Isoelectric focusing of basic proteases in immobilized pH gradients

By exploiting a new, alkaline immobilized pH gradient spanning the pH 10-11 interval, it has been possible to focus and to detect, by in situ zymogramming with cellulose acetate foils impregnated with fluorogenic substrates, 2 alkaline proteases, namely elastase and trypsin. Elastase gave a sharp array of 3 bands, with the following pIs (at 10 degrees C): 10.60 (major component), 10.53 (intermediate species) and 10.45 (minor isoform). Trypsin was resolved into 2, about equally abundant, species having pIs of 10.70 and 10.53. However, the latter enzyme gave smears in between these 2 forms and also anodic to the lower pI species. As hydrophobic interaction with the Immobiline matrix was excluded, it is suggested that these smears represent product of auto-digestion due to the very alkaline pH during the focusing process.     

Isoelectric focusing in immobilized pH gradients: Generation of extended pH intervals

A new technique for generatiing extended pH gradients (3–4 pH units) in Immobiline gels for isoelectric separations is described. A five-chamber gradient mixer has been built, based on the ‘Varigard’-type mixers of Peterson and Sober (Anal. Chem. 31, 1959, 857–862). Each chamber contains one of the following Immobilines, in this order: pK values 4.4, 4.6, 6.2, 7.0 and 8.5, titrated in the pH 4–8 interval with non-buffering Immobilines pK 9.3 (in the case of the two acidic Immobilines) and pK 3.6 (in the case of the three basic Immobilines). In this way it is possible to cast, in a highly reproducible way, an immobilized pH gradient in thepH range 4.0 to 7.5, which should be ideal for isoelectric separations in the first dimension of two-dimensional techniques. A computer program is also described which, given the molarities and pK values of the different Immobilines in the chambers of the Varigrad mixer, can generate the theoretical pH profile, together with the buffering capacity (β) and ionic strength (I) courses.     

Isoelectric focusing in immobilized pH gradients in the pH 10-11 range

With the synthesis of a new, strongly basic Immobiline (pK 10.3 at 10 degrees C) it has been possible to formulate a new pH 10-11 recipe for focusing very alkaline proteins, not amenable to fractionation with conventional isoelectric focusing in carrier ampholyte buffers. In this formulation, water is added as an acidic Immobiline having pK = 14 and a unit molar concentration (or with a pK = 15.74 and standard 55.56 molarity) since around pH 11 its buffering power becomes significant. The gel contains a 'conductivity quencher', i.e. a density gradient incorporated in the matrix, with the dense region located on the cathodic side (pH 11) for (a) smoothing the voltage gradient on the separation cell and (b) reducing the anodic electrosmotic flow due to the net positive charge acquired by the matrix at pH 11 (1 mM excess protonated amino groups to act as counterions to the 1 mm OH- groups in the bulk water solution generated by the local value of pH 11). Excellent focusing is obtained for such alkaline proteins as lysozyme (pI 10.55), So-6 (a leaf protein, pI 10.49), cytochrome c (pI 10.45) and ribonuclease (pI 10.12).     

Isoelectric focusing of tryptic peptides from hemoglobin subunits by immobilized pH gradients

The technique of isoelectric focusing on immobilized pH gradients (IPG) has been applied to the analysis of tryptic digests of alpha- and beta-chains of human hemoglobin. Using peptides purified by RP-HPLC as a reference, it was possible to create a peptide map in the single IEF dimension. Unfortunately, it was not possible to find experimental conditions (medium for migration and staining) which would allow the detection of peptides of less than 10-12 residues. Almost all the bands visible on the gel could be assigned to known peptides. In order to obtain these results the IPG runs were performed in 8 M urea containing 0.5% carrier ampholytes and the gel stained with colloidal Coomassie brilliant blue G-250, in the presence of a high-salt concentration and at acidic pH.     

Isoelectric focusing in immobilized pH gradients: Principle,methodology and some applications

A new technique for generating pH gradients in isoelectric focusing is described, based on the principle that the buffering groups are covalently linked to the matrix used as anticonvective medium. For the generation of this type of pH gradient in polyacrylamide gels, a set of buffering monomers, called Immobiline (in analogy with Ampholine), is used. The pH gradient gels are cast in the same way as pore gradient gels, but instead of varying the acrylamide content, the light and heavy solutions are adjusted to different pH values with the aid of the Immobiline buffers. Available Immobiline species make it possible to generate any narrow linear pH gradient between pH 3 and 10. The behaviour of these types of gradients in isoelectric focusing is described.Immobilized pH gradients show a number of advantages compared with carrier ampholyte generated pH gradients. The most important are: (1) the cathodic drift is completely abolished; (2) they give higher resolution and higher loading capacitu; (3) they have uniform conductivity and buffering capacity; (4) they represent a milieu of known and controlled ionic strenght.     

