A nonchromatographic assay for expression of the chloramphenicol acetyltransferase gene in eucaryotic cells

A rapid procedure is described for assaying chloramphenicol acetyltransferase (CAT) enzyme activity following transfection of the CAT gene into eucaryotic cells. CAT enzyme activity in cell extracts catalyzes the transfer of [14C]acetyl groups from labeled acetyl coenzyme A to unlabeled chloramphenicol. Labeled reaction product is quantitated by liquid scintillation counting after extraction into ethyl acetate. The method is valid for use with transfected cell extracts only if the extracts are first heated to 65 degrees C to remove a factor which degrades acetyl coenzyme A. The revised procedure offers considerable advantages in speed and ease of performance over the chromatographic assay in current use.     

Inhibition of CAT enzyme activity in Arabidopsis thaliana

Previously it was shown that transient chloramphenicol acetyltransferase (CAT) marker gene expression in Arabidopsis thaliana and Nicotiana tabacum resulted in significant differences in the accumulation of the CAT reaction products in radioactive CAT assays. Compared to Nicotiana tabacum, conversion of chloramphenicol to the acetylated products in Arabidopsis thaliana extracts was rather low. Here we report that the low CAT enzyme activity can be attributed in part to a heat sensitive CAT inhibitory effect in extracts of Arabidopsis thaliana. CAT enzyme activity in transgenic tobacco is inhibited by extracts from Arabidopsis. This inhibitory effect diminishes when Arabidopsis extracts were heat incubated. CAT activity in transgenic Arabidopsis lines was very low and was only detected in heat incubated extracts. Alternatively, enzyme-linked immunosorbent assays (ELISAs) can be used to detect the CAT protein in transgenic Arabidopsis.Abbreviations CAT chloramphenicol acetyltransferase - CAM chloramphenicol - ELISA enzyme linked immunosorbent assay     

A method to detect transfected chloramphenicol acetyltransferase gene expression in intact animals

A rapid procedure is described for assaying chloramphenicol acetyltransferase (CAT, EC 2.3.1.28) enzyme activity in intact animals following transfection of the RSV CAT plasmid into mouse bone marrow cells by electroporation. The reconstituted mice were injected with [14C]chloramphenicol and ethyl acetate extracts of 24-h urine samples were analyzed by TLC autoradiography for the excretion of 14C-labeled metabolites. CAT expression in vivo can be detected by the presence of acetylated 14C-labeled metabolites in the urine within 1 week after bone marrow transplantation and, under the conditions described, these metabolites can be detected for at least 3 months. CAT expression in intact mice as monitored by the urine assay correlates with the CAT expression in the hematopoietic tissues assayed in vitro. This method offers a quick mode of screening for introduced CAT gene expression in vivo without sacrificing the mice.     

Metabolism of N8-monoacetylspermidine in rat hepatoma cells. Investigation of its effect on the activity of L-ornithine decarboxylase

Recent evidence has indicated a role for the acetyl derivatives of polyamines, particularly N8-monoacetylspermidine, as activators of L-ornithine decarboxylase in rat hepatoma tissue culture (HTC) cells. This is in contrast with the well-described negative regulatory control of ornithine decarboxylase exerted by their non-acetylated counterparts. Because of the possibility of a rapid extracellular and intracellular catabolism of the acetyl derivatives of polyamines, the metabolism of N8-monoacetylspermidine and its effect on HTC cell ornithine decarboxylase have been investigated, under conditions which eliminate its extracellular catabolism. Differing from previous reports, we demonstrate that N8-monoacetylspermidine does not elevate ornithine decarboxylase activity when added at low concentrations to the culture medium of HTC cells. Higher concentrations decrease ornithine decarboxylase activity in a dose-dependent manner. This effect cannot be unambiguously attributed to the effect of the acetyl derivative itself, because of the presence in situ of a very active N8-monoacetylspermidine deacetylase, which generates spermidine intracellularly.     

Direct gene transfer into Actinidia deliciosa protoplasts: analysis of transient expression of the CAT gene using TLC autoradiography and a GC-MS-based method

Chloramphenicol acetyl transferase (CAT) gene was used as a reporter gene to assess the conditions for polyethylene glycol (PEG)-mediated transfection of kiwifruit protoplasts. The effect of plasmid concentration and the presence of carrier DNA were each assessed by analysing CAT activity in transfected protoplasts using thin-layer chromatography (TLC) autoradiographic detection of acetylated chloramphenicol. A gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) non-radioactive method was developed for monitoring CAT gene activity. This method provides a high speed of analysis (30 min) and precise means of detecting acetylated products at the nanomolar level, enabling quantification at very low transfection rates. Using this method we optimized plasmid and PEG concentration and also assessed the effect of heat shock on transfection. The best CAT activity was obtained using 30% polyethylene glycol 4000 and by submitting protoplasts to heat shock (45 °C, 5 min) prior to transfection.     

