of rabbit platelets. These in vitro data were compared to similar data obtained for a number of known PAF antagonists. The compounds were evaluated in vivo, in the mouse, for their ability to prevent death induced by a lethal challenge of PAF. The relationships between the biological activity and the nature, lipophilicity, and position of substituents of the aromatic rings were studied. Best activity was observed for compounds having linkages of the type -CH2CONH-, -CH2N(COR)-, and -CH2NHCO-. Many of these compounds inhibit PAF-induced platelet aggregation with IC50's under 1 microM.

Multidentate Acyclic Neutral Ligands and Their Complexation

AbstractThe properties of cyclic crown ethers are approximated by acyclic neutral ligands (popdands), which are compared and contrasted with open‐chain bioionophores and acidic chelating agents in this article. Variations of the endo‐polarophilicity/exo‐lipophilicity balance, complex stability, ion slectivity can often be accomplished more easily, with greater versatility, and at less expense with acyclic polyethers than with their cyclic counterparts; complexation and decomplexation are generally faster in acyclic systems; and the pseudocavity usually has greater conformational flexibility. Acyclic crown ethers and open‐chain cryptands stiffened by rigid “terminal groups” containing donor atoms readily form crystalline complexes of alkali and alkaline earth metals. Some oppen‐chain neutral ligands form helical conformations in their crycstalin complexes. The observed coordination numbers and geometries are of theoretical interst. Attractive terminal group interactions lead to pseudocyclic species occupying a position intermediate between cyclic and acyclic ligands. It has recently proved possible to isolate crystalline complexes of alkali and alkaline earth metal ions with weakly donating oligo(ethylene glycol ethers) and with glycols; such complexes have also been obtained with sugars. Acyclic neutral ligands can serve as simple models of nigericin‐type bioionophores and be used analytically in microelectrodes. The recently discoverd crystalline stoichiometric complexes formed by some acyclic neutral ligands with guest molecules such as urea, thiourea, and water provide a fresh insight into weak interactions between neutral molecules and for the development of urea receptors.