Endpoint fluorometric assays for determining activities of carnitine palmitoyltransferase and citrate synthase

Enlightening the Path to Protein Engineering: Chemoselective Turn-On Probes for High-Throughput Screening of Enzymatic Activity

Many successful stories in enzyme engineering are based on the creation of randomized diversity in large mutant libraries, containing millions to billions of enzyme variants. Methods that enabled their evaluation with high throughput are dominated by spectroscopic techniques due to their high speed and sensitivity. A large proportion of studies relies on fluorogenic substrates that mimic the chemical properties of the target or coupled enzymatic assays with an optical read-out that assesses the desired catalytic efficiency indirectly. The most reliable hits, however, are achieved by screening for conversions of the starting material to the desired product. For this purpose, functional group assays offer a general approach to achieve a fast, optical read-out. They use the chemoselectivity, differences in electronic and steric properties of various functional groups, to reduce the number of false-positive results and the analytical noise stemming from enzymatic background activities. This review summarizes the developments and use of functional group probes for chemoselective derivatizations, with a clear focus on screening for enzymatic activity in protein engineering.

Graphene-based biomimetic materials targeting urine metabolite as potential cancer biomarker: application over different conductive materials for potentiometric transduction

This works presents a novel surface Smart Polymer Antibody Material (SPAM) for Carnitine (CRT, a potential biomarker of ovarian cancer), tested for the first time as ionophore in potentiometric electrodes of unconventional configuration. The SPAM material consisted of a 3D polymeric network created by surface imprinting on graphene layers. The polymer was obtained by radical polymerization of (vinylbenzyl)trimethylammonium chloride and 4-styrenesulfonic acid (signaling the binding sites), and vinyl pivalate and ethylene glycol dimethacrylate (surroundings). Non-imprinted material (NIM) was prepared as control, by excluding the template from the procedure. These materials were then used to produce several plasticized PVC membranes, testing the relevance of including the SPAM as ionophore, and the need for a charged lipophilic additive. The membranes were casted over solid conductive supports of graphite or ITO/FTO. The effect of pH upon the potentiometric response was evaluated for different pHs (2-9) with different buffer compositions. Overall, the best performance was achieved for membranes with SPAM ionophore, having a cationic lipophilic additive and tested in HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid) buffer, pH 5.1. Better slopes were achieved when the membrane was casted on conductive glass (-57.4mV/decade), while the best detection limits were obtained for graphite-based conductive supports (3.6×10-5mol/L). Good selectivity was observed against BSA, ascorbic acid, glucose, creatinine and urea, tested for concentrations up to their normal physiologic levels in urine. The application of the devices to the analysis of spiked samples showed recoveries ranging from 91% (± 6.8%) to 118% (± 11.2%). Overall, the combination of the SPAM sensory material with a suitable selective membrane composition and electrode design has lead to a promising tool for point-of-care applications.

A rapid fluorescence detecting platform: applicable to sense carnitine and chloramphenicol in food samples

A new long-wavelength latent florescent probe, termed “BCC” for sensitive determination of coenzyme A, carnitine and chloramphenicol.

Determination of l-carnitine using enantioselective, potentiometric membrane electrodes based on macrocyclic antibiotics

In order to determine the enantiopurity of l-carnitine three enantioselective, potentiometric membrane electrodes were proposed for the assay of l-carnitine. The electrodes were designed using macrocyclic glycopeptide antibiotics-vancomycin and teicoplanin. Acetonitrile was added to the teicoplanine to design a modified teicoplanine based electrode. The linear concentration ranges for the proposed enantioselective membrane electrodes were 10(-4) to 10(-2)moll(-1) for electrodes based on vancomycin and teicoplanin and 10(-5) to 10(-2)moll(-1) for electrode based on teicoplanin modified with acetonitrile. The slopes of the electrodes were 56.5mV per pl-carnitine; 54.5mV per pl-carnitine and 58.3mV per pl-carnitine for vancomycin-, teicoplanin- and teicoplanin modified with acetonitrile-based electrodes, respectively. The enantioselectivity was determined over d-carnitine. The proposed electrodes could be employed reliably for the assay of l-carnitine raw material and its pharmaceutical formulation, Carnilean((R)) capsules. The surfaces of the electrodes are stable and easily renewable by polishing on alumina paper.

