Antibiotic chloramphenicol mechanism

A reaction in which the oxidized or reduced form of a compound isomerizes via a first-order process on the voltammetric time-scale is common for a wide range of organometallic and organic compounds (for example Bard etal., 1973 Bond et al., 1986,1988,1992). An example from the field of organic chemistry involves the reduction of diethyl maleate to its radical anion which then isomerizes to the diethyl fumarate anion, again an overall EC mechanism (Bard et al., 1973). There is a wide range of examples of other EC mechanisms such as the reduction of the antibiotic chloramphenicol in which a nitro unit (-NO2) is reduced to a hydroxylamine (-NHOH) (E step) which rapidly converts into a nitroso (-NO) species (C step) (Kissinger and Heineman, 1983). 

Mechanism-based inactivation of CYP450 (or suicide inhibition) occurs when a non-toxic drug is metabolised by CYP450 to generate a metabolite that can bind irreversibly with the enzyme. The mechanism of inhibition usually involves free-radical alkylation or acylation of the active site and results in destruction of enzyme activity. Examples of drugs that act in this way include the antibiotic chloramphenicol and the anticancer agent cyclophosphamide. 

The antibiotic chloramphenicol is oxidized by CYP monooxygenase to chloramphenicol oxamyl chloride formed by the oxidation of the dichloromethyl moiety of chloramphenicol followed by elimination of hydrochloric acid " (Figure 33.6). The reactive metabolite reacts with the e-amino group of a lysine residue in CYP and inhibits the enzymatic reaction progressively with time. This type of inhibition is a time-dependent inhibition or a mechanism-based inhibition or inactivation, and the substrate involved historically has been called a suicide substrate because the enzymatic reaction yields a reactive metabolite, which destroys the enzyme. ... 

Lack of Repression of the Shildmic Acid Pathway.—It is well established that a control mechanism on the shikimic acid pathway is by repression of enzymes early in the pathway by the end products, e.g. by phenylalanine. Cases where this type of control appear to have broken down have now been reported. Lowe and Westlake, in studying the biosynthesis of the phenolic antibiotic chloramphenicol in Streptomyces sp. 3022a, examined several enzymes of the pathway, notably chorismate mutase and anthranilate synthetase, but could find no evidence that end-product control on chloramphenicol synthesis was operating in this organism. Similarly, Chu and Widholm fed phenylalanine and tyrosine to a range of tissue cultures of higher plants, but were not able to observe any evidence of feedback control on chorismate mutase levels. 

Three ofher mechanisms of chloramphenicol resisfance have been described. Firsf, a fransposon-encoded chloramphenicol efflux protein has been idenfified in E. coli. Second, some bacterial sfrains have been found to possess drug-resisfanf ribosomes, and fhird, low level resisfance can arise by chromosomal mufafions which reduce fhe amounf ofporins and fherefore impair uptake. This last mechanism is essentially that described for the AG AC antibiotics. 

A third resistance mechanism is akin to that described for the AGAC antibiotics and chloramphenicol, whereby changes in the outer membrane porins of Gram-negative bacteria reduce the penetration of /3-lactams resulting in low levels of resistance. 

Antibiotics may be classified by chemical structure. Erythromycin, chloramphenicol, ampicillin, cefpodoxime proxetil, and doxycycline hydrochloride are antibiotics whose primary structures differ from each other (Fig. 19). Figure 20 shows potential oscillation across the octanol membrane in the presence of erythromycin at various concentrations [23]. Due to the low solubility of antibiotics in water, 1% ethanol was added to phase wl in all cases. Antibiotics were noted to shift iiB,sDS lo more positive values. Other potentials were virtually unaffected by the antibiotics. On oscillatory and induction periods, there were antibiotic effects but reproducibility was poor. Detailed study was then made of iiB,sDS- Figure 21 (a)-(d) shows potential oscillation in the presence of chloramphenicol, ampicillin, cefpodoxime proxetil, and doxycycline hydrochloride [21,23]. Fb.sds differed according to the antibiotic in phase wl and shifted to more positive values with concentration. No clear relationship between activity and oscillation mode due to complexity of the antibacterium mechanism could be discovered but at least it was shown possible to recognize or determine antibiotics based on potential oscillation measurement. 

