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 28,634 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The effect of various lipids on the enzymatic activity of pig heart mitochondrial malate dehydrogenase monomolecular films was studied using the subphase exchange technique described previously. Surface pressure-surface area (pi-A) curves of mixed films of enzyme with dipalmitoyllecithin, egg lecithin, cholesterol, and phospholipids extracted from pig hearts showed that the enzyme interacted with all of the lipids and that the enzyme remained in the film at pressures well above the collapse pressure of malate dehydrogenase in the absence of lipid. The surface enzyme activity was dependent on surface pressure for each lipid; in all cases, the lipids greatly broadened the range of surface pressures where surface enzyme activity was observed. The pi-A and enzyme activity data showed good correlation. Although the simple model system employed does not simulate the complexity of the biological membrane, it gives some evidence for the role of lipids in the stability of membrane-bound enzymes.
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PMID:Effect of lipids on enzymatic activity of pig heart mitochondrial malate dehydrogenase monomolecular films. 92 5

When exposed to CO, the aerobic respiratory system of the marine bacterium Pseudomonas nautica strain 617, previously reduced with dithionite, undergoes reoxidation. When dealing with the purified oxidase (dithionite reduced) exposure of the enzyme to CO induces its reoxidation (collapse of its alpha band). Under our experimental conditions, this form of the oxidase could not be reduced again by dithionite. Addition of formaldehyde to the native oxidized enzyme resulted in full inhibition of the oxidase reduction by dithionite, presumably due to complex formation. We hypothesized a reduction of CO into formaldehyde and a locking of the active site by the reaction product. By using flash photolysis, it was possible to turn over the enzyme, accumulate the reaction product and identify it as formaldehyde. When using the membrane-bound enzyme, formaldehyde accumulated without the help of flash photolysis. This unusual reduction of CO to formaldehyde could be related to the previously reported uncommon features of the P. nautica oxidase, in particular O2 reduction into H2O2 as end product [(1989) FEBS Lett. 247, 475-479].
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PMID:Reduction of carbon monoxide to formaldehyde by the terminal oxidase of the marine bacterium Pseudomonas nautica strain 617. 153 99

The rate of protonophore-mediated decay of pH gradient across lipid vesicular membranes was found to be enhanced by orders of magnitude by valinomycin-K+. Experiments in the presence of gramicidin have shown that the observed rate enhancement by valinomycin-K+ is not due to collapse of the diffusion potential alone. The enhancement of the rate showed hyperbolic dependence on the concentration of valinomycin. Rate enhancement was observed in the presence of the membrane permeant cation tetraphenylphosphonium (TTP+) also. Several factors which might enhance the intrinsic H+ conductivity of protonophores were analyzed. The level of partitioning of the protonophore into the membrane and the pK of membrane-bound protonophores were measured. Valinomycin-K+ did not alter both these parameters significantly. TPP+ increased the partitioning of protonophores and decreased the pK values of membrane-bound protonophores. However, these changes were too small to explain the observed rate enhancements. We suggest that valinomycin-K+ and TPP+ enhance the H+ conductivity of protonophores by increasing the permeability of the ionized form of protonophores by forming an ion pair.
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PMID:Enhancement of transmembrane proton conductivity of protonophores by membrane-permeant cations. 169 58

Fluoride inhibition of carbohydrate metabolism by the acidogenic plaque microflora is well-established, although it has not always been appreciated that oral bacteria vary considerably in their susceptibility to fluoride. Early studies demonstrated that the F-induced reduction in acid production was due, in part, to the inhibition of the glycolytic enzyme, enolase, which converts 2-P-glycerate to P-enolpyruvate. The decreased output of PEP in the presence of F, in turn, results in the inhibition of sugar transport via the PEP phosphotransferase system (PTS). Bacterial accumulation of fluoride involves the transport of HF, a process requiring a transmembrane pH difference or pH gradient, which is generated only by metabolically active cells. The uptake of HF into the more alkaline cytoplasm results in the dissociation of HF to H+ and F- and, if allowed to continue, the accumulation of protons acidifies the cytoplasm, causing a reduction in both the proton gradient and enzyme activity. Current information indicates that in addition to enolase, F- also inhibits the membrane-bound, proton-pumping H+/ATPase, which is involved in the generation of proton gradients through the efflux of protons from the cell at the expense of ATP. Thus, fluoride has the dual action of dissipating proton gradients and preventing their generation through its action on H+/ATPase. The collapse of transmembrane proton gradient, in turn, reduces the ability of cells to transport solutes via mechanisms involving proton motive force. In spite of these known effects on the bacterial cell, there is no general agreement that the anti-microbial effects of F contribute to the anti-caries effect of fluoride.(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:Biochemical effects of fluoride on oral bacteria. 217 27

