The Pharmacology of Semisynthetic Antibiotics

The chemical structures and reactions of penicillins and cephalosporins are reviewed in relation to their effects upon pharmacodynamic properties. The reactive betalactam ring is common to all penicillins and cephalosporin C analogues. This ring opens during acylation of the bacterial wall-building enzymes, but previous opening of the ring by acid or beta-lactamases destroys antibiotic activity.Semisynthetic substitutions may protect the ring by steric hindrance; this may actually inactivate certain penicillinases, so that resistant penicillins may potentiate penicillin G in some circumstances. However, the protective substitutions reduce the intrinsic activity of the synthetic penicillins themselves. Other properties which are affected include absorption, protein-binding, excretion, and possible allergenicity of the drugs. Effects on antibacterial spectrum may possibly be secondary to alteration of lipid solubility.     

THE NEWER PENICILLINS

The newer penicillins give high promise of overcoming some of the few disadvantages of penicillin-G.They fall into three groups: The alpha-phenoxy-penicillins; the penicillinase resistant penicillins; and the penicillins with enhanced activity against gram-negative bacteria.The newer alpha-phenoxy-penicillins offer little over alpha-phenoxy methyl penicillin (penicillin-V). As the length of the side chain is increased, absorption and attainable serum concentration is also increased, but these are questionable benefits and probably not significant for therapeusis.The penicillinase-resistant penicillins have once more brought almost all severe staphylococcal infections within therapeutic range. One of them, methicillin, must be administered parenterally. It is the agent of choice for the treatment of severe, penicillin-G resistant staphylococcal infections, and this is its only clinical indication. Another, oxacillin, which may be administered orally, is partially resistant to gastric acid degradation, but must be given on an empty stomach. It is most useful as prolonged therapy following methicillin, in the treatment of mixed hemolytic streptococcal-penicillin-G resistant staphylococcal infections, and as primary therapy for moderately severe penicillin-G resistant staphylococcal infections.The third group is still mostly in the experimental stage, but some strains of Proteus, E. coli, Salmonella and Shigella are highly vulnerable to their action.Toxic and allergic reactions to the newer penicillins, and crossed allergic reactions with penicillin-G, present unsolved problems.     

Antipseudomonal Activity of α-Sulfoaminopenicillins

A series of penicillins characterized by the presence of a sulfoamino or a modified sulfoamino group in the side chain was subjected to in vitro antimicrobial screening tests. Although the most potent members of the series were less active than benzylpenicillin against gram-positive bacteria and comparably active against most gram-negative bacteria, they were, on the average, 8 to 16 times more effective against strains of Pseudomonas aeruginosa. In other comparative laboratory tests against P. aeruginosa, these compounds were about as active as carbenicillin and four to eight times more active than ampicillin. An examination of structure-activity relationships indicated that maximal potency was obtained with penicillins having an alpha-(aromatic or heteroaromatic)-alpha-sulfoaminoacetamido side chain. The compound with an alpha-phenyl group was comparable in activity to those having an alpha-(2- or 3-thienyl) group, whereas any modification in position or structure of the alpha-sulfoamino group reduced activity. Results of studies with a cell-free P. aeruginosa beta-lactamase suggest that the marked inhibitory effects of alpha-sulfoamino penicillins for P. aeruginosa can be attributed, at least in part, to their high degree of resistance to this enzyme. Some derivatives, however, had weak antipseudomonal activity, despite possessing a high degree of beta-lactamase resistance.     

β-Lactam drug allergens: Fine structural recognition patterns of cephalosporin-reactive IgE antibodies

Lack of experimental findings on the spectrum of cephalosporin allergenic determinants has hindered diagnosis of adverse reactions to these drugs and retarded understanding of allergenic cross-reactions between cephalosporins and between cephalosporins and penicillins. Subjects allergic to the widely used cephalosporin antibiotic cefaclor have serum immuno globulin (Ig) E antibodies that react with the drug. Quantitative hapten inhibition studies employing sera from subjects allergic to cefaclor revealed fine structual recognition differences between the combining site specificities of cefaclor-reactive IgE antibodies in the sera of different subjects. Unlike penicillins, where discrete side chain or thiazolidine ring determinants alone may be recognized, IgE binding determinants on cefaclor encompassed the entire molecule. Fine structural recognition specificity differences at positions R₁ (side-chain) and R₂ (substituent attached to dihydrothiazine ring) were detected between IgE antibodies in different sera. Some antibodies showed clear preferential recognition of the aminobenzyl group at position R₁ and Cl at R₂ while with others, a greater degree of recognition tolerance was seen at R₁ where, for example, the aminohydroxybenzyl or aminodihydrobenzyl groups were recognized, and at R₂ where a methyl or even an ester group was tolerated. As with the penicillins, cephalosporins as allergens cannot simply be considered as a group of compounds with a common allergenic determinant structure. IgE antibodies that bind to cefaclor show great heterogeneity indicated by clear, fine structural differences in recognition of the R₁ and R₂ groups on the drug.     

