Hydrolytically stable, diaminocarboxylic acid-based membranes buffering in the pH range from 6 to 8.5 for isoelectric trapping separations

Controlled uptake and release of lysozyme from glycerol diglycidyl ether cross-linked oxidized starch microgel

A biodegradable microgel system based on glycerol-1,3-diglycidyl ether (GDGE) cross-linked TEMPO-oxidized potato starch polymers was developed for controlled uptake and release of proteins. A series of microgels were prepared with a wide range of charge density and cross-link density. We found both swelling capacity (SWw) and lysozyme uptake at saturation (Γsat) increased with increasing degree of oxidation (DO) and decreasing cross-link density. Microgel of DO100% with a low cross-link density (RGDGE/polymer (w/w) of 0.025) was selected to be the optimum gel type for lysozyme absorption; Γsat increased with increasing pH and decreasing ionic strength. It suggests that the binding strength was the strongest at high pH and low ionic strength, which was recognized as the optimum absorption conditions. The lysozyme release was promoted at low pH and high ionic strength, which were considered to be the most suitable conditions for triggering protein release. These results may provide useful information for the controlled uptake and release of proteins by oxidized starch microgels.

Proteomic analysis of 3T3-L1 adipocyte mitochondria during differentiation and enlargement

The increase in adipose tissue mass arises in part from progressive lipid loading and triglyceride accumulation in adipocytes. Enlarged adipocytes produce the highest levels of pro-inflammatory molecules and reactive oxygen species (ROS). Since mitochondria are the site for major metabolic processes (e.g., TCA cycle) that govern the extent of triglyceride accumulation as well as the primary site of ROS generation, we quantitatively investigated changes in the adipocyte mitochondrial proteome during different stages of differentiation and enlargement. Mitochondrial proteins from 3T3-L1 adipocytes at different stages of lipid accumulation (days 0-18) were digested and labeled using the iTRAQ 8-plex kit. The labeled peptides were fractionated using a liquid phase isoelectric fractionation system (MSWIFT) to increase the depth of proteome coverage and analyzed using LC-MS/MS. A total of 631 proteins in the mitochondrial fraction, including endoplasmic reticulum-associated and golgi-related mitochondrial proteins, were identified and classified into 12 functional categories. A total of 123 proteins demonstrated a statistically significant change in expression in at least one of the time points over the course of the experiment. The identified proteins included enzymes and transporters involved in the TCA cycle, fatty acid oxidation, and ATP synthesis. Our results indicate that cultured adipocytes enter a state of metabolic-overdrive where increased flux through the TCA cycle and increased fatty acid oxidation occur simultaneously. The proteomic data also suggest that accumulation of reduced electron carriers and the resultant oxidative stress may be attractive targets for modulating adipocyte function in metabolic disorders.

Combining isoelectric point-based fractionation, liquid chromatography and mass spectrometry to improve peptide detection and protein identification

The off-line coupling of an isoelectric trapping device termed membrane separated wells for isoelectric focusing and trapping (MSWIFT) to mass spectrometry-based proteomic studies is described. The MSWIFT is a high capacity, high-throughput, mass spectrometry-compatible isoelectric trapping device that provides isoelectric point (pI)-based separations of complex mixtures of peptides. In MSWIFT, separation and analyte trapping are achieved by migrating the peptide ions through membranes having fixed pH values until the peptide pI is bracketed by the pH values of adjacent membranes. The pH values of the membranes can be tuned, thus affording a high degree of experimental flexibility. Specific advantages of using MSWIFT for sample prefractionation include: (1) small sample volumes (approximately 200 microL), (2) customized membranes over a large pH range, (3) flexibility in the number of desired fractions, (4) membrane compatibility with a variety of solvents systems, and (5) resulting fractions do not require sample cleanup before MS analysis. Here, we demonstrate the utility of MSWIFT for mass spectrometry-based detection of peptides in improving dynamic range and the reduction of ion suppression effects for high-throughput separations of tryptic peptides.

Mass distribution, polydispersity, and focusing properties of carrier ampholytes for IEF. Part V: pH 9-11 interval

As a last part of an investigation on all 2-pH-unit intervals of carrier ampholytes (CAs) for IEF (see Electrophoresis 2006, 27, 3919-3934; 2006, 27, 4849-4858; 2007, 28, 715-723) two different lots of Servalyt CAs, in the pH 9-11 range, have been analyzed by a 2-D technique based on preparative Rotofor fractionation followed by capillary electrophoresis mass-spectrometry of 10 out of 20 fractions harvested, in the second dimension. The findings: the two lots contain 65 and 69 different M(r) compounds, in the M(r) interval of 232-667 Da, for a total of 341-387 isoforms, respectively. Since this is a chaotic organic synthesis, the high reproducibility (here demonstrated for the first time during the 40 years of existence of CAs) of the synthetic process (for two batches produced at 6 years of distance) is remarkable, considering that a 94% agreement for the individual chemicals and 88% agreement for the total number of isoforms for the two lots is found. It is additionally demonstrated that the lower pI species are accompanied by considerably more isoforms than the high pI forms and that in all cases such isoforms consist of family of compounds clustered around the pI of the parental form, with a pI spread of ca. 0.1-0.2 pH units.

