Conformational analysis of biologically active truncated linear analogs of endothelin-1 using NMR and molecular modeling.

Some linear truncated analogs of endothelin-1 display potent agonistic activity at the ET(B) receptor, especially when the side chain of Trp21 is N-formylated. Then, the three-dimensional arrangements of six structurally reduced linear analogs, three formylated and three nonformylated, have been investigated by high resolution NMR spectroscopy and molecular modeling, in order to pinpoint the conformational features related to the biological activity. Two-dimensional double-quantum-filtered correlation spectroscopy (DQFCOSY), total correlation spectroscopy (TOCSY) and nuclear Overhauser enhancement spectroscopy (NOESY) were recorded and analyzed for each molecule. Interspatial distance constraints were derived from the intensity of the NOESY connectivities. The formation of hydrogen bonding was monitored from the temperature dependence of the NH chemical shifts. Molecular models calculated by means of distance geometry, simulated annealing and energy minimization, using the NMR constraints, strongly suggested a global elongated structure for the formylated analogs exhibiting biological activity, and a folded arrangement for the unformylated derivatives. Homology comparisons allowed the identification of a beta-turn-like folding of the C-terminal segment Asp18-Trp21 as a probable key-factor for activity.     

Maitotoxin-Evoked γ-Aminobutyric Acid Release Is Due Not Only to the Opening of Calcium Channels

The effects of maitotoxin (MTX) on endogenous amino acid release were tested on highly purified striatal neurons differentiated in primary culture. MTX induced a large and concentration-dependent release of gamma-aminobutyric acid (GABA). This effect was abolished when experiments were performed in the absence of external Ca2+, and restored when Ca2+ ions were added after removing the MTX-containing Ca2+-free solution. MTX-induced amino acid release was not affected by 1 microM nifedipine and only slightly inhibited by 1 mM Co2+. MTX also induced a massive accumulation of 45Ca2+ in the neurons which, in contrast to the MTX-evoked GABA release, was totally blocked in the presence of 1 mM Co2+. Whereas 500 nM tetrodotoxin was without significant effect, MTX-evoked GABA release was dependent on the presence of external Na+ and sensitive to nipecotic acid, a GABA uptake inhibitor. It is concluded that, on striatal neurons, MTX induced Na+ influx only in the presence of external Ca2+. The increase in cytoplasmic Na+ ions then triggers the release of GABA.     

Release of Endogenous Amino Acids from Striatal Neurons in Primary Culture

Following partial purification, the characteristics of a cytosol protein kinase were investigated. The protein kinase was purified by ammonium sulfate precipitation and diethylaminoethyl-cellulose, ATP-agarose, and hydroxyapatite chromatography. Analysis of the purified protein kinase preparation by polyacrylamide gel electrophoresis revealed three major protein bands. The cytosol protein kinase was purified approximately 442-fold, as calculated from the cyclic nucleotide independent protein kinase activity in the 40,000 g supernatant. The activity of the kinase was found to be independent of either cyclic AMP or cyclic GMP. Moreover, the kinase activity was unaffected by the addition of the endogenous protein kinase inhibitor, or the regulatory subunit from the type II cyclic AMP-dependent protein kinase from bovine heart. The molecular weight of the enzyme was determined to be 95,000 by Sephadex G-200 gel filtration. The activity of the kinase was increased approximately twofold in the presence of 10 microM Ca+2 and calmodulin. This increase was reversed by the addition of EGTA. The subcellular distribution of the protein kinase was also examined. The soluble fraction from nerve terminal was found to have the highest concentration of the kinase activity.     

Poly(amidoamine) dendrimer peripherally modified with 4-N,N-dimethylaminoethyleneamino-1,8-naphthalimide as a sensor of metal cations and protons.

A new poly(amidoamine) dendrimer from second generation whose periphery comprises sixteen fluorescent 4-N,N-dimethylaminoethylamino-1,8-naphthalimide units has been synthesized and characterized. In DMF, the dendrimer shows sensitivity to the presence of Cu(2+), Fe(3+) and protons. The changes in the fluorescence intensity of the material are in opposite directions if acids or metals are present. Fluorescence enhancements (FE from 5 to 9 depending on solvent) are recorded when the photoinduced electron transfer (PET) originating from the donating amine to the electron accepting naphthalimide is inhibited by the protonation of the N,N-dimethylamino groups. In the case of Cu(2+) cations, a fluorescence quenching (FQ of 6) is first observed, followed by fluorescence partial restoration. In the Fe(3+) case, the same behaviour is observed with a final FE of 2. The successive complexations of these cations by the dendrimer core and by the external rim of the dendrimer may explain the results.