Kinetic and equilibrium characterization of vesamicol receptor-ligand complexes with picomolar dissociation constants.

Previous studies from this laboratory characterized 83 analogs of vesamicol by their potencies for inhibition of acetylcholine active transport by synaptic vesicles isolated from Torpedo electric organ. Examination of the more potent of these compounds, plus five new analogs, by kinetic and equilibrium measurements on complexes with the vesamicol receptor (VR) revealed nine analogs that are significantly more potent than vesamicol. Equilibrium measurements were performed at very low protein concentrations and extended incubation times, which allowed the characterization of very high affinity analogs. Better understanding of the structural binding requirements of the VR has resulted, and a spatial map of allowed hydrophobicity has been clearly established. Three analogs were resolved, and they displayed enantioselectivity ratios as high as 260 for binding to the VR (10-times higher than that of vesamicol). The most potent analog, 4-aminobenzovesamicol (ABV), was synthesized in tritiated form and shown to dissociate from the VR with a half-life of about 14 hr at 20 degrees. The estimated dissociation constant is < or = 6.5 +/- 0.5 pM. By reciprocal kinetic experiments with vesamicol and ABV, coincidence of the two binding sites on vesicles was established. The high affinity and enantioselectivity of ABV and other similar analogs, coupled with good chemical and radiochemical stability, make these ligands attractive for the study of the VR in complex tissues. The observed difference between the equilibrium dissociation constant for the vesamicol-VR complex as estimated by titration with [3H]vesamicol (7.6 nM) and by displacement of subsaturating [3H]vesamicol by nonlabeled vesamicol (1.0 nM) suggests that high and low affinity populations of the VR exist.

Alanine series of ovine corticotropin releasing factor (oCRF): a structure-activity relationship study.

Previous structure-activity relationship studies of CRF have shown that residues 1-4 were not necessary for receptor binding or transduction, that residues 4-8 were important for activation, and that residues 12-41 were mostly responsible for binding. Finally it was proposed that CRF assumed an alpha-helical structure when interacting with its receptor. By systematic substitution of each residue (except residues 1-4) in ovine CRF (oCRF) by Ala, we have investigated the role played by individual side chains in receptor recognition and activation. Out of 33 analogues (synthesized using SPPS on an MBHA resin, purified by RPHPLC and characterized by amino acid and mass spectral analyses), a significant loss of biological potency (less than 1% potency of native) was observed for 6 analogues ([Ala6], [Ala8], [Ala10], [Ala12], [Ala14], and [Ala38]); 12 analogues had biological potencies ranging from 1% to 60% and ranked as follows: [Ala35] less than [Ala16] less than [Ala9] less than [Ala19] less than [Ala15] less than [Ala13] less than [Ala7] less than [Ala23] less than [Ala11] less than or equal to [Ala21] less than [Ala27] less than or equal to [Ala18]; 8 analogues were found to be equipotent (greater than 60% and less than 150%) ([Ala5], [Ala17], [Ala26], [Ala29], [Ala30], [Ala34], [Ala36], and [Ala37]; and 7 analogues were found to be approximately 2-5 times more potent than native oCRF ([Ala25] = [Ala40] less than or equal to [Ala39] less than or equal to [Ala33] less than [Ala20] less than [Ala22] less than [Ala32], in an in vitro pituitary cell culture assay. In summary, the Ala substitutions which showed the greatest loss of potency (less than 1% of native oCRF) were those replacing hydrophobic residues while those showing the greatest increase in potency were replacing hydrophilic residues. Of the 22 Ala-containing analogues in the C-terminal half of the molecule, 17 analogues have equal or greater potencies than native oCRF. Substitution of Ala in the N-terminal region (residues 5-19) on the other hand is generally detrimental to biological activity. These results suggest that the side chains of residues 5-19 are very important for receptor binding and activation while, in the C-terminal region, the amino acid side chains may be more responsible for structural conservation than for functional expression.

Synthesis, in vitro acetylcholine-storage-blocking activities, and biological properties of derivatives and analogues of trans-2-(4-phenylpiperidino)cyclohexanol (vesamicol).

