Spectrophotometric assay for quantitative determination of 7-aminocephalosporanic acid from direct hydrolysis of cephalosporin C

Microenvironmental pH changes in immobilized cephalosporin C acylase during a proton-producing reaction and regulation by a two-stage catalytic process

Cephalosporin C acylase (CCA), a proton-producing enzyme, was covalently bound on an epoxy-activated porous support. The microenvironmental pH change in immobilized CCA during the reaction was detected using pH-sensitive fluorescein labeling. The high catalytic velocity of the initial stage of conversion resulted in a sharp intraparticle pH gradient, which was likely the key factor relating to low operational stability. Accordingly, a novel strategy for a two-stage catalytic process was developed to reduce the reaction rate of stage I at a low temperature to preserve enzymatic activity and to shorten the duration of catalysis at a high reaction temperature in stage II. The reaction using the two-stage catalytic process (10-37°C shift at 30min) showed significantly improved stability compared with that of the single-temperature reaction at 37°C (29 batches versus five batches, respectively) and a shorter catalytic period than the reaction at 10°C (40min versus 70min, respectively).

Engineering of a CPC acylase using a facile pH indicator assay

Cephalosporin C (CPC) acylase is important for the one-step production of 7-aminocephalosporanic acid (7-ACA), a key intermediate for cephalosporin antibiotics. However, its application is hampered by the low activity, substrate inhibition, and product inhibition. In this study, two rounds of combinatorial active-site saturation testing (CASTing) were carried out on the CPC acylase acyII from Pseudomonas SE83, and one mutant H57βA/H70βY with no substrate inhibition was obtained. For further engineering to reduce the product inhibition, a quick pH indicator assay was developed, allowing for real-time monitoring of the product inhibition in the presence of added 7-ACA. The utility of the assay was demonstrated by screening six libraries of site-directed saturation mutagenesis libraries of H57βA/H70βY. A new mutant H57βA/H70βY/I176βN was obtained, which showed a k  cat 3.26-fold and a K  IP 3.08-fold that of the wild type, respectively. Given the commercial value of the enzyme, both this pH indicator assay and the triple mutant should be useful for further engineering of the enzyme to increase the specific activity and to decrease the product inhibition.

Dual-lifetime referencing (DLR): a powerful method for on-line measurement of internal pH in carrier-bound immobilized biocatalysts

Background

Industrial-scale biocatalytic synthesis of fine chemicals occurs preferentially as continuous processes employing immobilized enzymes on insoluble porous carriers. Diffusional effects in these systems often create substrate and product concentration gradients between bulk liquid and the carrier. Moreover, some widely-used biotransformation processes induce changes in proton concentration. Unlike the bulk pH, which is usually controlled at a suitable value, the intraparticle pH of immobilized enzymes may deviate significantly from its activity and stability optima. The magnitude of the resulting pH gradient depends on the ratio of characteristic times for enzymatic reaction and on mass transfer (the latter is strongly influenced by geometrical features of the porous carrier). Design and selection of optimally performing enzyme immobilizates would therefore benefit largely from experimental studies of the intraparticle pH environment. Here, a simple and non-invasive method based on dual-lifetime referencing (DLR) for pH determination in immobilized enzymes is introduced. The technique is applicable to other systems in which particles are kept in suspension by agitation.

Results

The DLR method employs fluorescein as pH-sensitive luminophore and Ru(II) tris(4,7-diphenyl-1,10-phenantroline), abbreviated Ru(dpp), as the reference luminophore. Luminescence intensities of the two luminophores are converted into an overall phase shift suitable for pH determination in the range 5.0-8.0. Sepabeads EC-EP were labeled by physically incorporating lipophilic variants of the two luminophores into their polymeric matrix. These beads were employed as carriers for immobilization of cephalosporin C amidase (a model enzyme of industrial relevance). The luminophores did not interfere with the enzyme immobilization characteristics. Analytical intraparticle pH determination was optimized for sensitivity, reproducibility and signal stability under conditions of continuous measurement. During hydrolysis of cephalosporin C by the immobilizate in a stirred reactor with bulk pH maintained at 8.0, the intraparticle pH dropped initially by about 1 pH unit and gradually returned to the bulk pH, reflecting the depletion of substrate from solution. These results support measurement of intraparticle pH as a potential analytical processing tool for proton-forming/consuming biotransformations catalyzed by carrier-bound immobilized enzymes.