The Biflow: An instrument for transfer-loop mediated, continuous, preparative-scale isoelectric trapping separations

The Biflow, a new isoelectric trapping instrument was designed to obtain a narrow DeltapI fraction from a complex feed in one step. The Biflow contains two identical separation units, each unit houses: an anode and cathode compartment, an anodic and cathodic membrane, an anodic and cathodic separation compartment, and a separation membrane. The separation units are connected to independent power supplies. The anodic membranes in Units 1 and 2 typically buffer at the same pH value and so do the cathodic membranes. The separation membranes in Units 1 and 2 buffer at different pH values, these determine the pI range (DeltapI) of the product. The cathodic separation compartments in Units 1 and 2 contain the feed and harvest streams. The two anodic separation compartments, connected through an electrically insulating air gap, form the transfer loop through which the transfer stream is recirculated between Units 1 and 2. Ampholytic components in the feed, with pI values lower than the pH of the buffering membrane in Unit 1, pass into the transfer stream and are shuttled into Unit 2. In Unit 2, components in the transfer stream which have pI values higher than the pH of the buffering membrane in Unit 2, pass into the harvest stream. This double transfer of the target component, oppositely directed, guarantees the complete exclusion of products outside the desired DeltapI range from the harvest stream. The utility of the Biflow unit was demonstrated by isolating carnosine from a mixture of UV-absorbing ampholytes and ovalbumin isoforms as well as 4.4 <pI <4.5 and 5.4 <pI <5.5 minor constituents from chicken egg white.

Use of a preparative-scale, recirculating isoelectric trapping device for the isolation and enrichment of acidic proteins in bovine serum

A recirculating, preparative-scale isoelectric trapping device, developed for the binary isoelectric trapping separation of proteins has been used to desalt, isolate and enrich the pI<4 protein fraction from a 150 mL sample of bovine serum. Subsequent re-separation of the 2<pI<4 fraction with pH 3.0, 3.5 and 3.9 buffering membranes resulted in distinct, narrow pI fractions whose components could be readily analyzed by reversed-phase HPLC, even though they were below the detection limit in the original bovine serum sample. The entire isoelectric trapping process (from desalting to collection of the final, narrow pI fractions) took only 7h, indicating the potential of the recirculating, preparative-scale isoelectric trapping device as a front-end component in the proteomic work-flow when sufficiently large samples are available.

Rapid isoelectric trapping in a micropreparative-scale multicompartment electrolyzer

A multicompartmental electrolyzer called membrane-separated wells for isoelectric focusing and trapping (MSWIFT) has been developed and tested for micropreparative-scale isoelectric trapping separations. In the MSWIFT, the length of the heat conduction path from the center of the compartments to the wall is less than 1 mm. The compartments are made from 99.8% nonporous alumina that has a high heat conductivity and a high specific heat capacity, leading to adequate Joule heat dissipation even at power loads as high as 5 W. The length of any compartment parallel to the electric field (the intermembrane distance) can be selected to be multiples of 1.5 mm, leading to compartment volumes that are multiples of about 60 muL. A maximum of 20 (1.5 mm long) separation compartments can be readily assembled in the current version of MSWIFT. The MSWIFT has been used to desalt samples, isolate small ampholytic components (amino acids, peptides, and dyes), prefractionate complex protein mixtures and enrich minor components; these separations were achieved in 20-60 min.

pH transients during salt removal in isoelectric trapping separations: A curse revisited

The pH transients that occur during isoelectric trapping separations as a result of the removal of nonampholytic ionic components have been re-examined. Salts containing strong electrolyte anions and cations, both with equal and dissimilar mobilities, have been studied using anodic and cathodic buffering membranes whose pH values were both equidistant and nonequidistant from pH 7. The direction and magnitude of the pH transient (acidic or basic) was found to depend on both the mobilities of the anion and cation (mu(anion)/mu(cation)) and the pH difference between pH 7 and the pH of the buffering membranes (|pH(memb) (anodic) - 7|/|7 - pH(memb) (cathodic)|). When |pH(memb) (anodic) - 7|/|7 - pH(memb) (cathodic)| = 1, mu(anion)/mu(cation)<1 leads to an acidic pH transient, mu(anion)/mu(cation) = 1 eliminates the pH transient and mu(anion)/mu(cation)>1 leads to a basic pH transient. When mu(anion)/mu(cation) = 1, |pH(memb) (anodic) - 7|/|7 - pH(memb) (cathodic)|<1 leads to a basic pH transient, |pH(memb) (anodic) - 7|/|7 - pH(memb) (cathodic)| = 1 eliminates the pH transient and |pH(memb) (anodic) - 7|/|7 - pH(memb) (cathodic)|>1 leads to an acidic pH transient. By selecting appropriate anodic and cathodic buffering membranes to adjust the |pH(memb) (anodic) - 7|/|7 - pH(memb) (cathodic)| value, pH transients caused by dissimilar anion and cation mobilities can be avoided.

Synthesis of UV-absorbing and fluorescent carrier ampholyte mixtures and their application for the determination of the operational pH values of buffering membranes used in isoelectric trapping separations

Success in isoelectric trapping separations critically depends on the knowledge of the accurate operational pH value of the buffering membranes used. Currently, due to a lack of easy, rapid, accurate methods that can be used for the post-synthesis determination of the operational pH value of a buffering membrane, only nominal pH values calculated from the amounts of the reagents used in the synthesis of the membranes and their acid-base dissociation constants are available. To rectify this problem, UV-absorbing and fluorescent carrier ampholyte mixtures were prepared by alkylating pentaethylenehexamine with a chromophore and a fluorophore, followed by Michael addition of acrylic acid and itaconic acid to the resulting oligoamine. Carrier ampholyte mixtures, with evenly distributed absorbance values across the 3<pI<10 range, were prepared by blending. The master blend served as the feed mixture in binary isoelectric trapping separations that used the buffering membrane to be characterized. The pI value of the most basic UV-absorbing or fluorescent carrier ampholyte collected in the anodic separation compartment, determined by full-column imaging capillary isoelectric focusing analysis, indicates the operational pH value of the separation membrane.