Reduced water permeability and altered ultrastructure in thin descending limb of Henle in aquaporin-1 null mice.
It has been controversial whether high water permeability in the thin descending limb of Henle (TDLH) is required for formation of a concentrated urine by the kidney. Freeze-fracture electron microscopy (FFEM) of rat TDLH has shown an exceptionally high density of intramembrane particles (IMPs), which were proposed to consist of tetramers of aquaporin-1 (AQP1) water channels. In this study, transepithelial osmotic water permeability (Pf) was measured in isolated perfused segments (0.5-1 mm) of TDLH in wild-type (+/+), AQP1 heterozygous (+/-), and AQP1 null (-/-) mice. Pf was measured at 37 degrees C using a 100 mM bath-to-lumen osmotic gradient of raffinose, and fluorescein isothiocyanate (FITC)-dextran as the luminal volume marker. Pf was (in cm/s): 0.26 +/- 0.02 ([+/+]; SE, n = 9 tubules), 0.21 +/- 0.01 ([+/-]; n = 12), and 0.031 +/- 0.007 ([-/-]; n = 6) (P < 0.02, [+/+] vs. [+/-]; P < 0.0001, [+/+] vs. [-/-]). FFEM of kidney medulla showed remarkably fewer IMPs in TDLH from (-/-) vs. (+/+) and (+/-) mice. IMP densities were (in microm-2, SD, 5-12 micrographs): 5,880 +/- 238 (+/+); 5,780 +/- 450 (+/-); and 877 +/- 420 (-/-). IMP size distribution analysis revealed mean IMP diameters of 8.4 nm ([+/+] and [+/-]) and 5.2 nm ([-/-]). These results demonstrate that AQP1 is the principal water channel in TDLH and support the view that osmotic equilibration along TDLH by water transport plays a key role in the renal countercurrent concentrating mechanism. The similar Pf and AQP1 expression in TDLH of (+/+) and (+/-) mice was an unexpected finding that probably accounts for the unimpaired urinary concentrating ability in (+/-) mice. (+info)
The posterior nasal nerve plays an important role on cardiopulmonary reflexes to nasal application of capsaicin, distilled water and l-menthol in anesthetized dogs.
The sensory innervation of the cardiopulmonary reflexes to nasal application of capsaicin (CAPS), distilled water (DW) and l-menthol (LM) was studied in anesthetized dogs breathing through tracheostomy. A marked cardiopulmonary reflex was observed by CAPS and DW into the nasal cavity, while a prolongation of expiration was induced by LM. All these reflexes were significantly decreased by bilateral section of the posterior nasal nerve (PNN) and completely abolished by topical nasal anesthesia with lidocaine. Responses of the whole nerve activity of the PNN to these substances corresponded to the magnitude of the reflexes. These results indicate that PNN afferents play an important role on the reflex elicitation of the noxious, water and cold stimuli from the nasal cavity. (+info)
The accessibility of iron at the active site of recombinant human phenylalanine hydroxylase to water as studied by 1H NMR paramagnetic relaxation. Effect of L-Phe and comparison with the rat enzyme.
The high-spin (S = 5/2) Fe(III) ion at the active site of recombinant human phenylalanine hydroxylase (PAH) has a paramagnetic effect on the longitudinal relaxation rate of water protons. This effect is proportional to the concentration of enzyme, with a paramagnetic molar-relaxivity value at 400 MHz and 25 degrees C of 1. 3 (+/- 0.03) x 10(3) s-1 M-1. The value of the Arrhenius activation energy (Ea) for the relaxation rate was -14.4 +/- 1.1 kJ/mol for the resting enzyme, indicating a fast exchange of water protons in the paramagnetic environment. The frequency dependence of the relaxation rate also supported this hypothesis. Thus, the recombinant human PAH appears to have a more solvent-accessible catalytic iron than the rat enzyme, in which the water coordinated to the metal is slowly exchanging with the solvent. These findings may be related to the level of basal activity before activation for these enzymes, which is higher for human than for rat PAH. In the presence of saturating (5 mM) concentrations of the substrate L-Phe, the paramagnetic molar relaxivity for human PAH decreased to 0.72 (+/- 0.05) x 10(3) s-1 M-1 with no significant change in the Ea. Effective correlation times (tauC) of 1.8 (+/- 0.3) x 10(-10) and 1.25 (+/- 0.2) x 10(-10) s-1 were calculated for the enzyme and the enzyme-substrate complex, respectively, and most likely represent the electron spin relaxation rate (tauS) for Fe(III) in each case. Together with the paramagnetic molar-relaxivity values, the tauC values were used to estimate Fe(III)-water distances. It seems that at least one of the three water molecules coordinated to the iron in the resting rat and human enzymes is displaced from coordination on the binding of L-Phe at the active site. (+info)
Molecular dynamics study of substance P peptides in a biphasic membrane mimic.