Detection and side-chain specificity of IgE antibodies to flucloxicillin in allergic subjects

Unlike studies on the antigenicity of penicillins in laboratory animals, limited information is available on the allergenicity of penicillins in man, especially with regard to fine structural allergenic differences between the many different penicillins. Inconsistent with the earlier conclusions of others, our studies suggest that side-chain structures to flucloxacillin-reactive IgE antibodies. Quantitative hapten inhibition studies revealed potent inhibition by flucloxacillin and three structurally related penicillins: oxacillin, cloxacillin and dicloxacillin. Analysis of the results showed that the side-chain group of flucloxacillin, 3-2(-chloro-6-fluorophenyl)-5-methyl-4-isoxazolyl, is recognized by some antibodies and that the 5-methyl-3-phenyl-4-isoxazolyl group, with or without halogen substituents, accounts for the reactivity of other antibodies and for the cross-reactions seen with some other penicillins. Since it is the side-chain group that distinguishes the many different types of penicillin, and since IgE antibodies in many of the allergic reactions recognize the side-chain groups, it is becoming clear that specific assays are required for the detection of IgE antibodies to each of the different penicillins.     

Specificity of Penicillin Acylase of Fusarium and of Penicillium chrysogenum

Extracts containing penicillin acylase were obtained by shaking the mycelium of Fusarium avenaceum and of Penicillium chrysogenum in 0.2 M sodium acetate or sodium chloride solution. The optimum pH for conversion of penicillin V into 6-aminopenicillanic acid (6-APA) by the enzyme of Fusarium was about 7.5, and the reaction velocity was increased by a rise in temperature from 27 to 37 C. Penicillin G and penicillins with an aliphatic side chain were cleaved much less readily than was penicillin V. With the enzyme preparation obtained from a nonpenicillin-producing strain of P. chrysogenum, the reaction rate was higher at pH 8.5 than at pH 7.5 and pH 6.5. The acylase of P. chrysogenum hydrolyzes penicillin V more readily than penicillin G. In a series of aliphatic penicillins, the amount of 6-APA formed through the action of this enzyme increased with the number of carbon atoms of the side chain. Penicillins with a glutaryl or an adipyl group as side chain were unaffected by the enzyme of Fusarium and of Penicillium. No reaction was observed upon incubation of penicillin N (with a D-aminoadipyl side chain) or isopenicillin N (with an L-aminoadipyl side chain) with Fusarium and Penicillium extract. When the carboxy group of the side chain of these penicillins was esterified, formation of 6-APA was observed upon incubation with Penicillium extract, whereas no 6-APA or only very small amounts were obtained by acylase of Fusarium.     

Mutant Escherichia coli penicillin acylase with enhanced stability at alkaline pH

Increased stability at alkaline pH should be a valuable attribute for the utilization of penicillin acylase in bioreactors employed to convert penicillins into 6-aminopenicillanic acid, a precursor of semisynthetic penicillins. In these systems, base is added for pH control, which results in local alkaline conditions that promote enzyme inactivation. Hydrolysis and synthesis reactions are also pH dependent. Here, we report work in which the gene coding for Escherichia coli penicillin acylase was subjected to oligonucleotide-directed random mutagenesis at regions coding for amino acids predicted to be at the surface of the enzyme. The resulting mutant library, cloned in E. coli, was screened by a filter paper assay of the colonies for the presence of penicillin acylase activity with enhanced stability at alkaline pH. Characterization of one of the selected clones revealed the presence of a mutation, Trp431-Arg, which would presumably alter the surface charge of the protein. In vitro experiments demonstrated a near twofold increase in the half-life of the mutant enzyme when stored at pH 8.5 as compared with the wild-type enzyme, with a comparable specific activity at several pH values. In general, the mutant displayed increased stability toward the basic side in the pH-stability profile. (c) 1995 John Wiley & Sons, Inc.     

