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PubMed1982

PhoE protein pore of the outer membrane of Escherichia coli K12 is a particularly efficient channel for organic and inorganic phosphate.

This study was undertaken to investigate the proposed in vivo pore function of PhoE protein, an Escherichia coli K12 outer membrane protein induced by growth under phosphate limitation and to compare it with those of the constitutive pore proteins OmpF and OmpC. Appropriate mutant strains were constructed containing only one of the proteins PhoE, OmpF or OmpC, or none of these proteins at all. By measuring rates of nutrient uptake at low solute concentrations, the proposed pore function of PhoE protein was confirmed as the presence of the protein facilitates the diffusion of Pi through the outer membrane, such as a pore protein deficient strain behaves as a Km mutant. Comparison of the rates of permeation of Pi, glycerol 3-phosphate and glucose 6-phosphate through pores formed by PhoE, OmpF and OmpC proteins shows that PhoE protein is the most effective pore in facilitating the diffusion of Pi and phosphorus-containing compounds. The three types of pores were about equally effective in facilitating the permeation of glucose and arsenate. Possible reasons for the preference for Pi and Pi-containing solutes are discussed.

Bacterial Outer Membrane Proteins↗
PubMed1994

The control region of the pdu/cob regulon in Salmonella typhimurium.

The pdu operon encodes proteins for the catabolism of 1,2-propanediol; the nearby cob operon encodes enzymes for the biosynthesis of adenosyl-cobalamin (vitamin B12), a cofactor required for the use of propanediol. These operons are transcribed divergently from distinct promoters separated by several kilobases. The regulation of the two operons is tightly integrated in that both require the positive activator protein PocR and both are subject to global control by the Crp and ArcA proteins. We have determined the DNA nucleotide sequences of the promoter-proximal portion of the pdu operon and the region between the pdu and cob operons. Four open reading frames have been identified, pduB, pduA, pduF, and pocR. The pduA and pduB genes are the first two genes of the pdu operon (transcribed clockwise). The pduA gene encodes a hydrophobic protein with 56% amino acid identity to a 10.9-kDa protein which serves as a component of the carboxysomes of several photosynthetic bacteria. The pduF gene encodes a hydrophobic protein with a strong similarity to the GlpF protein of Escherichia coli, which facilitates the diffusion of glycerol. The N-terminal end of the PduF protein includes a motif for a membrane lipoprotein-lipid attachment site as well as a motif characteristic of the MIP (major intrinsic protein) family of transmembrane channel proteins. We presume that the PduF protein facilitates the diffusion of propanediol. The pocR gene encodes the positive regulatory protein of the cob and pdu operons and shares the helix-turn-helix DNA binding motif of the AraC family of regulatory proteins. The mutations cobR4 and cobR58 cause constitutive, pocR-independent expression of the cob operon under both aerobic and anaerobic conditions. Evidence that each mutation is a deletion creating a new promoter near the normal promoter site of the cob operon is presented.

Amino Acid Sequence↗
PubMed1986

Theoretical analysis of oxygen supply to contracted skeletal muscle.

Honig and collaborators reported striking contradictions in current understanding of O2 supply to working skeletal muscle. Therefore we re-examined the problem by means of a new composite computer simulation. As inclusion of erythrocytic O2 desaturation and oxygen transport and consumption inside the muscle cell into a single model would entail immense numerical difficulties, we broke up the whole process into its several components: O2 desaturation of erythrocytes O2 transport and consumption in muscle fiber capillary transit time characterizing the period of contact between red cell and muscle fiber. "Erythrocyte model" as well as "muscle fiber model" both consist of a central core cylinder surrounded by a concentric diffusion layer representing the extracellular resistance to O2 diffusion (Fig. 1). Resistance layers in both models are to be conceived of as one and the same anatomical structure--even though in each model their shape is adapted to the respective geometry. By means of this overlap region a spatial connexion between both is given, whereas temporal coherence governing O2 fluxes and red cell spacing is derived from capillary transit time. Analysis of individual components is outlined as follows: Assuming axial symmetry of the problem a numerical algorithm was employed to solve the parabolic system of partial differential equations describing red cell O2 desaturation. Hb-O2 reaction kinetics, free and facilitated O2 diffusion in axial and radial directions, and red cell movement in capillary were considered. Resulting time courses of desaturation, which are considerably faster than the ones computed by Honig et al., are given in the following table (see also Fig. 3). (Formula: see text) Furthermore, we studied the respective importance of the several processes included in our model: Omission of longitudinal diffusion increased desaturation time by 15% to 23%, whereas effects of reaction kinetics and axial movement were 5% and 2% respectively. For time courses see Fig. 2. Nature and magnitude of extra-erythrocytic resistance to O2 diffusion playing a prominent part in O2 desaturation are scarcely explored. Calculated desaturation times based upon our new estimates (line 3 of above table) correspond well, however, with findings by Sinha, who observed 1.75 to 4-fold prolongation in omental and mesenteric capillaries compared to desaturation through equivalent plasma layers. The 3-dimensional elliptic system of partial differential equations describing stationary O2 transport through resistance layer and subsequent free and facilitated O2 diffusion and O2 consumption in muscle fiber was solved analytically.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1986

