The present invention pertains to methods for measuring the activity of glutamine:fructose 6-phosphate amidotransferase and to methods for measuring the inhibitory activity of a test compound on glutamine:fructose 6-phosphate amidotransferase. The present invention also pertains to methods for measuring the formation of uridine-diphosphate-N-acetylglucosamine in extracts from human and animal tissue or cells in tissue cultures, to methods for measuring the inhibitory activity of a test compound on uridine-diphosphate-N-acetylglucosamine formation in cells, and to methods for measuring the inhibitory activity of a test compound on uridine-diphosphate-N-acetylglucosamine formation in a human or an animal.
Methods for analyzing glucosamine 6-phosphate and uridine-diphosphate-N-acetylglucosamine (UDP-N-acetylglucosamine, UDP-GlcNAc) are known. Glucosamine 6-phosphate is the first product of the enzyme glutamine:fructose 6-phosphate amidotransferase (GFAT), which is the rate-limiting enzyme in the hexosamine biosynthetic pathway. UDP-N-acetylglucosamine is the final product in the hexosamine biosynthetic pathway, which is then used, in the N-linked and O-linked glycosylation of proteins. In addition, it is acted upon by O-N-acetylglucosamine transferase to transfer a single N-acetylglucosamine residue onto serine or threonine amino acids. These pathways are important in the regulation of protein glycosylation and have been associated with insulin sensitivity and glucose regulation. The measurement of glucosamine 6-phosphate has been labor intensive using either of two methods for detection: 1) extraction and purification of the sample followed by HPLC/MS analysis (K. A. Robinson, M. L. Weinstein, G. E. Lindenmayer, and M. G. Buse (1995) Diabetes 44 1438-1446); or 2) a spectrophotometric method that requires either boiling the samples in water, or extracting the samples with perchloric acid followed by neutralization, and centrifugation steps. These methods are not amenable to high throughput assays.
This method of glucosamine 6-phosphate determination is used to identify GFAT inhibitors as well as their subsequent optimization. The measurement of UDP-GlcNAc is used to determine the effect of GFAT inhibitors in vivo, as well as the effects of various modulators of the enzymes in the hexosamine pathway.
The methods for analyzing glucosamine 6-phosphate and UDP-N-acetylglucosamine are based on the color reaction of N-acetylglucosamine with p-dimethylaminobenzaldehyde (Ehrlich's reagent) in dilute alkali solution (W. J. Morgan and L. A. Elson (1934) Biochem J. 28, 988-995). Modifications of this original procedure using a borate buffer for a more stable pH increased the yield by 2 fold and reduced the time of color development but also required high heat and ml quantities of reagents (J. L. Reissig, J. L. Strominger, and L. F. LeLoir (1955) JBC 217, 959-966). Acetylation of glucose 6-phosphate using a dilute acetic anhydride acetone solution followed by a borate alkali solution enabled quantitative conversion of glucosamine into N-acetylglucosamine but also required ml volumes and heating in boiling water (G. A. Levvy and A. McAllan (1959) Biochem J 73, 127-132). This method is capable of measuring glucosamine 6-phosphate in various tissues but requires the heating step in capped tubes (T. C. Richards and O. Greengard (1973) Biochemica et Biophysica Acta 304, 842-850). Others have measured GFAT activity in smaller quantities but the method requires perchloric acid to stop the reaction, neutralization with potassium hydroxide, centrifugation, and transfer of the supernatant for analysis (G. L. McKnight, S. L. Mudri, S. L. Mathewest, R. R. Traxinger, S. Marshall, P. O. Sheppard, and P. J. O'Hara (1992), JBC 267, 25204-25212). A spectrophotometric method for quantitation of UDP-N-acetylglucosamine has not been described. The current methods require purification of the sample and analysis by HPLC/MS (K. A. Robinson, M. L. Weinstein, G. E. Lindemnayer, and M. G. Buse (1995) Diabetes 44 1438-1446).