Conclusions

Fluorescein and Ru(dpp) constitute a useful pair of luminophores in by DLR-based intraparticle pH monitoring. The pH range accessible by the chosen DLR system overlaps favorably with the pH ranges at which enzymes are optimally active and stable. DLR removes the restriction of working with static immobilized enzyme particles, enabling suspensions of particles to be characterized also. The pH gradient developed between particle and bulk liquid during reaction steady state is an important carrier selection parameter for enzyme immobilization and optimization of biocatalytic conversion processes. Determination of this parameter was rendered possible by the presented DLR method.

Chromatographic studies of some cephalosporins on thin layers of silica gel G-zinc ferrocyanide

A simple, selective and precise thin-layer chromatographic method has been developed for the analysis of eight cephalosporin antibiotics, namely cephadroxil, cephalexin, cefixime, cefaclor, cefpodoxime proxetil, cefuroxime axetil, cefotaxime sodium and ceftriaxone sodium. The hR(F) values of these cephalosporins were investigated on silica gel G-zinc ferrocyanide layers. Mixing of zinc ferrocyanide with silica gel G resulted in a decrease in hR(F) values, removal of tailing and better resolutions. The influence of silica gel G-zinc ferrocyanide ratio and mobile phases on the chromatographic behavior of cephalosporins on thin layers was investigated. Cephalosporins were selectively separated in their binary and ternary synthetic mixtures and pharmaceutical formulations. Quantitative separations of cephalosporins from their synthetic mixtures were also achieved with good recoveries (97.8-100.3%).

Improvement of the glutaryl-7-aminocephalosporanic acid acylase activity of a bacterial gamma-glutamyltranspeptidase

7-Aminocephalosporanic acid (7-ACA) is an important material in the production of semisynthetic cephalosporins, which are the best-selling antibiotics worldwide. 7-ACA is produced from cephalosporin C via glutaryl-7-ACA (GL-7-ACA) by a bioconversion process using d-amino acid oxidase and cephalosporin acylase (or GL-7-ACA acylase). Previous studies demonstrated that a single amino acid substitution, D433N, provided GL-7-ACA acylase activity for gamma-glutamyltranspeptidase (GGT) of Escherichia coli K-12. In this study, based on its three-dimensional structure, residues involved in substrate recognition of E. coli GGT were rationally mutagenized, and effective mutations were then combined. A novel screening method, activity staining followed by a GL-7-ACA acylase assay with whole cells, was developed, and it enabled us to obtain mutant enzymes with enhanced GL-7-ACA acylase activity. The best mutant enzyme for catalytic efficiency, with a k(cat)/K(m) value for GL-7-ACA almost 50-fold higher than that of the D433N enzyme, has three amino acid substitutions: D433N, Y444A, and G484A. We also suggest that GGT from Bacillus subtilis 168 can be another source of GL-7-ACA acylase for industrial applications.

A colorimetric assay for the determination of acetyl xylan esterase or cephalosporin C acetyl esterase activities using 7-amino cephalosporanic acid, cephalosporin C, or acetylated xylan as substrate

A bromothymol blue-based colorimetric assay has been devised to screen for acetyl xylan esterase or cephalosporin C (CPC) deacetylase activities using 7-amino cephalosporanic acid (7-ACA), CPC, or acetylated xylan as substrate. These enzymes are not screened with their natural substrates because of the tedious procedures available previously. Acetyl xylan esterase from Bacillus pumilus CECT 5072 was cloned, expressed in Escherichia coli Rosetta (DE3), and characterized using this assay. Similar K(M) values for 7-ACA and CPC were obtained when compared with those described using HPLC methods. The assay is easy to perform and can be carried out in robotic high-throughput colorimetric devices normally used in directed evolution experiments. The assay allowed us to detect improvements in activity at a minimum of twofold with a very low coefficient of variance in 96-well plates. This method is significantly faster and more convenient to use than are known HPLC and pH-stat procedures.

Analysis of cephalosporin antibiotics

A comprehensive review with 276 references for the analysis of members of an important class of drugs, cephalosporin antibiotics, is presented. The review covers most of the methods described for the analysis of these drugs in pure forms, in different pharmaceutical dosage forms and in biological fluids.