Mass spectrometric characterization of low-molecular-mass color pI markers and their use for direct determination of pI value of proteins

The use of low-molecular-mass color pI markers for the determination of pI values of proteins in gel isoelectric focusing (IEF) in combination with mass spectrometry is described. Different types of substituted phenols of known pI values within the mass range 250-400 were used here as pI markers. The pure, synthesized pI markers were studied by MALDI-TOF/TOF MS. Fragmentation studies of the pI markers were also performed. Only stable and well-characterized pI markers were used in this work. The selected pI markers were mixed with proteins, deposited on a gel and separated in a pH gradient. Color pI markers enable supervision of progress of the focusing process and also estimation of the position of the invisible focused bands. The separated bands of the pI markers (containing separated proteins) were excised, and the pI markers were eluted from each gel piece by water/ethanol and identified by MALDI-TOF/TOF MS. From the washed gel pieces the remaining carrier ampholytes were then washed out and proteins were in-gel digested with trypsin. The obtained peptides were measured by MALDI-TOF/TOF MS and the proteins identified via a protein database search. This procedure allows avoiding time-consuming protein staining and destaining procedures, which shortens the analysis time roughly by half. For comparison, IEF gels were stained with Coomassie Brilliant Blue R 250 and proteins in the gel bands were identified according to the standard proteomic protocol. This work has confirmed that our approach can give information about the correct pI values of particular proteins and shorten significantly the time of analysis.

The Alpher, Bethe, Gamow of isoelectric focusing, the alpha-Centaury of electrokinetic methodologies. Part I

The birth and evolution of IEF in conventional carrier ampholyte buffers is reviewed here, from a shaky start during World War II, via desperate attempts of Svensson to create pH gradients by stationary electrolysis of salts, to the development of the IEF theory and the solution of the steady-state equation. The remarkable synthetic process of Ampholines, as ingeniously devised by Vesterberg, is additionally recalled, with a thorough description of the fundamental properties of these amphoteric buffers, creating and maintaining the pH gradient under strong electric fields. The review ends with a mention of the major contributions of B. J. Radola to this field, namely analytical and preparative IEF in granulated Sephadex layers and the development of ultrathin IEF, in polyacrylamide gels as thin as 20-100 mum. The latter technique paved the way to DNA sequencing gels and to CZE. The symptoms of decay are here presented through the simulations of Mosher and Thormann, clearly indicating an isotachophoretic mechanism for pH gradient decay with time. The decay of IEF was the birth of IPGs.

Colored pI standards and gel isoelectric focusing in strongly acidic pH

Colored, low molecular weight pI markers have been developed for isoelectric focusing (IEF) in acidic pH range. Their isoelectric points (pIs) were determined by direct measurement of the pH of the focused bands after completion of IEF on polyacrylamide gels. The practicable suitability of the proposed pI markers as pI standards for IEF was tested by applying gel IEF. The acidic pH gradient was created either by commercial synthetic carrier ampholytes or by mixture of simple buffers consisting of acids (non-ampholytes) and ampholytic buffers. By applying simple acids, it was possible to extend the acidic pH range beyond those achievable with commercial synthetic carrier ampholytes. By using an experimental arrangement without electrode electrolyte reservoirs with electrodes creating the fixed end of the gel, the strongly acidic pH gradient was stable even for prolonged focusing time.

Capillary zone electrophoresis of oligonucleotides and peptides in isoelectric buffers: theory and methodology

The use of isoelectric buffers in capillary zone electrophoresis is reviewed. Such buffers allow application of extremely high voltage gradients (up to 1000 V/cm in relatively high bore capillary, e.g. 75 to 100 microm internal diameter), permitting separations of the order of a few minutes and thus favoring high resolution due to minimal, diffusion-driven peak spreading. The fundamental properties of ampholytes are first discussed, such as buffering power (beta) as a function of delta pK, i.e. of the distance between the pI value and neighboring protolytic groups. The highest possible relative beta value (= 2) is obtained for amphoteres possessing a delta pK = 0.6, a condition not met by existing amphoteric species. A novel parameter for ampholyte evaluation is then proposed, namely the beta/lambda ratio, i.e. the ratio between the beta power and conductivity at the pI value. It is additionally shown that the pI is not a constant value, but depends on ampholyte concentration in solution. In addition, at constant concentration, the theoretical pI can change as a function of delta pK. Isoelectric His and, to a lesser extent, Lys have been found to offer unique separations of oligonucleotides in sieving liquid polymers. In the absense of sieving media, isoelectric Asp, in presence of 7 M urea (apparent pH 3.77), permits unique separations of oligonucleotides having the same length but different nucleotide composition. Isoelectric Asp (pI 2.77 at 50 mM concentration) provides a medium of high resolving power for generating peptide maps. In difficult cases, of coincident titration curves, the pH can be moved up to higher values (e.g. pH 3.0 for 30 mM Asp) thus eliciting separation of unresolved peptides at pH 2.77. This was illustrated by running peptide maps of tryptic digests of human beta-globin chains. Also imino diacetic acid (pI 2.33 at 50 mM concentration) allows generation of high resolution peptide maps.