WO 93/21330 (Zymogenetics) discloses DNA molecules encoding human glutamine:fructose-6-phosphate amidotransferase. The DNA molecules are used in methods to screen for glutamine:fructose-6-phosphate amidotransferase antagonists. The DNA molecules encoding human glutamine:fructose-6-phosphate amidotransferase are expressed in suitable host cells and recombinant glutamine:fructose-6-phosphate amidotransferase is produced. A test substance is exposed to the recombinant human glutamine:fructose-6-phosphate amidotransferase in the presence of fructose-6-phosphate and glutamine. A reduction in activity of the glutamine:fructose-6-phosphate amidotransferase in comparison to the activity in the absence of the test substance indicates a compound that inhibits human glutamine:fructose-6-phosphate amidotransferase.
U.S. Pat. No. 5,876,713 (Nishi et al.) discloses glutamine:fructose-6-phosphate amidotransferase; a DNA coding for the protein; a recombinant vector; a transformant; a method for producing the protein; a pharmaceutical composition comprising the protein; and an antibody against the protein or its partial peptide. The protein and the DNA are used as a prophylactic or therapeutic agent for hypoglycemia. The antibody is used in the assay of the protein and the protein is used as a reagent for the screening for candidate medical compounds.
The present invention relates to a method for measuring the activity of glutamine:fructose 6-phosphate amidotransferase, which comprises the steps of (a) forming a mixture of glutamine:fructose 6-phosphate amidotransferase, fructose 6-phosphate, and glutamine; (b) incubating the mixture in (a) under physiological conditions for a time sufficient to allow the formation of glucosamine 6-phosphate; (c) acetylating the glucosamine 6-phosphate in (b) to form N-acetylglucosamine 6-phosphate; (d) reacting the N-acetylglucosamine 6-phosphate in (c) with Ehrlich's reagent; and (e) measuring the amount of N-acetylglucosamine 6-phosphate present in (d) by determining the optical density at 500-610 nM of the mixture in (d) and comparing the amount of N-acetylglucosamine 6-phosphate in (d) with a control sample of N-acetylglucosamine 6-phosphate and a non-incubated control sample of glutamine:fructose 6-phosphate amidotransferase, or an incubated control sample of glutamine:fructose 6-phosphate amidotransferase without glutamine or without fructose 6-phosphate.
Preferably, the glutamine:fructose 6-phosphate amidotransferase is glutamine:fructose 6-phosphate amidotransferase-human alpha form having a Km of 1.49 mM for glutamine and a Km of 1.85 mM for fructose 6-phosphate or glutamine:fructose 6-phosphate amidotransferase-human beta form having a Km of 1.99 mM for glutamine and a Km of 6.8 mM for fructose 6-phosphate. The glutamine:fructose 6-phosphate amidotransferase may or may not be transfected into cells. Preferably, the mixture in (a) comprises glutamine:fructose 6-phosphate amidotransferase (0.25-25 μl), glutamine (2-40 mM), fructose 6-phosphate (2-40 mM), phosphate buffered saline pH 7.4, ethylenediaminetetraacetic acid (5 mM), and dithiothreitol (1 mM), the incubation step in (b) is carried out at 37° C. for a time period of 5 minutes to 5 hours, and the acetylation step in (c) is carried out with acetic anhydride 0.00625% to 0.625% in acetone followed by addition of potassium tetraborate 62.5 mM at 80° C. for 25 minutes. The Ehrlich's reagent in (d) preferably comprises 2 g p-dimethylaminobenzaldehyde, 0.3 ml water, 2.2 ml concentrated hydrochloric acid, and 17.4 ml acetic acid, which is diluted 1:2 in acetic acid. In a preferred embodiment, the mixture in (a) is formed in a microtiter plate and further comprises the step of adding a control sample of N-acetylglucosamine 6-phosphate and a non-incubated control sample of glutamine:fructose 6-phosphate amidotransferase, or an incubated control sample of glutamine:fructose 6-phosphate amidotransferase without glutamine or without fructose 6-phosphate to the microtiter plate, after the incubation step in (b).