Control of poly-beta-hydroxybutyrate synthase mediated by acetyl phosphate in cyanobacteria.

Poly-beta-hydroxybutyrate (PHB) synthesis in a cyanobacterium, Synechococcus sp. strain MA19, is controlled at the enzyme level and is dependent on the C/N balance in the culture medium. The control involves at least two enzymes. The first enzyme is PHB synthase. Little PHB synthase activity was detected in crude extracts from cells grown under nitrogen-sufficient conditions (MA19(+N)). The activity was detected exclusively in membrane fractions from nitrogen-deprived cells (MA19(-N)) under light but not dark conditions. The shift in the enzyme activity was insensitive to chloramphenicol, which suggests posttranslational activation. Acetyl phosphate activated PHB synthase in membrane fractions from MA19(+N). In vitro, the activation level of PHB synthase changed, depending on the concentration of acetyl phosphate. The second enzyme was phosphotransacetylase (EC 2.3.1.8), which catalyzes the conversion of acetyl coenzyme A (acetyl-CoA) to acetyl phosphate. The activity was detected in crude extracts from MA19(-N) but not in those from MA19(+N). The results suggested that intracellular acetyl phosphate concentration could be controlled, depending on C/N balance and intracellular acetyl-CoA concentration. Acetyl phosphate probably acts as a signal of C/N balance affecting PHB metabolism in MA19.     

Mode of action of Fusarium moniliforme endopolygalacturonase towards acetylated pectin

Endopolygalacturonase from Fusarium moniliforme was used to degrade acetylated homogalacturonan previously prepared from sugar beet pulp. The initial velocity and the final percentage of hydrolysis decreased very rapidly with increasing degree of acetylation, showing that acetyl substitution markedly affected the enzymatic activity. MALDI-TOF mass spectrometry was used to analyse the reaction products and to show acetyl groups on the oligogalacturonates. The results demonstrated that the enzyme was able to accommodate acetyl groups in its active site cleft. The influence of acetyl groups on the mode of action of the enzyme was discussed and compared to the influence of methyl groups.     

A rapid and specific assay for Nα-acetyltransferase activity

A rapid and specific assay for N^α-acetyltransferase activity is described. This assay makes use of acidic precipitation for removal of products resulting from acetylation of endogenous substrates from the enzyme preparation and of reverse-phase HPLC for isolation of the N-terminally acetylated synthetic substrate. The virtue of the assay lies in its fast, not too laborious, and reasonably accurate determination of activity during isolation and purification of the enzyme.     

Radiolabeling of mammalian cells in tissue culture by use of acetic anhydride. Potential value for studying the dynamics of protein turnover in living cells.

When Chinese hamster ovary cells, growing in monolayer culture, were treated with trace quantities of radioactive acetic anhydride, label was very rapidly introduced into both the plasma membrane and intracellular components, all of which reached similar specific radioactivities. The cells themselves showed no measurable loss in viability. The incorporated label was recovered predominantly in the form of acetyl groups on proteins. Since tyrosyl hydroxyls were not extensively acetylated, it seems likely that substitution occurred mainly on amino groups. There was no indication that the acetyl groups once introduced were labile or that the modified proteins were discriminated against by the cells, since, in dividing cells, the acetylated proteins were as long-lived as those labeled with radioactive l-leucine. Nor was there any evidence for reutilization of labeled acetate following protein turnover in cells maintained in a nonproliferating state on a medium lacking the essential amino acid l-isoleucine. We suggest that acetylation could provide an ideal means for introducing a very short “pulse” of radioactivity into proteins of living cells. In addition, the method should be extremely useful for studying processes such as enzyme induction and protein turnover where the problem of amino acid reutilization has long been recognized as a possible basis for artifacts.     

A simple quantitative assay for chloramphenicol acetyltransferase by direct extraction of the labeled product into scintillation cocktail.