Membrane integrity and amyloid cytotoxicity: a model study involving mitochondria and lysozyme fibrillation products

Recent findings implicate that fibrillation products, the protein aggregates formed during the various steps leading to formation of mature fibrils, induce neurotoxicity predominantly in their intermediate oligomeric state. This has been shown to occur by increasing membrane permeability, eventually leading to cell death. Despite accumulating reports describing mechanisms of membrane permeabilization by oligomers in model membranes, studies directly targeted at characterizing the events occurring in biological membranes are rare. In the present report, we describe interaction of the original native structure, prefibrils and fibrils of hen egg white lysozyme (HEWL) with mitochondrial membranes, as an in vitro biological model, with the aim of gaining insight into possible mechanism of cytotoxicity at the membrane level. These structures were first characterized using a range of techniques, including fluorescence, size-exclusion chromatography, dynamic light scattering, transmission electron microscopy, dot blot analysis and circular dichroism. HEWL oligomers were found to be flexible/hydrophobic structures with the capacity to interact with mitochondrial membranes. Possible permeabilization of mitochondria was explored utilizing sensitive fluorometric and luminometric assays. Results presented demonstrate release of mitochondrial enzymes upon exposure to HEWL oligomers, but not native enzyme monomer or mature fibrils, in a concentration-dependent manner. Release of cytochrome c was also observed, as reported earlier, and membrane stabilization promoted by addition of calcium prevented release. Moreover, the oligomer-membrane interaction was influenced by high concentrations of NaCl and spermine. The observed release of proteins from mitochondria is suggested to occur by a nonspecific perturbation mechanism.

Achiral liquid chromatography with circular dichroism detection for the determination of carnitine enantiomers in dietary supplements and pharmaceutical formulations

A simple and enantioselective method for the separation and determination of carnitine enantiomers in dietary supplements and pharmaceutical formulation samples is proposed. This method is based on achiral liquid chromatographic separation of carnitine enantiomers from interferences and direct circular dichroism (CD) detection. The calibration curve of the anisotropy factor (g) versus the enantiomeric excess was linear, with a correlation coefficient (R(2)) of 0.996. The precision evaluated by UV peak area and CD peak area was suitable (RSD <5% in all cases). The usefulness of the proposed method was demonstrated by analysing natural dietary supplements and pharmaceutical formulation samples. This method has the advantages of being rapid and precise, without using an expensive chiral column. The method was suitable for the simultaneous determination of both enantiomers and for assessing the chemical purity of carnitine.

C75 is converted to C75-CoA in the hypothalamus, where it inhibits carnitine palmitoyltransferase 1 and decreases food intake and body weight

Central nervous system administration of C75 produces hypophagia and weight loss in rodents identifying C75 as a potential drug against obesity and type 2 diabetes. However, the mechanism underlying this effect is unknown. Here we show that C75-CoA is generated chemically, in vitro and in vivo from C75 and that it is a potent inhibitor of carnitine palmitoyltranferase 1 (CPT1), the rate-limiting step of fatty-acid oxidation. Three-D docking and kinetic analysis support the inhibitory effect of C75-CoA on CPT1. Central nervous system administration of C75 in rats led to C75-CoA production, inhibition of CPT1 and lower body weight and food intake. Our results suggest that inhibition of CPT1, and thus increased availability of fatty acids in the hypothalamus, contribute to the pharmacological mechanism of C75 to decrease food intake.

Crystal structures of murine carnitine acetyltransferase in ternary complexes with its substrates

Carnitine acyltransferases catalyze the reversible exchange of acyl groups between coenzyme A (CoA) and carnitine. They have important roles in many cellular processes, especially the oxidation of long-chain fatty acids in the mitochondria for energy production, and are attractive targets for drug discovery against diabetes and obesity. To help define in molecular detail the catalytic mechanism of these enzymes, we report here the high resolution crystal structure of wild-type murine carnitine acetyltransferase (CrAT) in a ternary complex with its substrates acetyl-CoA and carnitine, and the structure of the S554A/M564G double mutant in a ternary complex with the substrates CoA and hexanoylcarnitine. Detailed analyses suggest that these structures may be good mimics for the Michaelis complexes for the forward and reverse reactions of the enzyme, representing the first time that such complexes of CrAT have been studied in molecular detail. The structural information provides significant new insights into the catalytic mechanism of CrAT and possibly carnitine acyltransferases in general.