Two neuropeptides, substance P (SP) and SP-tyrosine-8 (SP-Y8), have been studied by molecular dynamics (MD) simulation in a TIP3P water/CCl4 biphasic solvent system as a mimic for the water-membrane system. Initially, distance restraints derived from NMR nuclear Overhauser enhancements (NOE) were incorporated in the restrained MD (RMD) in the equilibration stage of the simulation. The starting orientation/position of the peptides for the MD simulation was either parallel to the water/CCl4 interface or in a perpendicular/insertion mode. In both cases the peptides equilibrated and adopted a near-parallel orientation within approximately 250 ps. After equilibration, the conformation and orientation of the peptides, the solvation of both the backbone and the side chain of the residues, hydrogen bonding, and the dynamics of the peptides were analyzed from trajectories obtained in the RMD or the subsequent free MD (where the NOE restraints were removed). These analyses showed that the peptide backbone of nearly all residues are either solvated by water or are hydrogen-bonded. This is seen to be an important factor against the insertion mode of interaction. Most of the interactions with the hydrophobic phase come from the hydrophobic interactions of the side chains of Pro-4, Phe-7, Phe-8, Leu-10, and Met-11 for SP, and Phe-7, Leu-10, Met-11 and, to a lesser extent, Tyr-8 in SP-Y8. Concerted conformational transitions took place in the time frame of hundreds of picoseconds. The concertedness of the transition was due to the tendency of the peptide to maintain the necessary secondary structure to position the peptide properly with respect to the water/CCl4 interface. (+info)
Molecular dynamics study of substance P peptides partitioned in a sodium dodecylsulfate micelle.
Two neuropeptides, substance P (SP) and SP-tyrosine-8 (SP-Y8), have been studied by molecular dynamics (MD) simulation in an explicit sodium dodecylsulfate (SDS) micelle. Initially, distance restraints derived from NMR nuclear Overhauser enhancements (NOE) were incorporated in the restrained MD (RMD) during the equilibration stage of the simulation. It was shown that when SP-Y8 was initially placed in an insertion (perpendicular) configuration, the peptide equilibrated to a surface-bound (parallel) configuration in approximately 450 ps. After equilibration, the conformation and orientation of the peptides, the solvation of both the backbone and the side chain of the residues, hydrogen bonding, and the dynamics of the peptides were analyzed from trajectories obtained from the RMD or the subsequent free MD (where the NOE restraints were removed). These analyses showed that the peptide backbones of all residues are either solvated by water or are hydrogen-bonded. This is seen to be an important factor against the insertion mode of interaction. Most of the interactions come from the hydrophobic interaction between the side chains of Lys-3, Pro-4, Phe-7, Phe-8, Leu-10, and Met-11 for SP, from Lys-3, Phe-7, Leu-10, and Met-11 in SP-Y8, and the micellar interior. Significant interactions, electrostatic and hydrogen bonding, between the N-terminal residues, Arg-Pro-Lys, and the micellar headgroups were observed. These latter interactions served to affect both the structure and, especially, the flexibility, of the N-terminus. The results from simulation of the same peptides in a water/CCl4 biphasic cell were compared with the results of the present study, and the validity of using the biphasic system as an approximation for peptide-micelle or peptide-bilayer systems is discussed. (+info)
Charge pairing of headgroups in phosphatidylcholine membranes: A molecular dynamics simulation study.