Role of bicarbonate at the acceptor side of Photosystem II

Besides being the substrate for the carboxylation reaction of photosynthesis, CO₂ (bicarbonate) is required for the activity of Photosystem II (water plastoquinone oxido-reductase). It plays a role on the electron donor side as well as the electron acceptor side. In this contribution, attention will mostly be focused on the history of research into the effects of bicarbonate on electron flow reactions on the acceptor side. Donor side reactions are discussed in this issue by Alan Stemler. This revised version was published online in June 2006 with corrections to the Cover Date.     

Hydrolysis of penicillins and related compounds by the cell-bound penicillin acylase of Escherichia coli

1. A method is given for the preparation of penicillin acylase by using Escherichia coli N.C.I.B. 8743 and a strain selected for higher yield. The enzyme is associated with the bacterial cells and removes the side chains of penicillins to give 6-amino-penicillanic acid and a carboxylic acid. 2. The rates of penicillin deacylation indicated that p-hydroxybenzylpenicillin was the best substrate, followed in diminishing order by benzyl-, dl-alpha-hydroxybenzyl-, 2-furylmethyl-, 2-thienylmethyl-, d-alpha-aminobenzyl-, n-propoxymethyl- and isobutoxymethyl-penicillin. Phenylpenicillin and dl-alpha-carboxybenzylpenicillin were not substrates and phenoxymethyl-penicillin was very poor. 3. Amides and esters of the above penicillins were also substrates for the deacylation reaction, as were cephalosporins with a thienylmethyl side chain. 4. For the deacylation of 2-furylmethylpenicillin at 21 degrees the optimum pH was 8.2. The optimum temperature was 60 degrees at pH7. 5. By using selection A of N.C.I.B. 8743 and determining reaction velocities by assaying yields of 6-amino-penicillanic acid in a 10min. reaction at 50 degrees and pH8.2, the K(m) for benzylpenicillin was found to be about 30mm and the K(m) for 2-furylmethylpenicillin, about 10mm. The V(max.) values were 0.6 and 0.24mumole/min./mg. of bacterial cells respectively.     

Possibility of the intrauterine sensitization to benzylpenicillin and ampicillin in Syrian hamsters]

Immunization of Syrian female hamsters with benzylpenicillin and ampicillin resulted in positive skin reactions of the retarded type and decreased titers of the blood antihistamine factor (AHF) in a part of the posterity. Penicillin allergy was in particular observed in the posterity of the female hamsters immunited before the pregnancy. The state of allergy to penicillins was found in the posterity of the female hamsters with both the positive and negative skin reactions on immunization during the 2nd half of the pregnancy.     

Specificities of haemagglutinating antibodies evoked by members of the cephalosporin C family and benzylpenicillin

1. Antisera have been produced in rabbits to benzylpenicillin and four members of the cephalosporin C family and to conjugates of these substances with bovine gamma-globulin. 2. Deacetoxycephalosporin C reacted less readily and deacetylcephalosporin C lactone more readily with bovine gamma-globulin than did benzylpenicillin, cephalosporin C or deacetylcephalosporin C. 3. Antisera to free or conjugated benzylpenicillin agglutinated red cells sensitized with a variety of penicillins, but only reacted to a significant extent with cells sensitized with the cephalosporins tested when the latter contained an N-phenylacetyl or chemically related side chain. 4. Antisera to members of the cephalosporin C family agglutinated cells sensitized with these cephalosporins or with penicillin N, but did not react with cephalosporins whose side chains were chemically unrelated to alpha-aminoadipic acid. 5. Members of the cephalosporin C family and products of hydrolysis of cephalosporin C behaved as hapten inhibitors of antisera to cephalosporin C, but 7-aminocephalosporanic acid was relatively ineffective. 6. These findings are discussed in relation to differences in the chemical properties of penicillins and cephalosporins.     

Immunological Studies on the Mechanism of Gold Hypersensitivity Reactions

Immunological studies were performed on 12 patients with rheumatoid arthritis who developed reactions to gold. IgE levels were found to be raised in 10 of 11 patients tested at the time of the gold reaction, returning to normal on stopping therapy. Two of 12 patients with gold reactions had positive in-vitro lymphocyte transformation responses to gold.It is suggested that dermatological side effects in particular are mediated by a type I hypersensitivity response.     

