Isoelectric focusing in immobilized pH gradients

Reducing Cathodic Drift during Isoelectric Focusing Using Microscale Immobilized pH Gradient Gels

Microfluidic analytical tools play an important role in miniaturizing targeted proteomic assays for improved detection sensitivity, throughput, and automation. Microfluidic isoelectric focusing (IEF) can resolve proteoforms in lysate from low-to-single cell numbers. However, IEF assays often use carrier ampholytes (CAs) to establish a pH gradient for protein separation, presenting limitations like pH instability in the form of cathodic drift (migration of focused proteins toward the cathode). Immobilized pH gradient (IPG) gels reduce cathodic drift by covalently immobilizing the pH buffering components to a matrix. To our knowledge, efforts to implement IPG gels at the microscale have been limited to glass microdevices. To adapt IEF using IPGs to widely used microfluidic device materials, we introduce a polydimethylsiloxane (PDMS)-based microfluidic device and compare the microscale pH gradient stability of IEF established with IPGs, CAs, and a hybrid formulation of IPG gels and CAs (mixed-bed IEF). The PDMS-based IPG microfluidic device (μIPG) resolved analytes differing by 0.1 isoelectric point within a 3.5 mm separation lane over a 20 min focusing duration. During the 20 min duration, we observed markedly different cathodic drift velocities among the three formulations: 60.1 μm/min in CA-IEF, 2.5 μm/min in IPG-IEF (∼24-fold reduction versus CA-IEF), and 1.4 μm/min in mixed-bed IEF (∼43-fold reduction versus CA-IEF). Lastly, mixed-bed IEF in a PDMS device resolved green fluorescent protein (GFP) proteoforms from GFP-expressing human breast cancer cell lysate, thus establishing stability in lysate from complex biospecimens. μIPG is a promising and stable technique for studying proteoforms from small volumes.

Understanding Leishmania parasites through proteomics and implications for the clinic

INTRODUCTION

Leishmania spp. are causative agents of leishmaniasis, a broad-spectrum neglected vector-borne disease. Genomic and transcriptional studies are not capable of solving intricate biological mysteries, leading to the emergence of proteomics, which can provide insights into the field of parasite biology and its interactions with the host. Areas covered: The combination of genomics and informatics with high throughput proteomics may improve our understanding of parasite biology and pathogenesis. This review analyses the roles of diverse proteomic technologies that facilitate our understanding of global protein profiles and definition of parasite development, survival, virulence and drug resistance mechanisms for disease intervention. Additionally, recent innovations in proteomics have provided insights concerning the drawbacks associated with conventional chemotherapeutic approaches and Leishmania biology, host-parasite interactions and the development of new therapeutic approaches. Expert commentary: With progressive breakthroughs in the foreseeable future, proteome profiles could provide target molecules for vaccine development and therapeutic intervention. Furthermore, proteomics, in combination with genomics and informatics, could facilitate the elimination of several diseases. Taken together, this review provides an outlook on developments in Leishmania proteomics and their clinical implications.

Taylor dispersion in equilibrium gradient focusing at steady state

An analytic expression is presented for the effective dispersion coefficient in the case where a solute is focused in a parabolic flow against a linear gradient in a restoring force. This expression was derived by employing a minor variation on the method of moments used by Aris in his development of the dispersion coefficients for a time-dependent, isocratic system. In the present case, dispersion is controlled by two dimensionless groups, a Peclet number which is proportional to the parabolic component of the flow, and a gradient number which is proportional to the slope of the restoring force. These results confirm that the Aris-Taylor expression for the dispersion coefficient should not be applied in cases where a solute is focused to a stationary steady state.

Bisalbuminemia in two Croatian families

BACKGROUND

Bisalbuminemia is a dysproteinemia characterized by the occurrence of two albumin fractions on serum protein separation by electrophoresis on cellulose acetate sheets. Bisalbuminemia may occur as a hereditary trait or as analytical interference with some drugs, especially penicillin.