Molecular dynamics simulation of the hydrated dimyristoylphosphatidylcholine (DMPC) bilayer membrane in the liquid-crystalline phase was carried out for 5 ns to study the interaction among DMPC headgroups in the membrane/water interface region. The phosphatidylcholine headgroup contains a positively charged choline group and negatively charged phosphate and carbonyl groups, although it is a neutral molecule as a whole. Our previous study (Pasenkiewicz-Gierula, M., Y. Takaoka, H. Miyagawa, K. Kitamura, and A. Kusumi. 1997. J. Phys. Chem. 101:3677-3691) showed the formation of water cross-bridges between negatively charged groups in which a water molecule is simultaneously hydrogen bonded to two DMPC molecules. Water bridges link 76% of DMPC molecules in the membrane. In the present study we show that relatively stable charge associations (charge pairs) are formed between the positively and negatively charged groups of two DMPC molecules. Charge pairs link 93% of DMPC molecules in the membrane. Water bridges and charge pairs together form an extended network of interactions among DMPC headgroups linking 98% of all membrane phospholipids. The average lifetimes of DMPC-DMPC associations via charge pairs, water bridges and both, are at least 730, 1400, and over 1500 ps, respectively. However, these associations are dynamic states and they break and re-form several times during their lifetime. (+info)
Localization and environment of tryptophans in soluble and membrane-bound states of a pore-forming toxin from Staphylococcus aureus.
The location and environment of tryptophans in the soluble and membrane-bound forms of Staphylococcus aureus alpha-toxin were monitored using intrinsic tryptophan fluorescence. Fluorescence quenching of the toxin monomer in solution indicated varying degrees of tryptophan burial within the protein interior. N-Bromosuccinimide readily abolished 80% of the fluorescence in solution. The residual fluorescence of the modified toxin showed a blue-shifted emission maximum, a longer fluorescence lifetime as compared to the unmodified and membrane-bound alpha-toxin, and a 5- to 6-nm red edge excitation shift, all indicating a restricted tryptophan environment and deeply buried tryptophans. In the membrane-bound form, the fluorescence of alpha-toxin was quenched by iodide, indicating a conformational change leading to exposure of some tryptophans. A shorter average lifetime of tryptophans in the membrane-bound alpha-toxin as compared to the native toxin supported the conclusions based on iodide quenching of the membrane-bound toxin. Fluorescence quenching of membrane-bound alpha-toxin using brominated and spin-labeled fatty acids showed no quenching of fluorescence using brominated lipids. However, significant quenching was observed using 5- and 12-doxyl stearic acids. An average depth calculation using the parallax method indicated that the doxyl-quenchable tryptophans are located at an average depth of 10 A from the center of the bilayer close to the membrane interface. This was found to be in striking agreement with the recently described structure of the membrane-bound form of alpha-toxin. (+info)
Polarization-modulated FTIR spectroscopy of lipid/gramicidin monolayers at the air/water interface.
Monolayers of gramicidin A, pure and in mixtures with dimyristoylphosphatidylcholine (DMPC), were studied in situ at the air/H2O and air/D2O interfaces by polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS). Simulations of the entire set of amide I absorption modes were also performed, using complete parameter sets for different conformations based on published normal mode calculations. The structure of gramicidin A in the DMPC monolayer could clearly be assigned to a beta6.3 helix. Quantitative analysis of the amide I bands revealed that film pressures of up to 25-30 mN/m the helix tilt angle from the vertical in the pure gramicidin A layer exceeded 60 degrees. A marked dependence of the peptide orientation on the applied surface pressure was observed for the mixed lipid-peptide monolayers. At low pressure the helix lay flat on the surface, whereas at high pressures the helix was oriented almost parallel to the surface normal. (+info)