Kinetics of ascorbate transport by cultured retinal capillary pericytes. Inhibition by glucose.

Accumulation of radioactive L-[carboxyl-14C]-ascorbic acid by cultured bovine retinal capillary pericytes was studied. Kinetic analysis of the transport showed a time-dependent, saturable system with an apparent Km of 76.0 microM and a Vmax of 42 pmole/micrograms DNA/min. A facilitated carrier diffusion process was established on the basis that the system was not sensitive to 2,4-dinitrophenol, ouabain, or reduced sodium concentration in the incubation media, and that the carrier system demonstrated stereospecificity for an ascorbate analogue, dehydroascorbate, and for sugar analogues such as alpha-D-glucose and 3-0-methyl-D-glucose (3-0-MG), but not for beta-D-fructose or L-glucose. Transport of ascorbate by cultured pericytes was insulin-insensitive. 3-0-Methyl-D-glucose inhibited ascorbate transport into pericytes in a non-competitive manner with a Ki of 22 mM. These results indicate that, in cultured retinal capillary pericytes, a common facilitated carrier diffusion system is involved in the transport of ascorbate and sugar analogues such as alpha-D-glucose or 3-0-MG.

Animals↗
PubMed2007

Anisotropy and temperature dependence of myoglobin translational diffusion in myocardium: implication for oxygen transport and cellular architecture.

Pulsed field gradient NMR methods have determined the temperature-dependent diffusion of myoglobin (Mb) in perfused rat myocardium. Mb diffuses with an averaged translational diffusion coefficient (DMb) of 4.24-8.37x10(-7)cm2/s from 22 degrees C to 40 degrees C and shows no orientation preference over a root mean-square displacement of 2.5-3.5 microm. The DMb agrees with the value predicted by rotational diffusion measurements. Based on the DMb, the equipoise diffusion PO2, the PO2 in which Mb-facilitated and free O2 diffusion contribute equally to the O2 flux, varies from 2.72 to 0.15 in myocardium and from 7.27 to 4.24 mmHg in skeletal muscle. Given the basal PO2 of approximately 10 mmHg, the Mb contribution to O2 transport appears insignificant in myocardium. In skeletal muscle, Mb-facilitated diffusion begins to contribute significantly only when the PO2 approaches the P50. In marine mammals, the high Mb concentration confers a predominant role for Mb in intracellular O2 transport under all physiological conditions. The Q10 of the DMb ranges from 1.3 to 1.6. The Mb diffusion data indicate that the postulated gel network in the cell must have a minimum percolation cutoff size exceeding 17.5 A and does not impose tortuosity within the diffusion root mean-square displacement. Moreover, the similar Q10 for the DMb of solution versus cell Mb suggests that any temperature-dependent alteration of the postulated cell matrix does not significantly affect protein mobility.

Animals↗
PubMed1992

Neurotransmitter release: facilitation and three-dimensional diffusion of intracellular calcium.

