Distance measurements using paramagnetic ion-induced relaxation in the saturation transfer electron spin resonance of spin-labeled biomolecules: Application to phospholipid bilayers and interdigitated gel phases

The saturation transfer electron spin resonance (STESR) spectra of spin-labeled phosphatidylcholines in gel phase lipid bilayers are shown to be sensitive to dipolar spin-spin interactions with paramagnetic ions in the aqueous phase. The reciprocal integrated intensity of the STESR spectrum is linearly dependent on aqueous Ni²⁺ ion concentration, hence, confirming the expectation that the STESR intensity is directly proportional to the spin-lattice relaxation time of the spin label. The gradient of the relaxation rate with respect to Ni²⁺ ion concentration decreases strongly with the position of the nitroxide group down the sn-2 chain of the spin-labeled lipid and is consistent with a 1/R³ dependence on the distance, R, from the bilayer surface. The values derived for the dimensions of the bilayer and lipid molecules in the case of dipalmitoyl phosphatidylcholine (DPPC) are in good agreement with those available from x-ray diffraction studies. Allowance for the multibilayer nature of the DPPC dispersions gives an estimate of the water layer thickness that is also consistent with results from x-ray diffraction. The profile of the paramagnetic ion-induced relaxation is drastically changed with DPPC dispersions in glycerol for which the lipid chains are known to be interdigitated in the gel phase. The terminal methyl groups of the lipid chains are located approximately in register with the C-3 atoms of the sn-2 chain of the oppositely oriented lipid molecules in the interdigitated phase. The thickness of the lipid layer and the effective thickness of the lipid polar group are reduced by ~40% in the interdigitated phase as compared with the bilayer phase. The calibrations of the distance dependence established by use of spin labels at defined chain positions should be applicable to STESR measurements on other biological systems.     

Glucosylenamines as glycosyl acceptors: synthesis of gentiobiosylenamines.

The preparation of 2,3,4-tri-O-benzyl- (3), 2,3,4-tri-O-acetyl- (4), and 2,3,4-tri-O-benzoyl-N-(2,2-diethoxycarbonylvinyl)-6-O-trityl-beta- D-glucopyranosylamine (5) is described. The reaction of 3-5 with 2,3,4,6-tetra-O-acetyl-alpha-D-glucopyranosyl bromide yields 2,3,4-tri-O-benzyl- (9), 2,3,4-tri-O-acetyl- (10), and 2,3,4-tri-O-benzoyl-N-(2,2-diethoxycarbonylvinyl)-6-O-(2,3,4,6-tet ra-O- acetyl-beta-D-glucopyranosyl)-beta-D-glucopyranosylamine (11), respectively. 2,3,4-Tri-O-benzyl- (6), 2,3,4-tri-O-acetyl- (7), and 2,3,4-tri-O- benzoyl-N-(2,2-diethoxycarbonylvinyl)-beta-D-glucopyranosylamine (8) are also described.     

Brefeldin A inhibits the formation of constitutive secretory vesicles and immature secretory granules from the trans-Golgi network.

The effects of brefeldin A (BFA) on membrane traffic between the trans-Golgi network (TGN) and the plasma membrane were investigated in intact PC12 cells and in a cell-free system derived from PC12 cells. In intact cells, BFA caused a virtually complete block of constitutive secretion, as indicated by the lack of release from, and accumulation in, the cells of a [35S]sulfate-labeled heparan sulfate proteoglycan (hsPG). Pulse-chase experiments with [35S]sulfate followed by subcellular fractionation showed that this block was due to the inhibition of formation of constitutive secretory vesicles (CSVs) from the TGN. BFA did not block the depolarization-induced release of [35S]sulfate-labeled chromogranin B (CgB) and secretogranin II (SgII) from secretory granules formed prior to the addition of the drug, showing that BFA does not block secretory granule fusion with the plasma membrane. The presence of BFA did, however, prevent the appearance of [35S]sulfate-labeled CgB and SgII in secretory granules, indicating that the drug inhibits the formation of secretory granules from the TGN. Evidence for a direct block of vesicle formation by BFA was obtained using a cell-free system derived from [35S]sulfate-labeled PC12 cells. In this system, low concentrations of BFA (5 micrograms/ml) inhibited the formation of the hsPG-containing CSVs and that of the SgII-containing secretory granules from the TGN to the same extent (50-60%) as, and in a non-additive manner with, the nonhydrolyzable GTP analogue GTP gamma S. Consistent with the inhibitory effects of BFA on vesicle formation from the TGN, BFA treatment of intact PC12 cells led to the hypersialylation of CgB, which presumably was due to the increased residence time of the protein in the TGN. In conclusion, our data are consistent with, and allow the generalization of, the concept that the BFA-induced block of anterograde membrane traffic results from the inhibition of vesicle formation from a donor compartment.     

A comparison of the Xenopus laevis oocyte acetylcholinesterase with the muscle and brain enzyme suggests variations at the post-translational level.

1. Xenopus laevis oocytes express endogenously two components of the cholinergic system: the muscarinic receptors and the acetylcholinesterase (AChE). 2. A biochemical characterization of this enzyme was carried out. 3. The results established that the activity found in the oocytes correspond to 'true' AChE with a molecular weight of 65,000 Da and a sedimentation coefficient of 3-4 S. 4. The enzyme aggregates in the absence of detergent suggesting that it possess an hydrophobic character; despite that, it is not sensitive to PIPLC. 5. A comparison with the Xenopus brain and muscle AChE shows different post-translational modifications and catalytic properties with the oocyte AChE.     