The present invention also relates to a method for measuring the inhibitory activity of a test compound on glutamine:fructose 6-phosphate amidotransferase, which comprises the steps of (a) forming a mixture of a test compound, glutamine:fructose 6-phosphate amidotransferase, fructose 6-phosphate, and glutamine; (b) incubating the mixture in (a) under physiological conditions for a time sufficient to allow the test compound to inhibit glutamine:fructose 6-phosphate amidotransferase from forming glucosamine 6-phosphate; (c) acetylating the glucosamine 6-phosphate in (b) to form N-acetylglucosamine 6-phosphate; (d) reacting the N-acetylglucosamine 6-phosphate in (c) with Ehrlich's reagent; and (e) measuring the amount of N-acetylglucosamine 6-phosphate in (d) by determining the optical density at 500-610 nm of the mixture in (d) and comparing the amount of N-acetylglucosamine 6-phosphate in (d) with a control sample of glutamine:fructose 6-phosphate amidotransferase not containing the test compound and a control sample of N-acetylglucosamine 6-phosphate, thereby determining the inhibitory activity of the test compound.
Preferably, the mixture in (a) comprises test compound, glutamine:fructose 6-phosphate amidotransferase (0.25-25 μl), glutamine (10 mM), fructose 6-phosphate (10 mM), phosphate buffered saline pH 7.4, ethylenediaminetetraacetic acid (5 mM), and dithiothreitol (1 mM), the incubation step in (b) is carried out at 37° C. for 1 hour, and the acetylation step in (c) is carried out with acetic anhydride 0.09375% in acetone followed by addition of potassium tetraborate 62.5 mM at 80° C. for 25 minutes.
The present invention further relates to a method for measuring the formation of uridine-diphosphate-N-acetylglucosamine in extracts from human and animal tissue or cells in tissue cultures, which comprises the steps of (a) hydrolyzing an extract from human or animal tissue or cells in tissue culture to convert uridine-diphosphate-N-acetylglucosamine to N-acetylglucosamine; (b) reacting the N-acetylglucosamine in (a) with Ehrlich's reagent; and (c) measuring the amount of N-acetylglucosamine present in (b) by determining the optical density at 500-610 nm of the mixture in (b) and comparing the amount of N-acetylglucosamine in (b) with a control sample of N-acetylglucosamine, thereby determining the formation of uridine-diphosphate-N-acetylglucosamine.
Preferably, the extracts are from human tissue or cells. Preferably, the hydrolyzing step in (a) is carried out with hydrochloric acid 0.1N, followed by addition of potassium tetraborate 62.5 mM at 80° C. for 10 minutes and the Ehrlich's reagent in (b) comprises 2 g p-dimethylaminobenzaldehyde, 0.3 ml water, 2.2 ml concentrated hydrochloric acid, and 17.4 ml acetic acid, which is diluted 1:2 in acetic acid. In a preferred embodiment, the hydrolyzing step in (a) is carried out in a microfuge tube and the measuring step in (c) is carried out in a microtiter plate.
The present invention still further relates to a method for measuring the inhibitory activity of a test compound on uridine-diphosphate-N-acetylglucosamine formation in cells, which comprises the steps of (a) incubating a test compound with cells in tissue culture under physiological conditions for a time sufficient to allow the test compound to inhibit glutamine:fructose 6-phosphate amidotransferase and reduce cellular levels of uridine-diphosphate-N-acetylglucosamine; (b) extracting the uridine-diphosphate-N-acetylglucosamine in (a); (c) hydrolyzing the uridine-diphosphate-N-acetylglucosamine in (b) to form N-acetylglucosamine; (d) reacting the N-acetylglucosamine in (c) with Ehrlich's reagent; and (e) measuring the amount of N-acetylglucosamine in (d) by determining the optical density at 500-610 nm of the mixture in (d) and comparing the amount of N-acetylglucosamine in (d) with the amount in cells in tissue culture not incubated with the test compound and a control sample of N-acetylglucosamine, thereby determining the inhibitory activity of the test compound.