In order to account for the time courses of both evoked release and facilitation, in the framework of the Ca2+ hypothesis, Fogelson and Zucker (1985, Biophys. J. 48, 1003-1017) suggested treating diffusion of Ca2+, once it enters through the Ca2+ channels, as a three-dimensional process (three-dimensional diffusion model). This model is examined here as a refined version of the "Ca(2+)-theory" for neurotransmitter release. The three-dimensional model was suggested to account for both the time course of release and that of facilitation. As such, it has been examined here as to its ability to predict the dependence of the amplitude and time course of facilitation under various experimental conditions. It is demonstrated that the three-dimensional diffusion model predicts the time course of facilitation to be insensitive to temperature. It also predicts the amplitude and time course of facilitation to be independent of extracellular Ca2+ concentration. Moreover, it predicts that inhibition of the [Na+]o in equilibrium with [Ca2+]i exchange does not alter facilitation. These predictions are not upheld by the experimental results. Facilitation is prolonged upon reduction in temperature. The amplitude of facilitation declines and its duration is prolonged upon increase in extracellular Ca2+ concentration. Finally, inhibition of the [Na+]o in equilibrium with [Ca2+]i exchange prolongs facilitation but does not alter the time course of evoked release after an impulse.

Animals↗
PubMed1987

Comparison of intracellular PO2 and conditions for blood-tissue O2 transport in heart and working red skeletal muscle.

1. Neither anoxic nor hypoxic cells were found in epicardium of anaesthetized dogs, cats, rabbits and rats despite heterogeneity of flow (Wieringa et al., 1982) and haematocrit (Honig et al., in press) in the coronary capillary network. 2. Median PO2 in unstressed dog heart and cat heart are 4.8 and 5.2 torr, respectively. These values are close to the P50 of the oxymyoglobin dissociation curve, and well above PcritO2. 3. A dense, interconnected capillary network and high capillary haematocrit appear essential to achieve high O2 extraction at flows characteristic of maximally working myocardium. 4. Mb promotes O2 transport in myocardium by: a) maximizing the driving force for transcapillary diffusion, b) minimizing spatial variability in PmbO2, c) facilitating O2 diffusion in myocytes and, d) permitting close capillary packing without a diffusion shunt for O2. 5. The O2 conductance of the red cell-capillary system is a major determinant of O2 mass transfer in red muscle.

Animals↗
PubMed1997

Myoglobin and hemoglobin rotational diffusion in the cell.

The detection of the 1H NMR signal of myoglobin (Mb) in tissue opens an opportunity to examine its cellular diffusion property, which is central to its purported role in facilitating oxygen transport. In perfused myocardium the field-dependent transverse relaxation analysis of the deoxy Mb proximal histidyl NdeltaH indicates that the Mb rotational correlation time in the cell is only approximately 1.4 times longer than it is in solution. Such a mobility is consistent with the theory that Mb facilitates oxygen diffusion from the sarcoplasm to the mitochondria. The microviscosities of the erythrocyte and myocyte environment are different. The hemoglobin (Hb) rotational correlation time is 2.2 longer in the cell than in solution. Because both the overlapping Hb and Mb signals are visible in vivo, a relaxation-based NMR strategy has been developed to discriminate between them.

Animals↗
PubMed1990

The effect of different agitation modes on platelet metabolism, thromboxane formation, and alpha-granular release during platelet storage.

Platelet concentrates (PCs), prepared by plateletpheresis, were stored in aliquots in polyvinylchloride blood bags for 5 days at 22 degrees C under rapid, slow, or no agitation. Nonagitated PCs were also stored in a 98-percent oxygen atmosphere. In nonagitated PCs, pO2, lactate production, and platelet factor 4 (PF 4) concentration increased, whereas the ATP level and pH dropped rapidly. These changes were somewhat minimized in nonagitated PCs stored in oxygen. There was no significant difference between the two agitated groups. The increase in PF 4 correlated inversely to the decrease in ATP: r = -0.91, p less than 0.001, n = 24. The formation of thromboxane B2 (TxB2) after stimulation with arachidonic acid or collagen was significantly higher in slowly agitated PCs on Day 5 than on Day 0 (p less than 0.01). Nonagitated PCs produced lower levels of TxB2 (collagen stimulation) on Day 5 (p less than 0.05). In unstimulated PCs, the levels of TxB2 and ATP were inversely correlated on Day 5 (r = -0.70, p less than 0.001, n = 20). In vivo survival was performed after 72 hours of storage; mean survival (+/- SD) was 6.5 (+/- 0.3) days for nonagitated oxygenated PCs and 6.8 (+/- 0.7) days for agitated PCs. In nonagitated PCs, anaerobic metabolism increased, although oxygen diffusion through the container wall was sufficient. Agitation seems to facilitate the diffusion of oxygen through the storage medium. Nonagitated PCs were stored safely for 24 hours; this period can be extended to at least 72 hours when aerobic metabolism is maintained.