A simple, rapid, sensitive, quantitative, and inexpensive assay for chloramphenicol acetyltransferase (CAT) is described. The assay is based on the direct extraction of the products of the reaction into toluene-based liquid scintillation cocktail. The assay is carried out in 7-ml scintillation vials using 1 mM chloramphenicol and either 100 microM acetyl-CoA and 0.1 microCi of [3H]acetyl-CoA or 1 mM acetyl-CoA and 0.5 microCi of [3H]acetyl-CoA. After incubation, the reaction is terminated with 0.5 ml of 0.1 M sodium borate-5 M NaC, pH 9. The acetylchloramphenicols are extracted with 5 ml of 0.4% 2,5-diphenyloxazole-0.005% 1,4-bis(5-phenyloxazol-2-yl)benzene in toluene by a 30-s shaking. After a short centrifugation to clarify the layers, the vials are counted in a liquid scintillation counter. Extracted products are stable in the organic layer. Under these conditions, nearly 100% extraction of acetylchloramphenicols is shown using nonlabeled compounds and spectrophotometric methods. Using pure enzyme in the assay, linearity of activity with enzyme concentration, time, and temperature of incubation is demonstrated. Assays may even be carried out at 60 degrees C, where the enzyme activity is 3.4-fold higher than that at 23 degrees C. The increase in enzyme activity with increasing temperature is due to the increased formation of predominantly 3-acetyl and 1-acetylchloramphenicols and not to 1,3-diacetylchloramphenicol. The present assay compared very well with the standard assay using [14C]chloramphenicol and TLC. Using this assay, we measured quantitatively the CAT activity in extracts of pSV2-CAT-transfected CV-1 cells in 10 min and NIH 3T3 cell extracts in 60 min at 60 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)     

Vargula hilgendorfii luciferase: a secreted reporter enzyme for monitoring gene expression in mammalian cells

The small marine ostracod crustacean, Vargula hilgendorfii, produces a bright blue luminous secretion which is ejected into seawater. The luminescence is due to a simple enzyme-catalyzed reaction involving only luciferase, luciferin (substrate), and molecular oxygen. Thus, V. hilgendorfii luciferase (VL) should be useful as a reporter enzyme in studies of gene expression in mammalian cells. Expression plasmids consisting of VL cDNA (vl) linked to the promoters simian virus 40 early region, Rous sarcoma virus long terminal repeat, human elongation factor, or mouse granulocyte colony-stimulating factor were introduced into a series of mammalian cell lines. Following transfection, VL activities in cell extracts and culture media were determined by a rapid light emission assay with V. hilgendorfii luciferin. Parallel experiments were carried out with the chloramphenicol acetyltransferase (CAT)-encoding gene. In all cell lines tested, VL was secreted, allowing the reporter activity to be determined directly from a small aliquot of the culture medium. The results indicate that the secreted VL enzyme is superior to CAT, firefly luciferase, and bacterial luciferase as a convenient and versatile indicator of gene expression in mammalian cells.     

3-(Bromoacetyl)chloramphenicol, an active site directed inhibitor for chloramphenicol acetyltransferase

Bacterial resistance to the antibiotic chloramphenicol is normally mediated by chloramphenicol acetyltransferase (CAT), which utilizes acetyl coenzyme A as the acyl donor in the inactivation reaction. 3-(Bromoacetyl)chloramphenicol, an analogue of the acetylated product of the forward reaction catalyzed by CAT, was synthesized as a probe for accessible and reactive nucleophilic groups within the active site. Extremely potent covalent inhibition was observed. Affinity labeling was demonstrated by the protection afforded by chloramphenicol at concentrations approaching Km for the substrate. Inactivation was stoichiometric, 1 mol of the inhibitor covalently bound per mole of enzyme monomer, with complete loss of both the acetylation and hydrolytic activities associated with CAT. N3-(Carboxymethyl)histidine was identified as the only alkylated amino acid, implicating the presence of a unique tautomeric form of a reactive imidazole group at the catalytic center. The proteolytic digestion of CAT modified with 3-(bromo[14C]-acetyl)chloramphenicol yielded three labeled peptide fractions separable by reverse-phase high-pressure liquid chromatography. Each peptide fraction was sequenced by fast atom bombardment mass spectrometry; the labeled peptide in each case was found to span the highly conserved region in the primary structure of CAT, which had been tentatively assigned as the active site. The rapid, stoichiometric, and specific alkylation of His-189, taken together with the high degree of conservation of the adjacent amino acid residues, strongly suggests a central role for His-189 in the catalytic mechanism of CAT.     

Herpes simplex virus thymidine kinase enzymatic assay in transient transfection experiments using thymidine kinase-deficient cells.

