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Query: UMLS:C0276640 (TEM)
 20,729 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

We describe a new technique, time-resolved cryotransmission electron microscopy (TRC-TEM), that can be used to study changes in microstructure occurring during dynamic processes such as phase transitions and chemical reactions. The sample is prepared on an electron microscope grid maintained at a fixed temperature in a controlled atmosphere. The dynamic process is induced on the grid by a change in pH, salt, or reactant concentration by rapid mixing with appropriate solutions. Alternatively, induction is by rapid change of specimen temperature, or by controlled evaporation of a volatile component. We call such procedures on-the-grid processing. The dynamic process is permitted to run for a defined time and then the thin-film specimen is thermally fixed by plunging into liquid ethane at its freezing point, producing a cryotransmission electron microscopy specimen. By repeating this procedure with varying delays between induction and sample fixation, we can observe transient microstructures. We demonstrate the use of TRC-TEM to study the intermediate structures that form during the transitions between L alpha, III, and HII liquid crystalline phases in phospholipid systems. We also identify several other possible applications of the technique.
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PMID:Time-resolved cryotransmission electron microscopy. 229 19

Compound U-76,252 (Upjohn) is a cephalosporin ester that enhances oral absorption of the active free acid cephem, U-76,253. The active form structurally resembles parenteral aminothiazolyl-methoxyimino cephalosporins such as cefotaxime and its desacetyl metabolite. The g-negative antimicrobial activity of U-76,253 A (sodium salt of U-76,253) was most similar to that of cefixime and more potent than that of cefaclor or cefuroxime among the orally administered cephalosporins. Against g-positive bacteria, U-76,253 A was more active than cefixime. U-76,253 A was relatively stable to hydrolysis by five beta-lactamases (Type Ia, TEM-1, K1, CARB-2, and OXA-1), a stability most similar to cefotaxime and superior to that of cefaclor. Only the Type Ia (P99) enzyme was significantly inhibited by U-76,253 (IC 50 = 2.0 microM).
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PMID:In vitro evaluations of U-76,252 (CS-807): antimicrobial spectrum, beta-lactamase stability, and enzyme inhibition. 285 64

Two novel C10-(dipeptidyl)cephalosporin esters (3-(beta-chloro-L-alanyl-beta-chloro-L-alanyloxymethyl)-7 beta-(2-thienylacetamido)-3-cephem-4-carboxylic acid (7) and sodium 3-(L-alanyl-L-alanyloxymethyl)-7 beta-(2-thienylacetamido)-3-cephem-4-carboxylate, toluene-sulfonic acid salt (18] were synthesized, and their reactions with Escherichia coli TEM beta-lactamase were examined. Kinetic parameters determined for the enzymatic reactions of 7 (Km = 0.32 mM; Vmax = 338 mumol min-1 (mg protein)-1) and of 18 (Km = 0.33 mM, Vmax = 338 mumol min-1 (mg protein)-1) demonstrate that both of the peptidyl esters are good substrates for the lactamase. In fact, the Vmax rates for 7 and 18 are each more than 4-fold greater than that obtained for cephalothin, 1 (Vmax = 78 mumol min-1 (mg protein)-1), a well characterized substrate for the lactamases. Analysis of the enzymatic reactions by high field (500 MHz) 1H NMR revealed similar patterns for fragmentation of the cephem nucleus of 1, 7, and 18. However, while hydrolysis of 1 produces acetate, cleavage of 7 and 18 releases beta Cl-LAla-beta Cl-LAla and LAla-LAla, respectively, from the dipeptidyl cephalosporin esters. Based on these findings, a strategy for co-opting the beta-lactamases of Gram-negative bacteria for "delivery" of bactericidal agents is described, and an explanation for the previously reported (Mobashery, S., Lerner, S.A., and Johnston, M. (1986) J. Am. Chem. Soc. 108, 1685) antibacterial activity of 7 is offered.
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PMID:Reactions of Escherichia coli TEM beta-lactamase with cephalothin and with C10-dipeptidyl cephalosporin esters. 351 15

Out of 15 selected enterobacterial strains resistant to ampicillin, 12 were able to transfer resistance to mecillinam to Escherichia coli K-12. This resistance to mecillinam was found to be coupled to the presence of beta-lactamase. One strain contained a beta-lactamase characterized as a class IV beta-lactamase, whereas the other 14 strains possessed a class III (TEM-like) beta-lactamase. The specific activity of the class IV beta-lactamase against mecillinam was 55%, and those of the class III beta-lactamase sensitivity of mecillinam, the minimal inhibitory concentrations were lower than might be expected. However, after enzymatic hydrolysis of mecillinam, no antibacterial activity was found. At increasing salt or buffer concentrations the minimal inhibitory concentrations of mecillinam increase to a varying extent for all strains, independently of beta-lactamase production. This study indicates that the increase in minimal inhibitory concentration is dependent on the salt concentration. The study also shows that this increase is not due to salt-mediated hydrolysis or to stimulation either of beta-lactamase activity or of beta-lactamase production. To explain the difference between ampicillin and mecillnam resistance in the beta-lactamase-positive strains, a hypothetical model is presented and discussed.
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PMID:Effect of beta-lactamase and salt on mecillinam susceptibility of enterobacterial strains. 625 65

The stability of low concentrations of amoxycillin in the presence of clavulanic acid (potassium salt) was determined for a wide range of clinically important beta-lactamases including the staphylococcal and TEM plasmid mediated enzymes. Even with enzyme preparations which completely hydrolysed the amoxycillin within a minute, clavulanic acid provided significant protection. The time course of the protection of amoxycillin reflected the time dependent action of clavulanic acid.
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PMID:The beta-lactamase stability of amoxycillin with the beta-lactamase inhibitor, clavulanic acid. 660 Jul 41