Blood Cells↗
PubMed1987

Does myoglobin contribute significantly to diffusion of oxygen in red skeletal muscle?

We have examined the role of myoglobin to facilitate O2 diffusion to active mitochondria in skeletal muscle by constructing computer-simulation experiments. Steady-state mitochondrial O2 consumption under different conditions of supply partial pressure of O2 (PO2) in a system with and without myoglobin were examined for a one-dimensional slab of tissue. O2 consumption by mitochondria was saturable with the mitochondria located in bands at uniform intervals throughout the tissue. Under these conditions, myoglobin provides a measurable increase in O2 transport for supply PO2 below 10 Torr and diffusion lengths expected for skeletal muscle fibers. We conclude that under circumstances where hypoxia lowers PO2 below 10 Torr that myoglobin begins to provide a measurable increase in O2 delivery to mitochondria.

Homeostasis↗
PubMed1986

Myoglobin-dependent oxidative metabolism in the hypoxic rat heart.

The role of myoglobin in facilitating O2 diffusion for oxidative energy production was investigated at high (0.9 mM) and low (0.1 mM) O2 tensions in the Langendorff-perfused rat heart. 31P nuclear magnetic resonance was used to monitor the intracellular pH and concentrations of high energy phosphates. NaNO2 or phenylhydrazine was used to inactivate greater than 85% of intracellular myoglobin. During hypoxia, ATP and phosphocreatine were depleted significantly more rapidly in hearts with reduced concentrations of functional myoglobin than in control hearts. However, at 0.9 mM O2, myoglobin inactivation did not limit oxidative energy metabolism. It is concluded that facilitation of O2 diffusion by cardiac myoglobin plays a significant role in O2 delivery to the mitochondria at low O2 tensions.

Animals↗
PubMed1991

Use of slime dispersants to promote antibiotic penetration through the extracellular polysaccharide of mucoid Pseudomonas aeruginosa.

Agents with the potential to reduce Pseudomonas aeruginosa alginate viscosity (slime dispersants) were shown to promote the diffusion of antipseudomonal antibiotics through alginate but were more effective in facilitating the diffusion of gentamicin than that of ceftazidime. EDTA increased the diffusion rates of these antibiotics by factors of 4.0 and 1.5, respectively, although sodium chloride significantly reduced viscosity and enhanced gentamicin diffusion.

Alginates↗
PubMed1990

Role of geometry and anisotropic diffusion for modelling PO2 profiles in working red muscle.

A 3-dimensional analytical model of O2 diffusion in heavily working muscle is proposed which considers anisotropic, myoglobin (Mb)-facilitated O2 diffusion inside the muscle fiber and a carrier-free layer separating erythrocytes and fiber. The model is used to study the effects of some commonly applied simplifying assumptions (reduced dimensionality, neglected anisotropy) on the resulting PO2 distributions: (1) In order not to underestimate PO2 drops near erythrocytes, modelling O2 transport in 3 dimensions is important. (2) For a capillary-to-fiber ratio of 1, the results from the 2-dimensional version of the present model and from a Krogh-type model which incorporates a carrier-free layer agree well. (3) This is not true if the capillary-to-fiber ratio is 2. (4) In neither case, a Hill-type model furnishes a good description of the PO2 distributions. (5) Anisotropic diffusion may become important under critical O2 supply conditions. For a capillary-to-fiber ratio of 1, a Krogh-type model in which the O2 fluxes within the carrier-free layer are adapted according to Hellums (Microvasc. Res. 13: 131, 1977) yields almost identical PO2 distributions as the present 3-dimensional model.

