Effects of organic antagonists of Ca(2+), Na(+), and K(+) on chemotaxis and motility of escherichia coli.
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Various Ca(2+) antagonists used in animal research, many of them known to be Ca(2+) channel blockers, inhibited Escherichia coli chemotaxis (measured as entry of cells into a capillary containing attractant). The most effective of these, acting in the nanomolar range, was omega-conotoxin GVIA. The next most effective were gallopamil and verapamil. At concentrations around 100-fold higher than that needed for inhibition of chemotaxis, each of these antagonists inhibited motility (measured as entry of cells into a capillary lacking attractant). Various other Ca(2+) antagonists were less effective, though chemotaxis was almost always more sensitive to inhibition than was motility. Cells treated with each of these Ca(2+) antagonists swam with a running bias, i.e., tumbling was inhibited. Similarly, some Na(+) antagonists used in animal research inhibited bacterial chemotaxis. E. coli chemotaxis was inhibited by saxitoxin at concentrations above 10(-7) M, while more than 10(-4) M was needed to inhibit motility. Cells treated with saxitoxin swam with a tumbling bias. In the case of other Na(+) antagonists in animals, aconitine inhibited bacterial chemotaxis 10 times more effectively than it inhibited motility, and two others inhibited chemotaxis and motility at about the same concentration. In the case of K(+) antagonists used in animal research, 4-aminopyridine blocked E. coli chemotaxis between 10(-3) M and, totally, 10(-2) M, while motility was not affected at 10(-2) M; on the other hand, tetraethylammonium chloride failed to inhibit either chemotaxis or motility at 10(-2) M. (+info)
Mutations in a tRNA import signal define distinct receptors at the two membranes of Leishmania mitochondria.
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Nucleus-encoded tRNAs are selectively imported into the mitochondrion of Leishmania, a kinetoplastid protozoan. An oligoribonucleotide constituting the D stem-loop import signal of tRNA(Tyr)(GUA) was efficiently transported into the mitochondrial matrix in organello as well as in vivo. Transfer through the inner membrane could be uncoupled from that through the outer membrane and was resistant to antibody against the outer membrane receptor TAB. A number of mutations in the import signal had differential effects on outer and inner membrane transfer. Some mutants which efficiently traversed the outer membrane were unable to enter the matrix. Conversely, restoration of the loop-closing GC pair in reverse resulted in reversion of transfer through the inner, but not the outer, membrane, and binding of the RNA to the inner membrane was restored. These experiments indicate the presence at the two membranes of receptors with distinct specificities which mediate stepwise transfer into the mitochondrial matrix. The combination of oligonucleotide mutagenesis and biochemical fractionation may provide a general tool for the identification of tRNA transport factors. (+info)
Red meat and colon cancer: dietary haem, but not fat, has cytotoxic and hyperproliferative effects on rat colonic epithelium.
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High intake of red meat is associated with an increased risk of colon cancer. It has been suggested that fat from red meat is responsible, because high fat intake increases the concentration of cytotoxic lipids in the colon. Experimental studies have not unequivocally supported such a role for fat, however. Recently, we showed that dietary haem, which is abundant in red meat, increased colonic cytotoxicity and epithelial proliferation. In this study, we wanted to clarify whether dietary fat affects colon cancer risk by itself or by modulating the detrimental effects of haem on the colonic epithelium. Rats were fed control or haem-supplemented diets with 10%, 25% or 40% of the energy derived from fat for 14 days. Faeces were collected for biochemical analyses. Colonic cytotoxicity was determined from the degree of lysis of erythrocytes by faecal water. Colonic epithelial proliferation was measured in vivo using [(3)H]thymidine incorporation. Increasing the fat content of the control diets stimulated faecal disposal of both fatty acids and bile acids. It also increased the concentration of fatty acids, but not that of bile acids, in faecal water in control rats. The cytolytic activity of faecal water and colonic epithelial proliferation were unaffected. Dietary haem increased faecal cation content and cytolytic activity of faecal water at all fat levels, suggesting that the colonic mucosa was exposed to high amounts of luminal irritants. This effect was smaller in rats on the low-fat diet. Dietary haem also increased colonic epithelial proliferation at all fat levels. The haem-induced effects were independent of fatty acids or bile acids in the faecal water. In western societies, 30-40% of ingested energy is supplied by dietary fat, so our results suggest that the association between consumption of red meat and risk of colon cancer is mainly due to its haem content, and is largely independent of dietary fat content. (+info)
Effect of dimethyl adipimidate on K+ transport and shape change in red blood cells from sickle cell patients.