Preferably, the cells in (a) are selected from the group consisting of type 293 human kidney cells, type 293 human kidney cells transfected with GFAT-beta, or COS monkey kidney cells, COS monkey kidney cells transfected with GFAT-beta, mammalian cells, yeast cells, and bacterial cells. Preferably, the hydrolyzing step in (c) is carried out with hydrochloric acid 0.1N, followed by addition of potassium tetraborate 62.5 mM at 80° C. for 10 minutes and the Ehrlich's reagent in (d) comprises 2 g p-dimethylaminobenzaldehyde, 0.3 ml water, 2.2 ml concentrated hydrochloric acid, and 17.4 ml acetic acid, which is diluted 1:2 in acetic acid.
The present invention still further relates to a method for measuring the inhibitory activity of a test compound on uridine-diphosphate-N-acetylglucosamine formation in a human or an animal, which comprises the steps of (a) dosing a human or an animal with a test compound under physiological conditions for a time sufficient to allow the test compound to inhibit glutamine:fructose 6-phosphate amidotransferase from forming uridine-diphosphate-N-acetylglucosamine; (b) removing tissue from the human or animal in (a) and extracting the uridine-diphosphate-N-acetylglucosamine from the tissue; (c) hydrolyzing the uridine-diphosphate-N-acetylglucosamine in (b) to form N-acetylglucosamine; (d) reacting the N-acetylglucosamine in (c) with Ehrlich's reagent; and (e) measuring the amount of N-acetylglucosamine in (d) by determining the optical density at 500-610 nm of the mixture in (d) and comparing the amount of N-acetylglucosamine in (d) with the amount in a human or an animal not dosed with the test compound and a control sample of N-acetylglucosamine, thereby determining the inhibitory activity of the test compound.
Preferably, the dosing in (a) is carried out orally or intravenously 1 to 2 times per day over a period of 1 day to 2 weeks and the animal in (a) is a human, mouse, or rat. Preferably, the hydrolyzing step in (c) is carried out with hydrochloric acid 0.1N, followed by addition of potassium tetraborate 62.5 mM at 80° C. for 10 minutes.
The present invention relates to rapid spectrophotometric methods for high throughput microtiter plate analysis of glucosamine 6-phosphate levels. The methods also enable the simple measurement and quantitation of UDP-GlcNAc from a variety of sources. The spectrophotometric methods described are preferably performed in a microtiter plate and are based on the color reaction of N-acetylglucosamine with p-dimethylaminobenzaldehyde (Ehrlich's reagent) in dilute alkali solution. The spectrophotometric methods do not require the many purification steps needed for HPLC/MS methods and are capable of assaying many samples at a time. The methods include the hydrolysis of UDP-N-acetylglucosamine to N-acetylglucosamine, which is then measured by the spectrophotometric method for N-acetylglucosamine.
This invention is more fully described in copending patent application entitled “Methods for Measuring Levels of Uridine-Diphosphate-N-Acetylglucosamine And Activity of Inhibitors Thereof” filed by applicant concurrently with the present patent application and assigned to the assignee of this application, which is hereby incorporated by reference.
As used herein, the term “physiological conditions” refers to concentration, temperature, pH, ionic strength, viscosity, time, and various biochemical parameters which are compatible with a viable organism, and/or which typically exist intracellularly in a viable cultured animal cell or mammalian cell. Suitable reaction conditions for in vitro enzyme reactions are generally physiological conditions. The incubation of the mixture of glutamine:fructose 6-phosphate amidotransferase, fructose 6-phosphate, and glutamine is generally carried out under physiological conditions for a time sufficient to allow the formation of glucosamine 6-phosphate. For the measurement of glucosamine 6-phosphate, the preferred physiological conditions comprise NaCl 150 mM at pH 7.4 and 37° C. For the measurement of uridine-diphosphate-N-acetylglucosamine, the cells are preferably grown in Delbecco's Modified Eagles Medium (DMEM) at pH 7.4 with 10% fetal. bovine serum. Particular aqueous conditions may be selected by the practitioner according to conventional methods with the optional addition of divalent cation(s) and/or metal chelators and/or non-ionic detergents and/or membrane fractions and/or anti-foam agents.