An enzymatic assay for herpes virus simplex type 1 thymidine kinase (HSV-TK) that was sensitive enough to quantitate intracellular levels of enzyme transiently expressed after transfection of HSV-TK vectors into TK-deficient cells using the DNA-calcium phosphate coprecipitation technique is described. TK activity in extracts of transfected cells was determined by binding of [methyl-3H]thymidylate product to thin layers of polyethyleneimine (PEI)-impregnated cellulose. The assay used high-specific-activity [methyl-3H]thymidine as substrate, which required removal of anionic material on a column of PEI-cellulose to enhance the signal-to-noise ratio. The assay was linear over a wide range with respect to the amount of HSV-TK plasmid transfected or content of HSV-TK enzyme in cell extracts. To validate the assay in transient expression experiments, HSV-TK and chloramphenicol acetyltransferase (CAT) plasmids were cotransfected into NIH/3T3 tk- fibroblasts. Transient TK and CAT levels were concordant in cell extracts prepared from replicate plates of transfected cells. Normalizing the transient TK activity for CAT activity from the cotransfected "internal standard" CAT plasmid improved precision significantly, reducing the sample-to-sample coefficient of variation from 41 to 19%. CAT normalization reduced experimental variability mostly by correcting outlying results in transfection efficiency. The HSV-TK reporter gene system based on TK enzymatic assay was thus subject to experimental variation similar to that of the well-established CAT reporter function, demonstrating its utility in transient gene expression analysis.     

PEG-mediated expression of GUS and CAT genes in protoplasts from embryogenic suspension cultures of Picea glauca

ß-Glucuronidase (GUS) and chloramphenicol acetyl transferase (CAT) were used as reporter proteins in protoplasts from embryogenic suspension cultures of Picea glauca (Moench) Voss (white spruce). Plasmid DNA enclosing chimeric GUS and CAT constructs, using the cauliflower mosaic virus 35S promoter, was introduced into Picea glauca protoplasts using polyethylene glycol (PEG). Transient expression was detected 12 to 40 h after PEG-mediated DNA delivery. Dose-response curves using covalently closed circular plasmid DNA, in the absence of carrier DNA, have been obtained for each of these reporter genes. Linearized plasmid DNA gave lower levels of expression than covalently closed circular plasmid DNA when assayed 40 h after PEG-mediated DNA transfer. The use of carrier DNA (herring sperm DNA), in combination with covalently closed circular plasmid DNA, increased the level of expression of GUS by about 50%. CAT expression was enhanced if PEG-mediated delivery was performed on ice rather than at room temperature. The highest level of expression for CAT, and the lowest signal-to-noise ratio, was found 24 h after PEG-mediated DNA transfer. Both GUS and CAT provided results that were quantifiable and can therefore be used as reporter genes in Picea glauca.Abbreviations CAT chloramphenicol acetyl transferase - GUS ß-glucuronidase - CaMV cauliflower mosaic virus - NOS nopaline synthase - CCC covalently closed circular DNA - L linear DNA - PEG polyethylene glycol - HS herring sperm DNA - P protoplasts - PCM protoplast culture medium - MES morpholinoethane-sulfonic acid - Cm chloramphenicol - Ac acetylated - MUG 4-methyl umbelliferyl ß-D-glucuronide - TLC thin layer chromatography     

A highly sensitive, mixed-phase assay for chloramphenicol acetyltransferase activity in transfected cells

We describe a simple, rapid yet extremely sensitive assay for chloramphenicol acetyltransferase (CAT) activity in extracts from transfected eukaryotic cells. Using our modified reaction conditions and the mixed-phase assay, less than 0.000010 unit of CAT activity in transfected cells can be reliably detected. The mixed-phase assay is based on the inability of the polar [3H]-acetyl-Coenzyme A (CoA) substrate to partition out of a urea containing aqueous phase into the nonpolar scintillation fluor, while the [3H]chloramphenicol reaction products partition into the toluene scintillation fluor and are quantitated by scintillation counting. The increased sensitivity of this assay is due to the optimization of the acetyl-CoA concentration, to a urea-containing aqueous phase which lowers the assay background, and to the use of extract blanks. The mixed-phase assay is simpler, is quantitative, uses less costly substrates, and is far more sensitive than the most widely used CAT assays, which require solvent extraction followed by thin-layer chromatography to separate the unreacted substrate from product.     

A simple phase-extraction assay for chloramphenicol acyltransferase activity

A simple and convenient phase extraction assay for chloramphenicol (Cm) acetyltransferase (CAT) activity has been developed, based on the enzymatic butyrylation of radiolabelled Cm. The assay is linear over two to three orders of magnitude of enzyme concentration, is highly sensitive, and substantially less expensive than all presently available alternatives. Methods for convenient CAT assay, adapted for mammalian cells, plant protoplasts, and mammalian cell culture supernatants, are described. These methods should also simplify measurement of CAT activity in other organisms, such as yeast and bacteria. In addition, a simple pre-extraction procedure is presented for purifying radiolabelled Cm which allows a 25-fold increase in sensitivity using tritiated substrates.