Effects of zidovudine, stavudine and beta-aminoisobutyric acid on lipid homeostasis in mice: possible role in human fat wasting

OBJECTIVE

Although mitochondrial DNA (mtDNA) depletion could play a role in nucleoside reverse transcriptase inhibitor-induced lipoatrophy, poor correlations between fat mtDNA levels and lipoatrophy suggest additional mechanism(s). Stavudine (d4T), zidovudine (AZT) and the thymine catabolite, beta-aminoisobutyric acid (BAIBA), but not zalcitabine (ddC) or didanosine (ddI), can increase fatty acid oxidation in liver mitochondria and plasma ketone bodies in mice. Since fat oxidation in non-adipose tissue can influence body adiposity, we sought to determine whether d4T, AZT and BAIBA can cause lipoatrophy in mice by this catabolic mechanism.

METHODS

Lean or obese ob/ob mice were treated for 6 weeks with d4T, AZT or BAIBA, and lean mice with ddC or ddI. Body fat mass was assessed by dual energy X-ray absorptiometry, and mtDNA by Slot blot hybridization in epididymal fat.

RESULTS

Whereas ddC or ddI did not change plasma beta-hydroxybutyrate and body fat mass, d4T, AZT and BAIBA increased plasma beta-hydroxybutyrate in lean mice suggesting increased hepatic fatty acid oxidation and ketogenesis. Despite unchanged food consumption, a supra-pharmacological dose of d4T tended to decrease, whilst AZT and BAIBA decreased body fat mass. Fat mtDNA and plasma triglycerides, cholesterol, glucose, insulin, leptin and adiponectin levels were unchanged. In obese mice, d4T, AZT and BAIBA did not increase plasma beta-hydroxybutyrate, and only AZT decreased body fat mass without reducing fat mtDNA.

CONCLUSIONS

d4T and AZT can enhance hepatic fat oxidation and cause fat wasting, without decreasing adipose tissue mtDNA and without causing insulin resistance in mice. BAIBA, a thymine catabolite, reproduces these effects. These catabolic effects could play a role in the lipoatrophy, which can occur in AZT- or d4T-treated patients.

Redesign of carnitine acetyltransferase specificity by protein engineering

In eukaryotes, L-carnitine is involved in energy metabolism by facilitating beta-oxidation of fatty acids. Carnitine acetyltransferases (CrAT) catalyze the reversible conversion of acetyl-CoA and carnitine to acetylcarnitine and free CoA. To redesign the specificity of rat CrAT toward its substrates, we mutated Met564. The M564G mutated CrAT showed higher activity toward longer chain acyl-CoAs: activity toward myristoyl-CoA was 1250-fold higher than that of the wild-type CrAT, and lower activity toward its natural substrate, acetyl-CoA. Kinetic constants of the mutant CrAT showed modification in favor of longer acyl-CoAs as substrates. In the reverse case, mutation of the orthologous glycine (Gly553) to methionine in carnitine octanoyltransferase (COT) decreased activity toward its natural substrates, medium- and long-chain acyl-CoAs, and increased activity toward short-chain acyl-CoAs. Another CrAT mutant, M564A, was prepared and tested in the same way, with similar results. We conclude that Met564 blocks the entry of medium- and long-chain acyl-CoAs to the catalytic site of CrAT. Three-dimensional models of wild-type and mutated CrAT and COT support this hypothesis. We show for the first time that a single amino acid is able to determine the substrate specificity of CrAT and COT.

Simultaneous determination of L- and D-carnitine using a sequential injection analysis/amperometric biosensors system

A sequential injection analysis (SIA) system is described for the simultaneous determination of L-and D-carnitine using amperometric biosensors as detectors. The SIA system was used, because of its high precision, accuracy and low sample and buffer consumption. The biosensors were designed using physical and chemical immobilization of L-amino acid oxidase and horseradish peroxidase (HRP) for the assay of L- carnitine, and D-amino acid oxidase and HRP for the assay of D-carnitine. The linear concentration ranges are in the pmol/l to nmol/l magnitude order, with very low limits of detection. The biosensors/SIA system was used reliably for on-line process control of the enantiopurity of carnitine with a frequency of 34 samples per hour.