Resistance is conferred by the presence of an R factor, which codes for an acetyl coenzyme A transferase that inactivates chloramphenicol. Another mechanism for resistance is associated with an inability of the antibiotic to penetrate the organism. This change in permeability may be the basis of multidrug resistance. 

The three antibiotic inhibitors of translation that will be used in this experiment are chloramphenicol, cycloheximide, and puromycin (Fig. 23-10). Chloramphenicol is specific for prokaryotic ribosomes, blocking the transfer of the peptide on the tRNA at the P site to the amino acid linked to the tRNA at the A site (the peptidyl transfer reaction). Since the source of the ribosomes used in this experiment is wheat germ (eukaryotic), we would predict that chloramphenicol would not have a great effect on translation. The mechanism of cycloheximide-mediated inhibition is the same as that described above for chloramphenicol, except for the fact that it is specific for the 80S eukaryotic ribosome. Puromycin is a more broad translational inhibitor, effective on both eukaryotic and prokaryotic ribosomes. It acts as a substrate analog of aminoacyl tRNA. When it binds at the A site of the ribosome, it induces premature termination of translation (Fig. 

Plasmid (or transposon)-encoded enzymes are thus responsible for the degradation of several different types of antibiotics. The inactivation of several /J-lactams, AGACs, 14-membered macrolides, other macrolides, lin-cosamides and streptogramis (MLS) and chloramphenicol is a major resistance mechanism it has yet to be shown that inactivation of other antibiotics falls into this category. 

Phosphorylation is a common mechanism resulting in resistance to the aminoglycoside antibiotics. This chemical strategy also has been associated with resistance to the macrolides such as erythromycin, the tuberactinomycins such as viomycin, and chloramphenicol. The aminoglycoside kinases share 3D structural similarity with the Ser/Thr/Tyr protein kinase family (36), and the conservation of kinase signature sequences in macrolide... 

Fluoroquinolones must penetrate bacteria to reach their target, DNA gyrase. The second mechanism of fluoroquinolone resistance is decreased cell wall permeability. The fluoroquinolones diffuse through porin channels in the outer membrane of Gram-negative bacteria. Mutation results in a decrease in porin channel proteins, resulting in decreased uptake of the fluoroquinolones into bacterial cells. Alterations in a wide range of outer membrane proteins in Pseudomonas spp. result in resistance. From these mutations, the increase in MIC of the fluoroquinolones is relatively low (2-to 32-fold). Flowever, there is cross-resistance with unrelated antibiotics, most frequently cefoxitin, chloramphenicol, trimethoprim and tetracycline. 

The result is the immediate termination of translation and the release of a truncated protein. Two potent antibiotics that specifically inhibit bacterial translation, are tetracycline, which blocks the A site and prevents the entry of aminoacyl-tRNAs, and chloramphenicol, which inhibits the peptidyl transferase activity of the 23 S rRNA. The mechanisms of action of these antibiotics, including streptomycin, which alters the fidelity of translation in bacteria, are listed in Table 26.1. 

Clindamycin binds exclusively to the 50S subunit of bacterial ribosomes and suppresses protein synthesis. Although clindamycin, erythromycin, and chloramphenicol are not structurally related, they act at sites in close proximity, and binding by one of these antibiotics to the ribosome may inhibit the interaction of the others. There are no clinical indications for the concurrent use of these antibiotics. Macrolide resistance due to ribosomal methylation by encoded enzymes also may produce resistance to clindamycin. However, because cUndamycin does not induce the methylase, there is cross-resistance only if the enzyme is produced con-stitutively. Clindamycin is not a substrate for macrolide efflux pumps thus, strains that are resistant to macrolides by this mechanism are susceptible to clindamycin. Altered metabolism occasionally causes clindamycin resistance. 

MECHANISM OF ACTION Chloramphenicol inhibits protein synthesis in bacteria, and to a lesser extent, in eukaryotic cells. It binds reversibly to the SOS ribosomal subunit (near the binding site for the macrolide antibiotics and clindamycin). The drug prevents the binding of the amino acid-containing end of the aminoacyl tRNA to the acceptor site on the SOS ribosomal subunit. The interaction between peptidyltransferase and its amino acid substrate is blocked, inhibiting peptide bond formation (Figure 46-2). 

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