pKa calculations for class A beta-lactamases: influence of substrate binding. (1/313)

Beta-Lactamases are responsible for bacterial resistance to beta-lactams and are thus of major clinical importance. However, the identity of the general base involved in their mechanism of action is still unclear. Two candidate residues, Glu166 and Lys73, have been proposed to fulfill this role. Previous studies support the proposal that Glu166 acts during the deacylation, but there is no consensus on the possible role of this residue in the acylation step. Recent experimental data and theoretical considerations indicate that Lys73 is protonated in the free beta-lactamases, showing that this residue is unlikely to act as a proton abstractor. On the other hand, it has been proposed that the pKa of Lys73 would be dramatically reduced upon substrate binding and would thus be able to act as a base. To check this hypothesis, we performed continuum electrostatic calculations for five wild-type and three beta-lactamase mutants to estimate the pKa of Lys73 in the presence of substrates, both in the Henri-Michaelis complex and in the tetrahedral intermediate. In all cases, the pKa of Lys73 was computed to be above 10, showing that it is unlikely to act as a proton abstractor, even when a beta-lactam substrate is bound in the enzyme active site. The pKa of Lys234 is also raised in the tetrahedral intermediate, thus confirming a probable role of this residue in the stabilization of the tetrahedral intermediate. The influence of the beta-lactam carboxylate on the pKa values of the active-site lysines is also discussed.  (+info)

Antibiotic synergy and antagonism against clinical isolates of Klebsiella species. (2/313)

Minimal inhibitory concentrations of kanamycin, gentamicin, amikacin, cephalothin, and chloramphenicol were determined in Trypticase soy broth for 70 clinical isolates of Klebsiella species. Gentamicin and amikacin were the most active on a weight basis. Chloramphenicol was more active than kanamycin, and cephalothin was the least active of all. Studies using a microtiter modification of the checkerboard technique were performed to evaluate the comparative activity of the three aminoglycosides in combination with either chloramphenicol or cephalothin. The cephalothin-aminoglycoside combinations demonstrated synergy in >80% of the isolates tested. No antagonism was noted. The chloramphenicol-aminoglycoside combinations showed antagonism in 35 to 45% of the isolates tested. The data suggest that the chloramphenicol-aminoglycoside combinations be used with caution when treating serious infections where Klebsiella is a potential pathogen.  (+info)

Transferability of cephalothin to the alveolar cavity in thoroughbreds. (3/313)

Five Thoroughbreds were classified into 4 groups according to the administration method used for saline solution (saline), ambroxol, and cephalothin sodium (cephalothin). In group A, cephalothin was injected intravenously after oral administration of ambroxol. In group B, cephalothin was injected intravenously after oral administration of saline. Groups C and D were used as control groups. The dose of cephalothin or ambroxol was clinically administrated. Venous blood and bronchoalveolar lavage fluid (BALF) were sampled from each group. In groups A and B, cephalothin concentrations in plasma reached their maximum level 5 min after cephalothin administration and then declined over time. In plasma obtained from groups A and B, there were no significant differences in pharmacokinetic parameters (T1/2, Kel, Vd). By contrast, cephalothin concentrations in BALF reached their peak at 180 min after cephalothin administration in both groups A and B and maintained a relatively high level even after 300 min. These findings indicate that cephalothin requires a relatively long period of time to move from the blood stream to the alveolar cavity, but once transferred to the alveolar cavity, it is preserved for a long time. In groups A and B, cephalothin concentrations in BALF were approximately at the same level. However, in group A, total protein in BALF was lower at 60, 180, and 300 min than the other groups. Then, cephalothin concentration was adjusted to total protein in BALF. After adjustment to total protein in BALF, group A showed a concentration level of cephalothin approximately 1.5-fold higher than that of group B. This suggests that the transferability of cephalothin to the alveolar cavity improves as a result of the oral administration of ambroxol.  (+info)

Alteration of methotrexate uptake in human leukemia cells by other agents. (4/313)