Isoelectric focusing in immobilized pH gradients: applications in clinical chemistry and forensic analysis

The applications of isoelectric focusing in immobilized pH gradients in clinical chemistry and forensic analysis are reviewed. Strong emphasis is given to the separation of serum proteins, in particular α₁-acidic glycoprotein, acid phosphatase, alkaline phosphatase, α₁-antitrypsin, apolipoproteins, complement component, factor B, factor XIIIB, group-specific component, lecithin:cholesterol acyltransferase, phosphoglucomutase, prealbumin, protein C and transferrin. The analysis of human parotid salivary proteins is discussed and an assessment is given of the state of the art in thalassaemia screening.     

Isoelectric focusing and two-dimensional electrophoresis of tubulin using immobilized pH gradients under denaturing conditions

Modifications of the LKB Immobiline isoelectric focusing (IEF) technique are described for use under conditions that solubilize and denature most proteins (8 M urea and 2% Nonidet-P40). This procedure permits pH gradients that are four- to fivefold shallower than previously available with conventional ampholine-IEF procedures. It can also be used as a first dimension in two-dimensional gel electrophoresis. The advantage of the stable ultranarrow pH gradient is demonstrated by directly comparing the resolution of vertebrate brain tubulins using (i) denaturing conventional ampholine-IEF and (ii) denaturing Immobiline-IEF. Analysis of tubulin on the Immobiline-IEF gel increases the separation distance between the individual tubulins and distinguishes differences among tubulin samples that could not be resolved by conventional ampholine isoelectric focusing.     

Isoelectric focusing in immobilized pH gradients: generation and optimization of wide pH intervals with two-chamber mixers

A new technique for generating extended pH gradients (5 pH units) in Immobiline gels is reported. The previously described (J. Biochem. Biophys. Methods 7, 1983, 123-142) five-chamber gradient mixer has been replaced by a two-vessel device. A single mixture of the available Immobilines (pK 3.6, 4.6, 6.2, 7.0, 8.5 and 9.3) is made, with relative concentrations adjusted so as to produce the most uniform buffering power throughout the desired pH interval. This mixture is then divided into two portions, which are titrated to the extremes of the required pH span with an acidic titrant (Immobiline pK approximately 1) and a basic species (Immobiline pK 9.95). Highly reproducible pH gradients (pH 4-9) are thus generated, which appear extremely useful for the first dimensioned of 2-dimensional techniques. Our previously reported computer program has been implemented with an optimization algorithm which, given any cocktail of Immobilines, automatically adjusts the relative initial concentrations until the smoothest possible beta power is found. For the first time it is possible to perform IEF under controlled physico-chemical parameters: pH span and linearity, beta power, ionic strength and molarity of the buffering species.     

Isoelectric focusing of sparingly soluble proteins in immobilized pH gradients, exemplified by microvillar membrane hydrolases

A novel fractionation technique is described for analysis of membrane-bound enzymes and sparingly soluble proteins: isoelectric focusing in a mixed-type matrix, containing a primary, immobilized pH gradient with a superimposed, secondary carrier ampholyte pH gradient. Three microvilli hydrolases: dipeptidyl peptidase IV, gamma-glutamyl transferase and alkaline phosphatase exhibit an array of sharply focused, enzyme active bands in the pH 4-6.5 range. The separation pattern obtained is by far superior to any separation achieved by either technique separately.     

High-resolution two-dimensional electrophoresis with isoelectric focusing in immobilized pH gradients

Due to the high reproducibility of pH gradient slope and width, immobilized pH gradients (IPG) have been used as the first dimension of two-dimensional techniques in order to generate maps of constant spot position in the $\text{p}\text{M}{}_{\mathrm{<mtext>r</mtext>}}$. However, when coupling IPG to SDS (sodium dodecyl sulphate) gels two problems were encountered: vertical streaking, due to incomplete zone solubilization in SDS, and horizontal streaking, due to spot fusion along the pH axis caused by the electroendosmosis of the charged Immobiline gels. Two methodical modifications are herewith described to overcome these drawbacks: (a) the SDS equilibrium time of the first-dimension gel has been prolonged to at least 30 min; (b) the SDS electrophoresis gel has been cast together with a starting gel, containing 2.5 mM of each Immobiline species used in the first dimension, which serves as a transition from the charged to the uncharged gel.     