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Dimethyl adipimidate (DMA) reduces K+ loss from, and dehydration of, red cells containing haemoglobin S (HbS cells). Three membrane transporters may contribute to these processes: the deoxygenation-induced cation-selective channel (Psickle), the Ca2+-activated K+ channel (or Gardos channel) and the K+-CI- cotransporter (KCC). We show that DMA inhibited all three pathways in deoxygenated HbS cells. The Gardos channel could be activated following Ca2+ loading. Considerable KCC activity was present in oxygenated HbS cells, showing a selective action of DMA on the transporter in deoxygenated cells. Inhibition of sickling correlated strongly with that of Psickle and moderately with that of KCC activity. We conclude that DMA does not inhibit the K+ pathways directly, but acts mainly by preventing HbS polymerisation and sickling. These findings are relevant to the development of novel chemotherapeutic agents for amelioration of sickle cell disease. (+info)
NorM of vibrio parahaemolyticus is an Na(+)-driven multidrug efflux pump.
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NorM of Vibrio parahaemolyticus apparently is a new type of multidrug efflux protein, with no significant sequence similarity to any known transport proteins. Based on the following experimental results, we conclude that NorM is an Na(+)-driven Na(+)/drug antiporter. (i) Energy-dependent ethidium efflux from cells possessing NorM was observed in the presence of Na(+) but not of K(+). (ii) An artificially imposed, inwardly directed Na(+) gradient elicited ethidium efflux from cells. (iii) The addition of ethidium to cells loaded with Na(+) elicited Na(+) efflux. Thus, NorM is an Na(+)/drug antiporting multidrug efflux pump, the first to be found in the biological world. Judging from the similarity of the NorM sequence to those of putative proteins in sequence databases, it seems that Na(+)/drug antiporters are present not only in V. parahaemolyticus but also in a wide range of other organisms. (+info)
Leishmania plasma membrane Mg2+-ATPase is a H+/K+-antiporter involved in glucose symport. Studies with sealed ghosts and vesicles of opposite polarity.
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Experiments from other laboratories conducted with Leishmania donovani promastigote cells had earlier indicated that the plasma membrane Mg2+-ATPase of the parasite is an extrusion pump for H+. Taking advantage of the pellicular microtubular structure of the plasma membrane of the organism, we report procedures for obtaining sealed ghost and sealed everted vesicle of defined polarity. Rapid influx of H+ into everted vesicles was found to be dependent on the simultaneous presence of ATP (1 mm) and Mg2+ (1 mm). Excellent correspondence between rate of H+ entry and the enzyme activity clearly demonstrated the Mg2+-ATPase to be a true H+ pump. H+ entry into everted vesicle was strongly inhibited by SCH28080 (IC50 = approximately 40 microm) and by omeprazole (IC50 = approximately 50 microm), both of which are characteristic inhibitors of mammalian gastric H+,K+-ATPase. H+ influx was completely insensitive to ouabain (250 microm), the typical inhibitor of Na+,K+-ATPase. Mg2+-ATPase activity could be partially stimulated with K+ (20 mm) that was inhibitable (>85%) with SCH28080 (50 microm). ATP-dependent rapid efflux of 86Rb+ from preloaded vesicles was completely inhibited by preincubation with omeprazole (150 microm) and by 5,5'-dithiobis-(2-nitrobenzoic acid) (1 mm), an inhibitor of the enzyme. Assuming Rb+ to be a true surrogate for K+, an ATP-dependent, electroneutral stoichiometric exchange of H+ and K+(1:1) was established. Rapid and 10-fold active accumulation of [U-(14)C]2-deoxyglucose in sealed ghosts could be observed when an artificial pH gradient (interior alkaline) was imposed. Rapid efflux of [U-(14)C]d-glucose from preloaded everted vesicles could also be initiated by activating the enzyme, with ATP. Taken together, the plasma membrane Mg2+-ATPase has been identified as an electroneutral H+/K+ antiporter with some properties reminiscent of the gastric H+,K+-ATPase. This enzyme is possibly involved in active accumulation of glucose via a H+-glucose symport system and in K+ accumulation. (+info)
Cation-pi effects in the complexation of Na+ and K+ with Phe, Tyr, and Trp in the gas phase.