GFAT inhibitors reduce or inhibit the production of glucosamine 6-phosphate and ultimately the production of UDP-N-acetylglucosamine. The in vitro method using glutamine:fructose 6-phosphate amidotransferase only permits the measurement of glucosamine 6-phosphate. Another consideration in identifying a GFAT inhibitor of potential use in treatment of patients is whether the test compound enters cells in vivo. Under physiological conditions in cells, biological agents, e.g., enzymes, are produced in connection with the hexosamine biosynthetic pathway, which are involved in the production of UDP-GlcNAc. Compounds that are found to inhibit GFAT enzyme activity in cells can be tested in animals to determine if they enter the tissues, inhibit GFAT activity, and inhibit production of UDP-GlcNAc.
For measurement of N-acetylglucosamine 6-phosphate, the GFAT enzyme reaction and analysis of the product are preferably carried out directly in a microtiter plate. The preferred reaction volume of 100 μl consists of the substrates glutamine (10 mM) and fructose 6-phosphate (10 mM), phosphate buffered saline (PBS) pH 7.4, ethylenediaminetetraacetic acid (EDTA) (5 mM), dithiothreitol (DTT) (1 mM), and the GFAT enzyme preparation or tissue extract (0.25-50 μl). The incubation mixture is added to the plate, the plate sealed, and placed floating on a 37° C. water bath for 1 hour. Additional incubation mixture is kept on ice to add to standard and control samples. After incubation, non-incubated control and glucosamine 6-phosphate standards of 2.5 to 30 nmoles along with the cold incubator mixture are added to the appropriate wells in the plate. The glucosamine 6-phosphate produced in the incubation mixtures is acetylated by first adding acetic anhydride 1.5% in acetone (10 μl), followed by potassium tetraborate 200 mM (50 μl). The plate is sealed, shaken for 2 minutes on a microshaker, and placed on an 80° C. water bath for 25 minutes. The plate is cooled by placing it on ice for 5 minutes and then Ehrlich's reagent is added (130 μl) (2 g Ehrlich's reagent+0.3 ml water+2.2 ml concentrated hydrochloric acid+17.4 ml acetic acid which is diluted 1:2 in acetic acid before use). The plate is then placed on a 37° C. water bath for 20 minutes and the OD determined at 500-610 nm, preferably 585 nm.
For measurement of UDP-N-acetylglucosamine in extracts from human and animal tissue or cells in tissue cultures, the extract sample is first hydrolyzed in 0.1N HCl at 80° C. for 10 minutes to produce N-acetylglucosamine. The sample is then cooled on ice for 5 minutes and neutralized with 0.1N KOH. Potassium tetraborate (62.5 mM[final]) is then added and the sample incubated at 80° C. for 25 minutes and then cooled on ice for 5 minutes. Ehrlich's reagent is then added and the sample is maintained for 20 minutes at 37° C. and then the OD is determined at 500-610 nm, preferably 585 nm.
Procedure for Microtiter Plate Assay for Glucosamine 6-Phosphate
The reaction mixture is made including enough for standard curve samples and kept on ice. The reaction is started by adding the incubation reagents to the microtiter plate, sealing the plate, and placing the plate in a water bath at 37° C. for 1 hr. Standard curve samples are added to the plate, 2.5 to 30 nmoles/well/10 μl, and the cold mix added to control and standard curve samples. Glucosamine 6-phosphate is acetylated with acetic anhydride 1.5%/acetone 10 μl+potassium tetraborate 200 mM 50 μl, by sealing the plate, shaking for 2 minutes and placing on an 80° C. water bath for 25 minutes, then cooling on ice for 5 minutes. Diluted Ehrlich's reagent 130 μl is added and the plate placed on a 37° C. water bath for 20 minutes. The OD is determined at 585 nm.