Lymphocyte transformation studies in drug hypersensitivity

In a group of patients with clinically diagnosed drug hypersensitivity the in vitro lymphocyte response to the suspected drug was assessed by the lymphocyte transformation test. The test gave positive results in all 15 patients with penicillin-induced immediate or accelerated allergic reactions and positive immediate skin-test reactivity to the major or the minor antigenic determinant of penicillin, or both, but in only 3 of the 12 patients with delayed-onset maculopapular rashes induced by penicillin, despite positive immediate reactivity to the skin-test reagents.Lymphocyte stimulation greater than five times the control level was demonstrated for five patients with penicillin-induced erythroderma, Stevens-Johnson syndrome or a serum-sickness-like illness, or with methicillin-induced interstitial nephritis, all of whom had negative reactions to the appropriate skin-test reagents. A low level of stimulation was seen in eight other skin-test-negative patients with possible allergic reactions induced by penicillins. However, in all subjects tested the stimulation was significantly greater than the mean for control subjects.For 9 of 11 patients with isoniazid-induced hepatitis or maculopapular rashes, but for only 8 of 31 patients with eruptions induced by a variety of drugs other than penicillins and isoniazid, significant stimulation occurred in the lymphocyte transformation test.It is concluded that the lymphocyte transformation test is useful in the detection of hypersensitivity to the penicillins (although in IgE-mediated reactions skin testing is clearly preferable) and isoniazid but is of limited value in the demonstration of hypersensitivity to other drugs.     

Amide bonds to the nitrogen atoms of cysteine and serine as "weak points" in the backbones of proteins

During the initial event in protein self-splicing, a peptide bond to the nitrogen atom of an internal cysteine or serine residue is usually cleaved by the side chain -SH or -OH group to yield a thioester or oxyester intermediate that undergoes further reactions. Self-splicing reactions also accompany the maturation of hedgehog signaling proteins, plant-type asparaginases, and pyruvoyl enzymes. It would be of interest to know whether peptide bonds that involve the nitrogen atoms of cysteine or serine are more susceptible to cleavage than peptide bonds to amino acids that lack reactive side chains. Extrapolations of the results of model reactions conducted at elevated temperatures indicate that the -SH group of N-acetylcysteine enhances the rate of its hydrolysis by a factor of 70, while the OH group of N-acetylserine enhances the rate of its hydrolysis 12-fold, compared with the rate of hydrolysis of N-acetylalanine in neutral solution at 25 °C. Several lines of evidence suggest that the rate-enhancing effects of these -SH and -OH side chains arise from their ability to act as intramolecular general acid-base catalysts for hydrolysis, rather than as nucleophilic catalysts. The protein environment within self-splicing proteins appears to redirect the actions of these side chains to nucleophilic attack, generating rate enhancements that approach the rate enhancements generated by conventional enzymes.     

Isolation and characterization of the acetyl-CoA synthetase from Penicillium chrysogenum. Involvement of this enzyme in the biosynthesis of penicillins.

Acetyl-CoA synthetase (ACS) of Penicillium chrysogenum was purified to homogeneity (745-fold) from fungal cultures grown in a chemically defined medium containing acetate as the main carbon source. The enzyme showed maximal rate of catalysis when incubated in 50 mM HCl-Tris buffer, pH 8.0, at 37 degrees C. Under these conditions, ACS showed hyperbolic behavior against acetate, CoA, and ATP; the Km values calculated for these substrates were 6.8, 0.18, and 17 mM, respectively. ACS recognized as substrates not only acetate but also several fatty acids ranging between C2 and C8 and some aromatic molecules (phenylacetic, 2-thiopheneacetic, and 3-thiopheneacetic acids). ATP can be replaced by ADP although, in this case, a lower activity was observed (37%). ACS in inhibited by some thiol reagents (5,5'-dithiobis(nitrobenzoic acid), N-ethylmaleimide, p-chloromercuribenzoate) and divalent cations (Zn2+, Cu2+, and Hg2+), whereas it was stimulated when the reaction mixtures contained 1 mM dithiothreitol, reduced glutathione, or 2-mercaptoethanol. The calculated molecular mass of ACS was 139 +/- 1 kDa, and the native enzyme is composed of two apparent identical subunits (70 kDa) in an alpha 2 oligomeric structure. ACS activity was regulated "in vivo" by carbon catabolite inactivation when glucose was taken up by cells in which the enzyme had been previously induced. This enzyme can be coupled "in vitro" to acyl-CoA:6-aminopenicillanic acid acyltransferase from P. chrysogenum, thus allowing the reconstitution of the functional enzymatic system which catalyzes the two latter reactions responsible for the biosynthesis of different penicillins. The ACS from Aspergillus nidulans can also be coupled to 6-aminopenicillanic acid acyltransferase to synthesize penicillins. These results strongly indicate that this enzyme can catalyze the activation (to their CoA thioesters) of some of the side-chain precursors required in these two fungi for the production of several penicillins. All these data are reported here for the first time.     