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Na+ and K+ gas-phase affinities of the three aromatic amino acids Phe, Tyr, and Trp were measured by the kinetic method. Na+ binds these amino acids much more strongly than K+, and for both metal ions the binding strength was found to follow the order Phe < or = Tyr < Trp. Quantum chemical calculations by density functional theory (DFT) gave the same qualitative ordering, but suggested a somewhat larger Phe/Trp increment. These results are in acceptable agreement with predictions based on the binding of Na+ and K+ to the side chain model molecules benzene, phenol, and indole, and are also in reasonable agreement with the predictions from purely electrostatic calculations of the side-chain binding effects. The binding energies were compared with those to the aliphatic amino acids glycine and alanine. Binding to the aromatic amino acids was found to be stronger both experimentally and computationally, but the DFT calculations indicate substantially larger increments relative to alanine than shown by the experiments. Possible reasons for this difference are discussed. The metal ion binding energies show the same trends as the proton affinities. (+info)
Functional expression in Escherichia coli of low-affinity and high-affinity Na(+)(Li(+))/H(+) antiporters of Synechocystis.
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Synechocystis sp. strain PCC 6803 has five genes for putative Na(+)/H(+) antiporters (designated nhaS1, nhaS2, nhaS3, nhaS4, and nhaS5). The deduced amino acid sequences of NhaS1 and NhaS2 are similar to that of NhaP, the Na(+)/H(+) antiporter of Pseudomonas aeruginosa, whereas those of NhaS3, NhaS4, and NhaS5 resemble that of NapA, the Na(+)/H(+) antiporter of Enterococcus hirae. We successfully induced the expression of nhaS1, nhaS3, and nhaS4 under control of an Na(+)-dependent promoter in Escherichia coli TO114, a strain that is deficient in Na(+)/H(+) antiport activity. Inverted membrane vesicles prepared from TO114 nhaS1 and TO114 nhaS3 cells exhibited Na(+)(Li(+))/H(+) antiport activity. Kinetic analysis of this activity revealed that nhaS1 encodes a low-affinity Na(+)/H(+) antiporter with a K(m) of 7.7 mM for Na(+) ions and a K(m) of 2.5 mM for Li(+) ions, while nhaS3 encodes a high-affinity Na(+)/H(+) antiporter with a K(m) of 0.7 mM for Na(+) ions and a K(m) of 0.01 mM for Li(+) ions. Transformation of E. coli TO114 with the nhaS1 and nhaS3 genes increased cellular tolerance to high concentrations of Na(+) and Li(+) ions, as well as to depletion of K(+) ions during cell growth. To our knowledge, this is the first functional characterization of Na(+)/H(+) antiporters from a cyanobacterium. Inverted membrane vesicles prepared from TO114 nhaS4 cells did not have Na(+)/H(+) antiport activity, and the cells themselves were as sensitive to Na(+) and Li(+) ions as the original TO114 cells. However, the TO114 nhaS4 cells were tolerant to depletion of K(+) ions. Taking into account these results and the growth characteristics of Synechocystis mutants in which nhaS genes had been inactivated by targeted disruption, we discuss possible roles of NhaS1, NhaS3, and NhaS4 in Synechocystis. (+info)