The morphology of human platelets spread on glass substrates is sensitive to the presence of calcium. In the absence of Ca2+, cells spread from buffered salt solution develop radially oriented filopodia and subsequently a broad hyalomere surrounding the central region of the cell from which granules are frequently exocytosed. In the presence of Ca2+ cell rounding and apparent withdrawal from the substrate occurs. Scanning (SEM) and transmission (TEM) electron microscopy of cells rounded in the presence of Ca2+ show fibrous elements connecting the cells to the substratum as well as adherent to the substrate in the vicinity of the rounded cells. Interference reflection microscope (IRM) images of these cells are heterogeneous: some contain small discrete darker regions suggesting the presence of focal specializations at the ventral cell surface. In contrast IRM images of cells spread in the absence of Ca2+ indicate predominantly broad areas of unspecialized contact with the substrate in agreement with TEM observations. These results suggest that Ca2+ may enhance platelet-substrate adhesion by initially promoting the formation of focal specializations which become more pronounced as cell rounding occurs possibly due to Ca2+ activation of an actomyosin-based contractile mechanism.
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PMID:Effect of calcium on the morphology of human platelets spread on glass substrates. 668 65

The destruction of amoxycillin by beta-lactamase action represents an important mechanism of bacterial resistance to the drug. Data is presented to illustrate that clavulanic acid used in the form of its potassium salt inhibits the amoxycillin destroying action of many different types of beta-lactamase for example: the staphylococcal enzyme, the clinically important plasmid mediated enzymes of the TEM, SHV, OXA and PSE types and the chromosomally controlled enzymes produced by Proteus mirabilis and Klebsiella pneumoniae. The mechanisms by which clavulanic acid inhibits beta-lactamases and potentiates the antibacterial action of amoxycillin are discussed.
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PMID:Biochemistry and action of clavulanic acid. 676 60

A combined transmission (TEM) and scanning (SEM) electron microscopic study was performed on aortae of deoxycorticosterone-salt (DOC-salt)-treated rats and spontaneously hypertensive rats (SHR) to compare the effects of hypertension as well as its reversal on the aortic intima. To best reproduce the in vivo state of the vasculature, rats were perfusion-fixed at pressures corrected for each individual animal (30 mm Hg below measured systolic pressure). The intimal alterations were focal and thus were best appreciated with the combined use of SEM and TEM. Qualitatively, both models of hypertension showed similar intimal changes, which consisted of subintimal thickening due to an accumulation of both extracellular material and cells. Subendothelial cells with a morphology indicating a blood-borne origin were present simultaneously with cells derived from the vessel wall. The increased subendothelial extracellular material included precipitated plasma proteins, reticulated basement membrane, collagen fibers, and fragments of elastin. Increase in the height of endothelial cells with distortion of nuclear shape was prominent. Withdrawal of DOC-salt combined with low-salt diet for 11 weeks did not result in a discernible regression of these intimal changes despite normalization of blood pressure. We conclude that vascular injury, once induced, may be difficult to reverse and suggest that areas of prior damage may serve as foci for later vascular complications.
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PMID:Effects of hypertension and its reversal on aortic intima lesions of the rat. 737 54

To examine the effect of disruption of the salt bridge (between Arg-164 and Asp-179 [numbering of Ambler et al. (Biochem J. 267:269-272, 1991)]) that anchors the conserved omega-loop in class A beta-lactamases, we obtained mutant enzymes with each of the 19 other amino acid residues replacing Asp-179 in the TEM beta-lactamase encoded by pUC19 and studied the level of resistance to various beta-lactams conferred by each enzyme. All mutations of Asp-179 compromised the level of resistance to ampicillin, but most of them enhanced resistance to ceftazidime. In contrast, mutations of Asp-179 generally impaired the low levels of resistance to cefepime and aztreonam. One might expect to find clinical isolates with mutant TEM beta-lactamases with replacements of Asp-179 that express an expanded spectrum of resistance to beta-lactams including ceftazidime.
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PMID:Effects of Asp-179 mutations in TEMpUC19 beta-lactamase on susceptibility to beta-lactams. 748 39

TEM-35 (inhibitor resistant TEM (IRT)-4) and TEM-36 (IRT-7) clavulanic acid-resistant beta-lactamases have evolved from TEM-1 beta-lactamase by two substitutions: a methionine to a leucine or a valine at position 69 and an asparagine to an aspartic acid at position 276. The substitutions at position 69 have previously been shown to be responsible for the resistance to clavulanic acid, and they are the only mutations encountered in TEM-33 (IRT-5) and TEM-34 (IRT-6). However, the N276D substitution has never been found alone in inhibitor-resistant beta-lactamases, and its role in resistance to clavulanic acid was thus unclear. The N276D mutant was constructed, purified, and kinetically characterized. It was shown that the substitution has a direct effect on substrate affinities and leads to slightly decreased catalytic efficiencies and that clavulanic acid becomes a poor substrate of the enzyme. Electrospray mass spectrometry demonstrated the simultaneous presence of free and inhibited enzymes after incubation with clavulanic acid and showed that a cleaved moiety of clavulanic acid leads to the formation of the major inactive complex. The kinetic properties of the N276D mutant could be linked to a salt-bridge interaction of aspartic acid 276 with arginine 244 that alters the electrostatic properties in the substrate binding area.
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PMID:The asparagine to aspartic acid substitution at position 276 of TEM-35 and TEM-36 is involved in the beta-lactamase resistance to clavulanic acid. 762 42


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