Rhinosporidium seeberi, the causative organism of rhinosporidiosis of the nasal mucosa and skin was reviewed with regard to its pathogenesis and histopathology, histochemistry, ultrastructure, life cycle, and cultivation. The pathological findings from infected tissues reveal a granulomatous reaction comprising mixed cell granuloma, pseudocystic abscesses, fibrosis around the causative organism (R. seeberi), and transepidermal elimination. The cell walls of trophocytes and sporangia exhibit the presence of cellulose. The spore wall is encapsulated with granular fibrillary substances consisting of acid mucopolysaccharides. Spheroid bodies have proved to be DNA surrounded by a thin membrane-bound layer. In the cytoplasm of the organism, various substances can be detected by histochemical methods (e.g., glycogen, glycoprotein, acid mucopolysaccharides, neutral lipids, and phospholipids). The walls of the sporangia are found to be trilaminated, whereas those of trophocytes are bilaminated. There is a myriad of curvilinear structures around the outer wall of both forms. The ultrastructure of a trophocyte shows it to be comprised of sporoblasts containing oval or round membrane-bound nuclei with nucleoli, mitochondria, endoplasmic reticulum, chromatin granules, vacuoles, lipid bodies, and spherules. We suggest that the multilamellar bodies are precursors of trophocytes and sporangia. Abortive trophocytes without cytoplasmic organelles are seen, and they collapse at the end of the maturation process. Rhinosporidium seeberi fails to grow in any of the artificial media used but can be maintained through its life cycle in tissue cultures.
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PMID:Rhinosporidiosis. 268 23

Rightside-out membrane vesicles of Streptococcus cremoris were fused with proteoliposomes containing the light-driven proton pump bacteriorhodopsin by a low-pH fusion procedure reported earlier [Driessen, A.J.M., Hellingwerf, K.J. & Konings, W.N. (1985) Biochim. Biophys. Acta 808, 1-12]. In these fused membranes a proton motive force, interior positive and acid, can be generated in the light and this proton motive force can drive the uptake of Ca2+. Collapsing delta psi with a concomitant increase in delta pH stimulates Ca2+ uptake while dissipation of the delta pH results in a reduced rate of Ca2+ uptake. Also an artificially generated delta pH, interior acid, can drive Ca2+ uptake in S. cremoris membrane vesicles. Ca2+ uptake depends strongly on the presence of external phosphate while Ca2+-efflux-induced proton flux is independent of the presence of external phosphate. Ca2+ accumulation is abolished by the divalent cation ionophore A23187. Calcium extrusion from intact cells is accelerated by lactose. Collapse of the proton motive force by the uncoupler carbonylcyanide p-trifluoromethoxyphenylhydrazone or inhibition of the membrane-bound ATPase by N,N'-dicyclohexylcarbodiimide strongly inhibits Ca2+ release. Further studies on Ca2+ efflux at different external pH values in the presence of either valinomycin or nigericin suggested that Ca2+ exit from intact cells is an electrogenic process. It is concluded that Ca2+ efflux in S. cremoris is mediated by a secondary transport system catalyzing exchange of calcium ions and protons.
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PMID:Calcium transport in membrane vesicles of Streptococcus cremoris. 301 12

Storage of 5-hydroxytryptamine (5HT) in membrane-bound vesicles (dense bodies) of platelets has been proposed to occur as a result of the formation of macromolecular complexes between nucleotides and 5HT, or because of the existence of an electrochemical proton gradient (delta mu H+) across the vesicle membrane. Tests of the applicability of these hypotheses to 5HT storage in the dense bodies of human platelets have been made by examining the disposition of quinacrine and 5HT in these organelles following varying treatments. Binding seems unlikely, since solid analogues of the dense body core (calcium, adenine nucleotides, and pyrophosphate) do not significantly bind 5HT or quinacrine. Incubation of platelets with substances which disrupt delta mu H+ releases a large percentage of the intra-platelet quinacrine. A much smaller fraction of the total platelet 5HT is released by similar treatment, suggesting that the delta mu H+ may not be required for 5HT storage. Because inhibition of the de novo uptake of 5HT into dense bodies fails to cause the significant loss of the 5HT stored in this compartment, 5HT stores do not appear to be maintained by active 5HT uptake. Several substances which enter the dense bodies equally well at 0 degrees C and 37 degrees C cause release of 5HT at 37 degrees C but not at 0 degrees C. The release observed at 37 degrees C thus cannot be attributable to collapse of delta mu H+ or to the displacement of 5HT from intra-granular binding sites, but may be related to increased membrane permeability to 5HT at 37 degrees C. Based on these observations, it appears as if 5HT taken up into the dense bodies of human platelets is retained because the dense body membrane has a very low passive permeability for 5HT, and that many compounds which cause 5HT release at 37 degrees C may act by increasing this permeability.
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PMID:Exploration of mechanisms of amine storage in the dense bodies of human platelets. 609 85