The uptake of methotrexate (MTX) and the effect of drugs known to either inhibit or enhance MTX transport in L1210 murine leukemia were studied in man using blast cells from patients with acute myelogenous leukemia in vitro. MTX uptake was found to proceed slowly, requiring at least 160 min for cells to reach a "steady state" when extracellular MTX concentrations were 1 muM. Efflux of MTX from preloaded cells required 80 to 120 min and the nonexchangeable or tightly bound fraction was 40% of the total intracellular drug. Utilizing doses that are estimates of achievable peak blood levels following single i.v. injection, cephalothin (21 mug/ml) and hydrocortisone (20 mug/ml) inhibited net MTX accumulation by 20 and 28%, respectively. Vincristine sulfate at 8.3 and 0.083 mug/ml enhanced MTX uptake by 54 and 33%, respectively, by inhibiting MTX efflux, thus increasing the level of intracellular drug in excess of the tightly bound fraction. The potential clinical implications of using MTX in combination with the aforementioned drugs for cancer chemotherapy are discussed.  (+info)

The exocellular DD-carboxypeptidase-endopeptidase of Streptomyces albus G. Interaction with beta-lactam antibiotics. (5/313)

Kinetically, the three-step model proposed for the interaction between beta-lactam antibiotics and the exocellular DD-carboxypeptidases-transpeptidases of Streptomyces R61 and Actinomadura R39 [Frere, Ghuysen & Iwatsubo (1975) Eur. J. Biochem. 57, 343--357; Fuad, Frere, Ghuysen, Duez & Iwatsubo (1976) Biochem. J. 155, 623--629] applies to the interaction between the much less penicillin-sensitive exocellular DD-carboxypeptidase-endopeptidase of Streptomyces albus G and at least phenoxymethylpenicillin, cephalothin and cephalosporin C. The penicillin resistance of the albus G enzyme is mainly due to the low efficiency with which the first reversible complex formed with the antibiotic (complex EI) undergoes transformation into a second more stable complex EI*. Analysis of the ternary interaction between enzyme, NalphaNepsilon-diacetyl-L-lysyl-D-alanyl-D-alanine (Ac2-L-Lys-D-Ala-D-Ala) and cephalosporin C indicates a non-competitive mechanism.  (+info)

Interpretive criteria for cefamandole and cephalothin disk diffusion susceptibility tests. (6/313)

A multi-center study of 1,838 clinical isolates established the accuracy of diffusion susceptibility tests with 30-mug cephalothin disks and 30-mug cefamandole disks. The same interpretive zone standards can be applied to tests with either disk but the two drugs cannot be tested interchangeably.  (+info)

Helicobacter mesocricetorum sp. nov., A novel Helicobacter isolated from the feces of Syrian hamsters. (7/313)

A spiral-shaped bacterium with bipolar, single, nonsheathed flagella was isolated from the feces of Syrian hamsters. The bacterium grew as a thin spreading film at 37 degrees C under microaerobic conditions, did not hydrolyze urea, was positive for catalase and alkaline phosphatase, reduced nitrate to nitrite, did not hydrolyze hippurate, and was sensitive to nalidixic acid but resistant to cephalothin. Sequence analysis of the 16S rRNA gene and biochemical and phenotypic criteria indicate that the novel bacterium is a helicobacter. The novel bacterium is most closely related to the recently described mouse enteric helicobacter, Helicobacter rodentium. This is the first urease-negative Helicobacter species with nonsheathed flagella isolated from feces of asymptomatic Syrian hamsters. We propose to name this novel helicobacter Helicobacter mesocricetorum. The type strain is MU 97-1514 (GenBank accession number AF072471).  (+info)

Massive pulmonary gangrene: a severe complication of Klebsiella pneumonia. (8/313)

SUMMARY: Massive pulmonary gangrene developed in two patients. Review of the literature reveals 10 other case reports of pulmonary gangrene complicating lobar pneumonia. Among the total of 12 patients whose cases have now been reported, all 4 patients who were treated nonsurgically died and the 8 who underwent surgical resection of the gangrenous lung survived. The present report emphasizes the necessity of early recognition and appropriate surgical treatment for a successful outcome.  (+info)

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