Procedure for Spectrophotometric Assay for Measuring UDP-N-Acetylglucosamine
The reaction mixture is made as described above for the glucosamine 6-phosphate assay except the reaction is preferably started in a microfuge tube. If the sample contains too much protein, acetonitrile is added to first precipitate the protein to clarify the solution. The sample is then hydrolyzed in HCl 0.1N at 80° C. for 10 minutes, cooled on ice, and centrifuged 5 minutes to remove debris if the high protein content. The sample is neutralized with KOH 0.1N. K2B407 (59 mM[final]) is added and the sample incubated at 80° C. for 25 minutes and then cooled on ice for 5 minutes. Ehrlich's reagent is added for 20 minutes at 37° C., the samples centrifuged for 2 minutes, and 200 μl of the supernatant is added to a microtiter plate. The OD is determined at 585 nm.
Throughout this disclosure, applicant will suggest various theories or mechanisms by which applicant believes the present methods function. While applicant may offer various mechanisms to explain the present invention, applicant does not wish to be bound by theory. These theories are suggested to better understand the present invention but are not intended to limit the effective scope of the claims.
Throughout this application, various publications have been referenced. The disclosures in these publications are incorporated herein by reference in order to more fully describe the state of the art.
The present invention is further illustrated by the following examples that are presented for purposes of demonstrating, but not limiting, the preparation of the compounds and compositions of this invention.
Enzyme preparation. GFAT-alpha (GenBank Accession No. M90516) and GFAT-beta (Accession No. AB016789) are cloned into pET12 and pEF-BOS C vectors (New England BioLabs) for transfection into SF9 and Hi5 insect cells, COS, type 293 cells, and E. coli. Endogenous activity for each cell line was also measured in non-transfected cells. The enzymes are expressed in each cell line by growing in large 24×24 cm plates and harvesting 20 hours after transfection. The media is removed and the plates are washed in 15 ml PBS. 1.5 ml of a lysis buffer (PBS 1×, KCl 50 mM, EDTA 10 mM, with protease inhibitors leupeptin 10 ug/ml, PMSF 20 ug/ml, A-protinin 30 ug/ml, and pepstatin 10 ug/ml) is added to each plate and the cells are removed using a large plastic cell scraper. This results in a final volume of about 3 ml. The cell #/lysis volume is about 4×107 cell/ml. The suspension is sonicated on ice (Branson sonicator setting 2.5 to 3) for 15 seconds using a microtip. The sonicated cell extracts are stored in microfuge tubes at −80° C. Before use, the extracts are centrifuged at 15000×g for 5 minutes at 4° C. Each enzyme preparation is titrated to determine the volume that gives between 20 to 30 nmoles of glucosamine 6-phosphate in a reaction mixture in a 60-minute incubation at 37° C. (
Stimulation of GFAT activity. The enzyme reaction was found to be linear over time (
The activity of the GFAT-alpha and GFAT-beta enzymes expressed in SF9 insect cells were characterized with known inhibitors of GFAT (
Measurement of glucosamine 6-phosphate The reaction is started by adding the incubation mixture to the appropriate wells in a microtiter plate and placing the plate on a 37° C. water bath. After the appropriate incubation time, the reaction is stopped by adding 10 μl of 1.5% acetic anhydride (final 0.09375%) followed by 50 μl of potassium tetraborate 200 mM (final 62.5 mM). The plate is sealed and placed on a micro-shaker for 2 minutes. The plate is then floated on an 80° C. water bath for 30 minutes taking care to remove air bubbles under the plate. 0.09375% acetic anhydride (out of a range of 0.00625% to 0.625%) at 80° C. (out of a range of 20° to 80° C.) was found to be the best concentration for acetylation of glucosamine 6-phosphate (
Tissue extract. Tissue to be assayed is kept frozen at −80° C. and kept on dry ice during the preparation and weighing process. The tissue is pulverized to a fine powder by rapping the tissue in heavy foil, placing it on top of a metal block in a bed of dry ice, and rapidly hammering it until it is a fine powder. It is then placed in a tarred microfuge tube and kept on dry ice until weighed. The tissue is extracted in chloroform:water (1 mg+2 μl chloroform+2 μl water). The suspension is sonicated for about 5 seconds on ice (Branson sonicator setting 3 to 5 with micro-tip) to breakup any clumps. The extract can then be stored at −80° C. Before analysis, the tissue extract is then thawed and centrifuged at 15000×g for 20 minutes at 4° C.