In vitro fragments of male rat mediobasal hypothalami were superfused with Krebs--Ringer solution in the presence or absence of CaCl2. Infusions containing up to 60 mM potassium chloride were applied, at the end of which tissues were fixed in osmium tetroxide and prepared for transmission electron microscopy. Control superfusions were run in parallel. Quantitative measurements performed on electron micrographs of the outermost palisade region showed significant (20-30%) increase in caliber of axon endings after intensive potassium ion stimulation. Ultrastructurally, widespread depletion of granular vesicles and microvesicles was found. Vesicle shift to the outer zone of the terminals, formation of membrane-bound tubules of the same diameter as microvesicles, and images of attachment and collapse of vesicles into the axolemma were found, particularly after 1 min stimulation. These findings were interpreted as consistent with exocytosis. Longer stimulations were followed by the appearance of large pleomorphic vacuoles that are probably the result of post-exocytotic membrane retrieval. Axon enlargement and vesicle depletion were absent in specimens superfused with calcium-free medium containing high potassium. The functional significance of these ultrastructural changes are interpreted as supporting the hypothesis that exocytosis of calcium-loaded microvesicles can contribute to extrude this ion from median eminence nerve endings during secretion.
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PMID:Structural changes in nerve endings of rat median eminence superfused with media rich in potassium ions. 663 69

The unique electromotility of the outer hair cell (OHC) is believed to promote sharpening of the passive mechanical vibration of the mammalian basilar membrane. The cell also presents a voltage-dependent capacitance, or equivalently, a nonlinear gating current, which correlates well with its mechanical activity, suggesting that membrane-bound voltage sensor-motor elements control OHC length. We report that the voltage dependence of the gating charge and motility are directly related to membrane stress induced by intracellular pressure. A tracking procedure was devised to continuously monitor the voltage at peak capacitance (VpkCm) after obtaining whole cell voltage clamp configuration. In addition, nonlinear capacitance was more fully evaluated with a stair step voltage protocol. Upon whole cell configuration, VpkCm was typically near -20 mV. Negative patch pipette pressure caused a negative shift in VpkCm, which obtained a limiting value near the normal resting potential of the OHC (approximately -70 mV) at the point of cell collapse. Positive pressure in the pipette caused a positive shift that could reach values greater than 0 mV. Measures of the mechanical activity of the OHC mirrored those of charge movement. Similar membrane-tension dependent peak shifts were observed after the cortical cytoskeletal network was disrupted by intracellular dialysis of trypsin from the patch pipette. We conclude that unlike stretch receptors, which may sense tension through elastic cytoskeletal elements, the OHC motor senses tension directly. Furthermore, since the voltage dependence of the OHC nonlinear capacitance and motility is directly regulated by intracellular turgor pressure, we speculate that modification of intracellular pressure in vivo provides a mechanism for controlling the gain of the mammalian "cochlear amplifier".
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PMID:Membrane tension directly shifts voltage dependence of outer hair cell motility and associated gating charge. 761 63

Programmed cell death is an active process wherein the cell initiates a sequence of events culminating in the fragmentation of its DNA, nuclear collapse, and disintegration of the cell into small, membrane-bound apoptotic bodies. Examination of the death program in various models has shown common themes, including a rise in cytoplasmic calcium, cytoskeletal changes, and redistribution of membrane lipids. The calcium-dependent neutral protease calpain has putative roles in cytoskeletal and membrane changes in other cellular processes; this fact led us to test the role of calpain in a well-known model of apoptotic cell death, that of thymocytes after treatment with dexamethasone. Assays for calcium-dependent proteolysis in thymocyte extracts reveal a rise in activity with a peak at about 1 hr of incubation with dexamethasone, falling to background at approximately 2 hr. Western blots indicate autolytic cleavage of the proenzyme precursor to the calpain I isozyme, providing additional evidence for calpain activation. We have also found that apoptosis in thymocytes, whether induced by dexamethasone or by low-level irradiation, is blocked by specific inhibitors of calpain. Apoptosis of metamyelocytes incubated with cycloheximide is also blocked by calpain inhibitors. These studies suggest a required role for calpain in both "induction" and "release" models of apoptotic cell death.
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PMID:Calpain activation in apoptosis. 816 63


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