Capillary electrophoresis of oligonucleotides in sieving liquid polymers in isoelectric buffers

Analysis of oligonucleotides (especially in regard to assessing the purity of antisense compounds) is typically performed in 18% T sieving liquid polyacrylamide, in 30% formamide and 7 M urea. Up to 600 V/cm have been reported, with transit times, for 20 to 25 oligomers, of 15-20 min. We show that the same analysis can be performed in isoelectric buffers, typically histidine (His), and in more dilute linear polyacrylamides, e.g. 10% T (at 0% C), with much reduced analysis times. A series of His concentrations has been explored, ranging from 25 to 150 mM. Best performance is obtained in 100 mM His (at pH = pI, i.e., 7.47 at 25 degrees C), dissolved in 7 M urea, in presence of 10% sieving liquid linear polyacrylamide. Such a buffer allows delivering 800 V/cm without any loss of resolution due to Joule heating, with retention of very high resolving power down to fragments as short as tetranucleotides. Under these conditions, the analysis time for an antisense oligonucleotide containing fragments from a 10-mer to 18-mer is in a time window of 4-5 min. It is shown that the smallest fragment (10-mer) migrates in the capillary at the remarkable speed of 5 cm/min.

Steady state electrolysis and isoelectric focusing

The properties of pH gradients formed by stationary electrolysis of weak mobile or fixed electrolytes are analyzed. The model uses the appropriate balance equations and those of chemical equilibria. It is shown how the equation of the current density has to be modified for considering that fraction of current that is associated with the diffusion of neutral buffer molecules within a pH gradient. Furthermore it is shown that the pH gradients themselves give rise to water production within the gradient and that essential properties of the steady state are related to chemical reactions between the electrolyte constituents. The differential equations describing the gradients of the concentration of a given component, the pH, conductivity and potential are explicitly formulated in relation to those reactions. The equations are solved numerically and the significance of the results for isoelectric focusing is discussed. The experimental conditions to reach shallow and smooth pH gradients exhibiting sufficient ionic strength are formulated.

Column isoelectric focusing in natural pH gradients generated by biological buffers

Using 2 or 3 simple Good zwitterionic buffers at a 16 or 18 mmol/L final column concentration of the mixture, natural pH gradients of 4 to 8 and 3 to 9.5, respectively, were generated in a liquid LKB column. The pH gradients, stabilized by an anticonvective sucrose gradient, were linear, reproducible and stable in the electric field up to 5h. The pH gradients were used for isoelectric focusing of a number of impure proteins such as human hemoglobin, bovine serum albumin and chicken egg white lysozyme. The protein components could be well separated in the gradient, were easily recovered and appeared to be quite pure when analyzed by sodium dodecyl sulfate-gel electrophoresis. Furthermore, the pH gradient 4-8 was effectively used to isolate one of the acidic isozyme (pI 5.6) components of mouse liver alcohol dehydrogenase (EC 1.1.1.1) in an enzymatically active state, suggesting that the procedure does not denature proteins. The low cost, the ease with which the pH gradients are formed, their linearity, stability for a sufficient period to allow proteins to reach equilibrium and their subsequent recovery from buffer eluates should make the procedure interesting for electrofocusing of proteins.

pH gradients generated by polyprotic buffers. II. Experimental validation

The experimental validation refers to the computer program reported in the companion paper, able to simulate the course of pH, buffering power (beta) and ionic strength (I) of polyprotic buffers (either singly or in a mixture) titrated over any pH range. With simple oligoamines (up to five nitrogens) it is shown that it is impossible to generate linear pH gradients in the pH 4-10 interval, unless they are mixed in appropriate ratios. With pentaethylene hexamine, when used alone, it is possible to create a linear pH 4-10 interval, provided the molarity ratios are altered in the two chambers of the gradient mixer. The general rule operating for generation of linear pH intervals is constancy of buffering power throughout the titration. Local minima of beta produce steeper gradients, while local beta maxima flatten it. The ideal delta pK to arrange for linear pH gradients during titration is centred around 1 pH unit; thus polyprotic buffers with very large delta pK values (e.g., EDTA) appear to be totally useless for this purpose. The present computing algorithms should be quite efficient for optimizing existing buffer recipes for chromatofocusing or ampholyte displacement chromatography or for creating new, properly tailored, buffer mixtures.

Two-dimensional gel electrophoresis of proteins

The high-resolution capacity of two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) makes it an excellent tool for the analysis and characterisation of complex protein mixtures. The evolution of two-dimensional electrophoresis is briefly described. The various steps involved in 2-D PAGE, the identification and characterization of proteins separated by 2-D PAGE and the quantitative and qualitative analysis of 2-D patterns are discussed in detail and some new approaches are described. In the final section a brief outline of some of the biomedical applications of 2-D PAGE to screening of body fluids, genetic diseases, inborn errors of metabolism, cancer and neoplastic transformation are discussed.

Multiple hemoglobins in Triturus cristatus: their study by analytical isoelectrofocusing

Electrophoretic separation of the hemoglobin of healthy adult Triturus cristatus reveals four components. Isoelectric focusing of the same hemolysates in various commercial ampholytes of different chemical composition and pH range results in the separation of eight individual hemoglobin bands. The bands obtained by electrophoresis are not homogeneous as revealed by individual gel electrofocusing. They finally separate into the same eight components, as in the whole hemolysate. From the above findings it is concluded that this species has not four but eight individual hemoglobin molecular forms. Our results demonstrate lack of hemoglobin polymorphism in this species.

Isoelectric focusing in agarose gel for detection and identification of hemoglobin variants

The use of isoelectric focusing in agarose gel was examined to test its usefulness for screening of cord blood hemoglobins and for the identification of hemoglobin variants in general. Advantages of agarose include: short running times, ease of preparation, and nontoxicity. In cord blood samples identification of Hb S, Hb C, and Hb Bart's was achieved. Separation of 16 hemoglobin variants, including six of clinical and geographical importance, was shown.