Analytical and preparative isoelectric focusing of peptides in immobilized pH gradients

A new method for peptide analysis and purification is described, based on isoelectric focusing in immobilized pH gradients. On the analytical scale, the peptide zones can now be revealed by an stain for primary and secondary amino group (e.g. ninydrin, fluorescamine, dansyl chloride) since the buffering species, unlike conventional carrier ampholytes, contain only carboxyl and tertiary amino groups. For preparative purposes, conditions have been described to remove most contaminants (e.g. unreacted monomers, non-cross-linked, short polyacrylamide chains) from the gel matrix before the electrophoretic run. However, ca. 2% of the gel dry mass is still present as extractable material. The focused peptides can be recovered in higly yields (ca. 90%) with a fairly high degree of purity (75%), the contaminants being mostly components eluted from the polyacrylamide gel.     

Preparative isoelectric focusing in immobilized pH gradients. II. A case report

The preparative aspects of isoelectric focusing (IEF) in immobilized pH gradients (IPG) have been investigated as a function of the following parameters: environmental ionic strength (I), gel geometry and shape of pH gradient. As model proteins, hemoglobin (Hb) A and a minor, glycosylated component (HbA1c), with a delta pI = 0.04 pH units, have been selected. The load capacity increases almost linearly, as a function of progressively higher I values, from 0.5 X up to 2 X molarity of buffering Immobiline (pK 7.0) to abruptly reach a plateau at 3 X concentration of buffering ion. The load capacity also increases almost linearly as a function of gel thickness from 1 to 5 mm, without apparently levelling off. When decreasing the pH interval from 1 pH unit (pH 6.8-7.8) to 1/2 pH unit (pH 7.05-7.55) the amount of protein loaded in the HbA zone could be increased by 40%. In 5 mm thick gels, at 2 X pK 7.0 Immobiline concentration, over a 1/2 pH unit span, up to 350 mg HbA (in a 12.5 X 11 cm gel) could be loaded in a single zone, the load limit of the system being around 45 mg protein/ml gel volume.     

Direct recovery of proteins into a free-liquid phase after preparative isoelectric focusing in immobilized pH gradients

A new method for electrophoretic retrieval of protein zones from Immobiline matrices is described, based on elution directly in a free liquid phase, rather than in ion-exchange beads or molecular sieves, as previously described. The chopped Immobiline gel is loaded on top of a 5% T stacking gel, 6-10 mm in height, and forced to transverse it and collect into a chamber, filled with 20% sucrose solution, closed on its anodic side by a dialysis sac. The transfer is practically quantitative, for most proteins, after 30-60 min of zone electrophoresis at 10 W (300 V potential differential). Recovery of protein mass is in general better than 90%, while for enzyme activity is in the range of 60-80%. For preserving enzyme integrity, the following precautions are recommended: short electrophoretic times; avoidance of anodic oxidation; chilling of the buffer in the anodic chamber; and use of low levels (2-5 mM) of the specific enzyme substrate throughout the entire electrophoretic system (cathode, anode and gel plug).     

pH measurements in ultranarrow immobilized pH gradients

It is possible to measure pH values in immobilized pH gradients (IPG) when the polyacrylamide matrix is made to contain an additional, carrier ampholyte-generated pH gradient. After an IPG run, 5 mm gel segments, along the separation axis, are cut and eluted in 300 microliter of 10 mM KCl and the pH read with a standard pH meter. When using ultranarrow pH gradients, larger gel segments (ca. 265 microliter) are eluted in 900 microliter of 100 mM KCl and the pH assessed with a differential pH meter. In the latter case, either internal or external standards are used as a reference, or starting point, to convert delta pH values into an actual pH curve. The reproducibility of the system is better than +/- 0.05 pH units, with a ca. 15% error over a 0.3 pH unit span. In ultranarrow pH gradients, it is imperative to use mixtures of all commercially available carrier ampholytes, so as to smoothen conductivity and buffering capacity gaps. By the present method, it is also possible to convert a wide (2-3 pH unit) carrier ampholyte interval into a narrow (0.2-0.3 pH unit) one.