Side effects of benoxaprofen.

A study was made of adverse dermatological reactions to the non-steroidal anti-inflammatory agent benoxaprofen. Photosensitivity was seen in several patients, confined to wavelengths less than 340 nm. Other cutaneous side effects were erythema multiforme, the Stevens-Johnson syndrome, milia, and onycholysis. One case of pancytopenia and toxic epidermal necrolysis was reported. patients were not rechallenged with the drug, but these reactions appear to be true side effects of benoxaprofen.     

Use of long-chain fatty acid-CoA ligase (AMP-forming) from Pseudomonas fragi for the ''in vitro'' synthesis of natural penicillins

Five different naturally occurring penicillins containing as side chains hexanoic, trans-3-hexenoic, heptanoic, octanoic or trans-3-octenoic acids have been synthesized 'in vitro' by coupling long-chain fatty acid-CoA ligase (AMP-forming) (EC 6.2.1.3) from Pseudomonas fragi (LFCoA-L) with acyl-CoA: 6-aminopenicillanic acid acyltransferase (AT) from Penicillium chrysogenum. The quantity of penicillin produced was directly related with the carbon length of the side chain precursor tested, being maximal with octanoic acid. Fatty acids with a lower length than C5 were not recognized as substrates and nor were certain aromatic molecules.     

Lysine epsilon-aminotransferase, the initial enzyme of cephalosporin biosynthesis in actinomycetes

Streptomyces clavuligerus, Streptomyces lipmanii and Nocardia (formerly Streptomyces) lactamdurans are Gram-positive mycelial bacteria that produce medically important beta-lactam antibiotics (penicillins and cephalosporins including cephamycins) that are synthesized through a series of reactions starting from lysine, cysteine and valine. L-lysine epsilon-aminotransferase (LAT) is the initial enzyme in the two-step conversion of L-lysine to L-alpha-aminoadipic acid, a specific precursor of all penicillins and cephalosporins. Whereas S. clavuligerus uses LAT for cephalosporin production, it uses the cadaverine pathway for catabolism when lysine is the nitrogen source for growth. Although the cadaverine path is present in all examined streptomycetes, the LAT pathway appears to exist only in beta-lactam-producing strains. Genetically increasing the level of LAT enhances the production of cephamycin. LAT is the key rate-limiting enzyme in cephalosporin biosynthesis in S. clavuligerus strain NRRL 3585. This review will summarize information on this important enzyme.     

Apparent inhibition of photoredox reactions of magnesium octaethylporphyrin at the lipid bilayer-water interface by neutral quinones.

Neutral quinones rapidly equilibrate across the lipid bilayer, hereby rendering the photoeffects seen in pigmented bilayers sensitive to the redox properties at both interfaces. The lack of photoeffect by quinones themselves and their apparent quenching reactions with aqueous acceptors is thus explained. An aqueous donor is needed on one side to break the symmetry and to allow vectorial electron transfer to be recorded. It is concluded that the neutral quinone accumulates on the polar side of the interface with respect to the hydrophobic pigment. The system may allow the study of kinetics of proton transfer accompanying the redox reactions of the quinones.     

The immunological aspects of predicting antibiotic therapy efficacy in peritonitis patients]

The analysis of the impact of various group antibiotics on the mechanisms of development and correction of pathogenetically heterogeneous immune deficiency in 235 patients at age of 17 to 85 years with local (80 patients) and general (155 patients) peritonitis is presented. Cephalosporins and fluoroquinolones promoted restoration of the immune and interleukin (IL-1 and IL-2 of donors, IL-1r and IL-2r of patients) status and lowered the immunodepressive effect of glucocorticoids (GC) at the organism (cortisol, ACTH and cortisol-binding globulin) and cellular (GC receptors III) levels. Aminoglycosides and penicillins had no significant action on the immune and interleukin status but lowered the EG effect at the organism (aminoglycosides) and cellular (penicillins) levels. It is recommended that the antibiotics be used with an account of their involvement in the systemic reactions of the host.