Cell extract: Cells in culture are incubated in 35 mm culture dishes with and without a GFAT inhibitor and treated with glucose to produce UDP-N-acetylglucosamine. Cells are normally grown to about 80% confluency in DMEM-glucose 4.5 mM+10% fetal bovine serum. The media is then changed to DMEM (no glucose)+10% fetal bovine serum for 12 to 16 hours. If a GFAT inhibitor is present, it is added for 30 minutes, and followed by addition of glucose 25 mM for time periods of 1 hour to 24 hours. The media is removed, cells washed 1× in PBS, and extracted in 200 μl of chloroform:water (1:1). The suspension is sonicated for about 5 seconds on ice to breakup any clumps. The extract can be stored at −80° C. Before analysis, the tissue extract is then thawed and centrifuged at 15000×g for 20 minutes at 4° C.
Analysis of UDP-N-Acetylglucosamine in tissue extracts. 50 μl of tissue extract (25 mg starting tissue) is placed in a microfuge tube and 50 μl of acetonitrile is added. This is needed to precipitate excess protein in the sample and help clarify the sample. To hydrolyze the UDP-N-acetylglucosamine in the sample, 10 μl of HCl 1N is added and the sample vortexed. This mixture is heated at 80° C. for 20 minutes. The sample is then centrifuged 15000×g at room temperature for 15 seconds to precipitate any condensation formed. The sample is neutralized with the addition of 10 μl KOH 1N. The resulting product, N-acetylglucosamine, is measured by the Morgan and Elson color reaction. 50 μl of potassium tetraborate 200 mM is added and the sample heated at 80° C. for 25 minutes followed by cooling on ice for 5 minutes. 150 μl of Ehrlich's reagent is added and mixed. After 20 minutes at 37° C., the sample is centrifuged 15000×g at room temperature for 2 minutes. 200 μl of the supernatant is added to a 96 well plate and the OD determined at 585 nm. The quantity of N-acetylglucosamine is calculated from standard samples of N-acetylglucosamine using a Softmax-Pro program. The standard curve samples are prepared from N-acetylglucosamine, 2.5 to 30 umoles/well, in a similar manner to the tissue samples. Comparison experiments were performed to confirm the analysis of N-acetylglucosamine produced by three different methods. The experiments demonstrated that: 1) glucosamine 6-phosphate acetylated with acetic anhydride+potassium tetraborate, 2) N-acetylglucosamine treated with potassium tetraborate, and 3) UDP-N-acetylglucosamine hydrolyzed with HCl 0.1N followed by treatment with potassium tetraborate all gave similar results (
The differentiation of the GFAT-alpha from the GFAT-beta can be determined by their sensitivity to UDP-N-acetylglucosamine. GFAT-alpha is inhibited by UDP-N-acetylglucosamine with an IC50 of 8 μM. GFAT-beta is inhibited by UDP-N-acetylglucosamine with an IC50 of 100 μM.
While a number of embodiments of this invention have been represented, it is apparent that the basic construction can be altered to provide other embodiments that utilize the invention without departing from the spirit and scope of the invention. All such modifications and variations are intended to be included within the scope of the invention as defined in the appended claims rather than the specific embodiments that have been presented by way of example.
This application claims the benefit of Provisional Application Ser. No. 60/434,615, filed Dec. 19, 2002.
Number | Date | Country | |
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60434615 | Dec 2002 | US |