Flavin-photosensitized oxidation of reduced c-type cytochromes. Reaction mechanism and comparison with photoreduction of oxidized cytochromes by flavin semiquinones

In order to compare the oxidation and reduction reactions of c-type cytochromes (cytochrome c552 from the green alga Monoraphidium braunii and horse heart cytochrome c) by different flavins (lumiflavin, riboflavin and FMN), laser flash photolysis studies have been carried out using either reduced or oxidized protein in the presence of triplet or semiquinone flavin, respectively. The reaction kinetics clearly demonstrate that cytochrome oxidation is mediated by the flavin triplet state. The rate constants for reduction are 20-100 times smaller than those for oxidation, indicating that the triplet state is a more effective reactant than is the semiquinone. This is attributed to its excited state nature and correspondingly high free energy content. The rate constants for both the reduction and oxidation of cytochrome c552 by riboflavin are significantly smaller than those obtained with lumiflavin, suggesting a steric interference of the ribityl side chain in the flavin-cytochrome interaction. The comparison between oxidation and reduction indicates that the former process is less affected by steric hindrance than the latter. Both reduction and oxidation of cytochrome c552 by FMN show an ionic strength dependence with the same sign, consistent with a negatively charged reaction site on the cytochrome. The magnitude of the electrostatic effect is slightly smaller for reduction than it is for oxidation. A pattern quite similar to that observed with cytochrome c552 was obtained when parallel experiments were carried out with horse cytochrome c, although differences were observed in the steric and electrostatic properties of the electron transfer site(s) in these two cytochromes. These results suggest that the same or closely adjacent sites on the proteins are involved in the oxidation and reduction reactions. The biochemical implications of this are discussed.     

Asymmetric reduction of ketones with resting cells of Sulfolobus solfataricus

The method of resting cells has been of interest in the development of biocatalysts applied to organic reactions.This article deals with the use of resting cells of a thermophilic archaebacterium Sulfolobus solfataricus, in the asymmetric reduction of acyclic, cyclic, and aromatic ketones. The system allows the continuous regeneration of endogenous coenzyme with the coupled substrate approach. The results indicate that the direction of hydride attack was equatorial on the re face of the carbonyl group of substrates producing (S)-alcohols with a good optical yield. A convenient system for the reuse of resting cells has been set out to synthesize (S)-alcohols on a preparative scale.     

Archaebacterial elongation factor Tu insensitive to pulvomycin and kirromycin

A spermine-dependent, polyphenylalanine-synthesizing cell-free system having an optimum activity at 75-85 degrees C, has been developed from the extremely thermoacidophilic archaebacterium Caldariella acidophila. The C. acidophila system is totally insensitive to the EF-Tu targeted antibiotics pulvomycin (at 40 degrees C) and kirromycin (at 47-72 degrees C) contrary to control systems derived from both mesophilic (Escherichia coli) and thermoacidophilic (Bacillus acidocaldarius) eubacteria. The archaebacterial EF-Tu-equivalent factor is also immunologically unrelated to eubacterial EF-Tu and does not cross react with antibodies against Escherichia coli EF-Tu. The pulvomycin and kirromycin reactions thus provide new phyletic markers for archaebacterial ancestry.     

The structure of hemoglobin Creteil (beta 89 Ser replaced by Asn) is similar to that of abnormal human hemoglobins having sequence changes at Tyr 145 beta

In normal deoxyhemoglobin A, the beta chain COOH-terminal peptide adopts a well ordered structure which is needed for the full expression of allosteric action. Our crystallographic studies of deoxyhemoglobin Creteil (beta 89 Ser replaced by Asn), a variant hemoglobin characterized by high oxygen affinity and a very low level of allosteric function, show that replacement of Ser 89 beta by asparagine causes severe disordering of the beta chain COOH-terminal tetrapeptide. This results, as shown by our spectroscopic studies, in the destabilization of the quaternary structure of deoxyhemoglobin Creteil. We find, furthermore, that the changes in tertiary structure observed in deoxyhemoglobin Creteil are common to other variant hemoglobins having similar functional abnormalities but very different changes in primary structure. In particular, direct comparison of the difference electron density map of deoxyhemoglobin Creteil with that of deoxyhemoglobin Nancy (beta 145 Tyr replaced by Asp) suggests that these two abnormal hemoglobins may have the same mechanism of dysfunction despite the very different nature of their respective sequence changes.     

Hydrogen exchange from identified regions of the S-protein component of ribonuclease as a function of temperature,pH, and the binding of S-peptide

A medium resolution hydrogen exchange method (Rosa & Richards, 1979) has been used to measure the average rates of amide hydrogen exchange for known segments of the S-protein portion of ribonuclease-S. The analytical procedure permitted exchange rates to be monitored for seven S-protein fragments distributed throughout the structure, including regions of α-helix and β-sheet. Kinetics were measured as a function of pH, temperature and S-peptide binding.The pH dependence of exchange from isolated S-protein between pH 2·8 and pH 7·0 was found to deviate significantly from a first-order dependence on hydroxide ion concentration. The protection against exchange with increasing pH appeared to be closely related to the electrostatic stabilization of S-protein. It is suggested that such favorable electrostatic interactions result in increased energy barriers to the conformational fluctuations that provide solvent access to the time-average crystallographic structure. This explanation of the observed correlation between stability and exchange kinetics is also consistent with the calculated apparent activation energies for exchange from S-protein between 5·5 and 20 °C.S-peptide binding dramatically slows exchange from many S-protein sites, even those distant from the area of S-peptide contact. Interestingly, the effects of complex formation are not evenly propagated throughout S-protein. The most significantly perturbed sites (≥10³-fold reduction in exchange rate constants) lie within fragments derived from regions of secondary structure. Exchange from several other fragments is not significantly affected. The S-peptide—S-protein dissociation constant at neutral pH is so small that the measured exchange must have occurred from the complex and not from the dissociated parts.