Animals↗
PubMed1983

Determination of molecular weight of membrane proteins by the use of low-angle laser light scattering combined with high-performance gel chromatography in the presence of a non-ionic surfactant.

An assessment study was carried out to evaluate the performance of the low-angle laser light scattering technique combined with high-performance gel chromatography in the presence of a nonionic surfactant, octaethyleneglycol n-dodecyl ether, precision differential refractometry and ultraviolet photometry. It was found that the combined technique is highly promising as a method for the determination of the molecular weight of a membrane protein solubilized by the surfactant. For trial, molecular weights of the following membrane proteins of Escherichia coli, both solubilized in oligomeric forms, were measured; porin that forms the transmembrane diffusion pore in the outer membrane, and lambda-receptor protein that facilitates the diffusion of maltose-maltodextrins across the outer membrane. The result obtained indicates that both porin and lambda-receptor protein exist as trimers in the surfactant solution.

Bacterial Outer Membrane Proteins↗
PubMed1996

The collagen skeleton of the human umbilical cord at term. A scanning electron microscopy study after 2N-NaOH maceration.

The organization of the collagen fibrils in the human umbilical cord at term is directly visualized by means of a scanning electron microscopy cell maceration method. This technique clearly reveals that there is a much more extensive collagen fibrillar architecture within the umbilical cord than that reported in the classical histological descriptions. The Wharton's jelly, in fact, appears as a spongy network of interlacing collagen fibres and small woven bundles apparently arranged at random and forming a continuous soft skeleton that encases the umbilical vessels. The collagen fibrillar network shows the presence of a wide system of interconnected cavities consisting of canalicular-like structures as well as cavernous and perivascular spaces. This system of cavities might play a mechanical role allowing the storing of the ground substance of the jelly and its diffusion during twisting or compression. Furthermore, it may have an important role facilitating the diffusion throughout the jelly of diffused materials (i.e. water and trophic metabolites) either from or to the umbilical vessels and the amniotic cavity, thus overcoming the lace of a proper vasculature with the jelly.

Collagen↗
PubMed1996

Polymers, Dk, and contact lenses: now and in the future.

PURPOSE: The polymers used in conventional contact lenses are briefly reviewed as are the factors that contribute to their gas permeability. The chemistry and gas permeability of future hydrogel contact lenses are discussed. RESULTS: Interchain packed zones in polymers are less permeable than the amorphous zones. Polymers with thick bulky groups in rigid gas permeable lenses hinder interchain packing, allowing oxygen and carbon dioxide to diffuse through the polymer voids. Also, the flexibility of the siloxane groups facilitates the diffusion of gases. In hydrogel contact lenses, the polymer segments are separated by the aqueous phase. The gas diffusivity increases with hydration, but is limited by the rigid polymer network. Research on new hydrogel contact lenses aims to develop hydrogels with gas permeability enhancing polymers. In these hydrogels, the gas permeability depends not only on their hydration but also on the permeation of the gas through the polymer phase. CONCLUSIONS: The new hydrogel contact lenses will contain highly permeable polysiloxane domains. A potential problem with the new hydrogels is decreased wettability due to the migration of hydrophobic moieties to the lens surface.

Compliance↗
PubMed1990

EM visualization of nucleocytoplasmic transport processes.

The nuclear envelope is strategically located between the nucleoplasm and cytoplasm, and, as such, can play a major role in controlling cellular activity by regulating the exchange of macromolecules between these two compartments. The nuclear pore complexes, which are located within circular areas formed by fusion of the inner and outer membranes of the envelope, represent the primary, if not the exclusive, exchange sites. Individual pores are able to function in both protein import and RNA efflux from the nucleus. Translocation of macromolecules occurs by either passive diffusion or facilitated transport through central channels within the pores. The functional size of the diffusion channel is approximately 9 to over 12 nm in diameter depending on the cell type. The width of the transport channel varies as a function of the number and effectiveness of the specific nuclear targeting signals contained within the permeant molecule. The maximum diameter of the channel can be over 26 nm. Nucleocytoplasmic exchanges can be regulated either by (1) differences in the properties of the transported molecule (molecular size and signal content) or (2) changes in the properties of the pore complexes, which can effect both diffusion and transport.

Amino Acid Sequence↗