Eighty-four analogues and derivatives of the acetylcholine-storage-blocking drug trans-2-(4-phenylpiperidino)-cyclohexanol (vesamicol) were synthesized, and their potencies were evaluated with the acetylcholine active-transport assay utilizing purified synaptic vesicles from Torpedo electric organ. The parent drug exhibits enantioselectivity, with (-)-vesamicol being 25-fold more potent than (+)-vesamicol. The atomic structure and absolute configuration of (+)-vesamicol were determined by X-ray crystallography. The absolute configuration of (-)-vesamicol is 1R,2R. Structure-activity evidence indicates that (-)-vesamicol does not act as an acetylcholine analogue. Alterations to all three rings can have large effects on potency. Unexpectedly, analogues locking the alcohol and ammonium groups trans-diequatorial or trans-diaxial both exhibit good potency. A potent benzovesamicol family has been discovered that is suitable for facile elaboration of the sort useful in affinity labeling and affinity chromatography applications. A good correlation was found between potencies as assessed by the acetylcholine transport assay and LD50 values in mouse.

Sidedness and chemical and kinetic properties of the vesamicol receptor of cholinergic synaptic vesicles.

Cholinergic synaptic vesicles isolated from Torpedo electric organ contain a receptor for the compound l-2-(4-phenylpiperidino)cyclohexanol (vesamicol, formerly AH5183), which when occupied blocks storage of acetylcholine (AcCh). The inside or outside orientation of the receptor and its chemical and ligand binding kinetics characteristics were studied. Binding of [3H]vesamicol to the receptor is inhibited efficiently by the protein modification reagents 4-(chloromercuri)benzenesulfonate and N,N'-dicyclo-hexylcarbodiimide and by protease treatment of cholate-solubilized receptor. The receptor in native vesicles is resistant to irreversible inactivation by proteases, elevated temperature, or pH extremes. [3H]Vesamicol binding depends on deprotonation of a group of pKa1 = 6.26 +/- 0.03 and protonation of a group of pKa2 = 10.60 +/- 0.04, which is probably the tertiary amine of the drug molecule itself. The membrane-impermeant zwitterionic vesamicol analogue dl-trans-4-oxo-4-[5,6,7,8-tetrahydro-6-hydroxy-7-(4-phenyl-1-piperidinyl )-1- naphthalenyl]amino]butanoic acid (TPNB) is an effective inhibitor of AcCh active transport with an IC50 value of (51 +/- 8) x 10(-9) M. At 23 degrees C, [3H]vesamicol bound to the receptor at a rate of (1.74 +/- 0.06) x 10(5) M-1 s-1, and excess unlabeled vesamicol displaced a low concentration of bound [3H]vesamicol at 0.29 +/- 0.01 min-1. At 0 degrees C, 10 microM unlabeled vesamicol displaced 36 +/- 2% of a low concentration of bound [3H]vesamicol at 0.16 +/- 0.02 min-1 and 64 +/- 2% at 0.013 +/- 0.001 min-1. One micromolar unlabeled vesamicol behaved similarly.(ABSTRACT TRUNCATED AT 250 WORDS)

Correlates of vincristine resistance in four murine tumor cell lines.

The mechanism(s) of cellular resistance to vincristine (VCR) are poorly understood. Four murine tumor cell lines with varying degrees of VCR resistance, as measured by prolonged survival of tumor-bearing mice following VCR treatment, were selected for study. These lines were P1534, P388, P388/VCR and L1210. Steady-state cellular VCR levels, bound intracellular VCR, displaceable intracellular VCR, influx velocities and efflux velocities following VCR preloading were all measured in vitro and correlated with augmentation of survival. Neither the influx velocity, efflux velocity nor the steady-state VCR level showed any apparent correlation with in vivo sensitivity. Moreover, the ratio of influx velocity to efflux velocity was highest in the most sensitive cell line (i.e. P1534) and lowest in the most resistant cell line (i.e. P388/VCR). Bound intracellular VCR correlated best with VCR sensitivity suggesting that high-affinity intracellular binding, presumably to tubulin (Ka congruent to 1 X 10(-7) M), is a critical determinant of VCR sensitivity.