Proteins, analysis precipitation reactions

The same derivatization was applied to the HPLC determination of STR and DHS in milk. The comparison of HPLC and ELISA methods was also performed for DIHS. After removal of fat by extracting a milk sample with oxalic acid and centrifuging, proteins were precipitated with TCA. The supernatant was treated by SPE on a Cl8 column. The cartridge was washed with water, and the analytes were eluted with ion-pair in MeCN. The eluate was reconcentrated by evaporating and dissolving in water. Postcolumn reaction took place at 65°C. Recoveries were dependent on the concentration level and the batch of SPE columns used, and independent of the fat content and homogenization. The sample cleanup was not sufficient for the analysis of cheese. The DIHS concentrations of incurred samples determined by ELISA were higher than those obtained by the LC method (107). 

Add 10 /Jb of 3H-Phe solution to tube 1 and pipette up and down rapidly several times to mix. Immediately add 750 pi of 5% TCA to stop the reaction. Place tube 1 on ice for later analysis. This reaction will serve as a time zero blank for the remaining reactions that will indicate the amount of soluble 3H-Phe in the reaction that will unavoidably contaminate the precipitated proteins. 

An LC-MS-MS method to monitor lonafarnib (a novel anticancer drug that inhibits farnesyl transferase) in human plasma. Deuterated internal standard is used proteins are precipitated by acetonitrile. Reverse-phase chromatographic separation is performed using acetonitrile/water/formic acid (50 50 0.05, v/v/v) mobile phase. Time of analysis 8 min. A triple quadrupole tandem mass spectrometer in the positive-ion mode with multiple reaction monitoring is used for detection. The cahbration curve has been established in the 2.5-2500 ng/ml concentration range. The validated method was successfully used in phase I trials of the drug. 

For analysis by titration with Fehling s solution, the sample is treated with lead acetate to precipitate protein and fat, filtered, and the filtrate titrated with alkaline CuS04, while heating. The reactions involved are summarized in Figure 2.37. 

Biophysical characterization showed that a single HRP II protein bound 17 molecules of heme [35]. In an in vitro heme polymerization assay, HRP II promoted the synthesis of hemozoin, while controls, such as the proteins bovine serum albumin and lysozyme or the homopeptides polyhistidine, polylysine, and polyasparagine, did not. FT-IR analysis of the reaction product showed the characteristic vibrations of hemozoin. The polymerization activity had a pH maximum near 4.0, which dropped off precipitously near the pKa of histidine. The heme polymerization... 

Macek et al. [120] developed a method to quantitate omeprazole in human plasma using liquid chromatography-tandem mass spectrometry. The method is based on the protein precipitation with acetonitrile and a reversed-phase liquid chromatography performed on an octadecylsilica column (55 x 2 mm, 3 /im). The mobile phase consisted of methanol-10 mM ammonium acetate (60 40). Omeprazole and the internal standard, flunitra-zepam, elute at 0.80 0.1 min with a total rim time 1.35 min. Quantification was through positive-ion made and selected reaction monitoring mode at m/z 346.1 —> 197.9 for omeprazole and m/z 314 —> 268 for flunitrazepam, respectively. The lower limit of quantification was 1.2 ng/ml using 0.25 ml of plasma and linearity was observed from 1.2 to 1200 ng/ml. The method was applied to the analysis of samples from a pharmacokinetic study. 

Figure 4.9 The preparation of a sample from a reaction mixture that contains an excess amount of protein prior to injection and analysis by HPLC. (A) Termination carried out by immersion of reaction sample in a sand bath maintained at 1SS°C (B) Removal of the precipitated protein by either centrifugation or filtration. (C) Injection and analysis of the clarified solution.

Setting up an HPLC assay for activities on intact cells requires only one major change since the reaction mixture will contain cells, the samples for HPLC analysis cannot be injected directly onto the analytical column thus the reactions must be terminated and any precipitated proteins removed (see Chapter 4). Termination can be accomplished by centrifugation at a low speed or by filtration (see Fig. 4.9B). However, care should be taken to avoid any cell breakage, particularly if the product of the primary or even secondary reactions can also be found as a naturally occurring intracellular component. 

The reaction mixture contained angiotensin I, a phosphate buffer at pH 8.0, Nad (chloride is required for activity), and the enzyme. Incubations were at 37°C for 30 minutes and were terminated by treatment with a boiling water bath for 5 minutes. Centrifugation was used to remove precipitated protein. Fluorescamine in acetone was added to the supernatant solution, and samples were injected for analysis. The enzymatically formed fragments were separated on HPLC as shown in Figures 9.24 and 9.25 the rate of His-Leu formation is shown in Figure 9.26. 

The assay developed for this activity involves the reaction of the P5C with o-aminobenzaldehyde (OAB) to form the reaction product dihydroquinozoli-nium (DHQ). The DHQ and unreacted OAB were separated by reversed-phase HPLC (LiChrosorb C18). The column was eluted isocratically with a mobile phase of 1 part methanol to 2 parts water, and the separation shown in Figure 9.37 obtained. The reaction mixture contained L-omithine (35 mAf), a-ketoglutarate, potassium phosphate (pH 7.4), and pryidoxyl phosphate in a total volume of 2 mL. The reaction was started by the addition of the homogenate and terminated by the addition of 1 mL of 3 N HO containing the OAB. Precipitated protein was removed by centrifugation (3000 rpm), and samples of the supernatant solution (10 nL) were injected for analysis. 

The reaction was carried out with e-lactose-lysine as the substrate using a sodium phosphate buffer (pH 7.5). The reaction was started by the addition of the enzyme after incubation for 1 hour, it was terminated with methanol containing /3-alanine as an internal standard. Precipitated protein was removed by centrifugation, and samples of the supernatant solution were injected and analyzed by HPLC. A chromatogram showing the analysis of a reaction mixture is given in Figure 9.64. 

The conditions for enzymatic analysis varied with the enzyme under study. In general, the reactions were started by the addition of the enzyme activity, and incubations were at 37°C. The reaction was stopped by the addition of 100% methanol, and precipitated protein was removed by centrifugation. Samples of the supernatant solution were analyzed by HPLC. A chromatogram of a glycosidase activity is shown in Figure 9.74. The peak of free 4-MU is noted. The enzyme was obtained from urine samples. 

Such a procedure adapted from Gross and Morell (1966) and Blumenfeld and Gallop (1962) is as follows. Visser et al. (1971) treated 2.5 to 10 mg of a modified elastase with 1 N NHjOH-HCl, adjusted to pH 9 by addition of sodium hydroxide, for 2 hr at 25°C. The excess hydroxylamine was removed either by dialysis or by precipitation of the protein at pH 3.0. The protein was then dissolved, brought to pH 8.0 by addition of NaOH and treated with an equal volume of a 1 % solution of l-fluoro-2,4-dinitrobenzene in ethanol. The pH of the solution was maintained at 8 by the continuous addition of NaOH. The reaction is complete when no additional alkali must be added for 5 min. The mixture is then extracted three times with ether and the aqueous phase subjected to the conditions of the Lossen rearrangement (i.e. heating to 100°C under alkaline condition (0.1 N NaOH) for 10 min). Acid hydrolysis followed by amino acid analysis permits the identification of either diaminopropionic or diaminobutyric acid which would result from either aspartate or glutamate modification, respectively. Diaminopropionic and diaminobutyric acids may be estimated on the short column of the amino acid analyzer. Diaminopropionic acid emerges with histidine. (Color values do not seem to be available.)... 

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