METHODS

Two patients with the finding of bisalbuminemia are presented. Both patients (patient 1 was a 4-1/2-month-old male infant, and patient 2 was a 15-year-old boy) were admitted for respiratory infection.

RESULTS

Bisalbuminemia was detected by serum protein electrophoresis and confirmed by isoelectric focusing in pH gradient gel (pH range 4.0-6.5). This finding was supported by simultaneous detection of abnormal albumin in the mother of patient 1, while the father had normal albumin. The abnormal fast albumin in both patients had an increased relative mobility of 1.08 when measured from the sample application position.

CONCLUSIONS

The results presented are the first description of albumin mutations in Croatia (that according to the CISMEL group could be classified as ZC/HZ), and present the first step in identification prior to determination of structural change and amino acid sequence in the albumin molecule.

Dynamics of protein isoelectric focusing in immobilized pH gradient gels

The protein migration velocity profile through the pH gradient in immobilized pH gradient gels (IPG) was investigated. Strong nonuniformities of migration velocity distribution, which could not be remedied by adding carrier ampholytes, were observed at the initial stages of a run (up to several thousand volt-hour products for wide pH gradients). The final (after a prerun) velocity distribution in IPG gels differs from the one containing carrier ampholytes, with maximum velocity being achieved not at the pH extremes but closer to neutrality. In some uniform pH media, created by either a single or by several Immobilines, essentially a nonconstant migration profile was also obtained immediately after a prerun. After a prerun, followed by a pause in the absence of current, the phenomenon of fast evolution of the final velocity profile was found.

Separation of arginase isoforms by capillary zone electrophoresis and isoelectric focusing in density gradient column

Four major arginase isoforms, I, II, III and IV, have been detected in Evernia prunastri thallus. They differ in terms of both physical and biochemical properties. The isoelectric point (pI) of these proteins has been determined by both isoelectric focusing in density gradient column and high-performance capillary electrophoresis (HPCE). Isoelectric focusing revealed charge microheterogeneity for isoforms II and IV whereas arginases I and II had the same pI value of 5.8. HPCE separation confirmed this charge microheterogeneity for isoform IV but not for isoform III, and provided evidence of microheterogeneity for isoforms I and II. The effect of various electrolyte buffers and running conditions on the HPCE separation of arginase isoform were investigated. Addition of 0.5 mM spermidine (SPD) to the running buffer reduced the electroosmotic flow (EOF) and permitted discriminating between the native proteins and protein fragments.

Altered blood-brain-barrier function in Alzheimer's disease?

Alzheimer's disease (AD) and vascular dementia (VD) are the two most common causes of dementia. As yet, no definitive biological antemortem marker has been established for differential diagnosis of AD or VD. In this study, proteins of cerebrospinal fluid (CSF) from AD, VD and control patients were analyzed by two-dimensional (2-D) electrophoresis with immobilized pH gradients in the first dimension. No specific changes for AD or VD could be detected in the 2-D CSF patterns. However, a spot of haptoglobin alpha-1 chains (13.5 kDa; approximate pI 4.6) was found to be present in the majority of 2-D CSF maps from the dementia cases, suggesting a high-molecular-weight transudate type of alteration in the blood-brain barrier with considerable frequency in AD.

Isoelectric focusing in immobilized pH gradients of the glucose transporter from human red cell membranes

Isoelectric focusing of the human red cell glucose transporter (a transmembrane protein) was performed in immobilized pH gradients. Isoelectric focusing of integral membrane proteins presents problems that are related to the amphiphilic nature of these proteins. Solubilizing additives must be used to counteract hydrophobic effects. In our case, urea and the nonionic detergent, Triton X-100, were used. Focusing was done at 15 degrees C. The isoelectric point (pI) of the glucose transporter (freshly purified by anion-exchange chromatography in the presence of octyl glucoside) was determined at 8.4 +/- 0.05 (n = 9), in good agreement with our earlier determinations by two-dimensional electrophoresis with isoelectric focusing in the presence of carrier ampholytes in the first dimension. The width of the focused zone was approximately 0.1 pH unit, more narrow than after focusing with carrier ampholytes. In an immobilized pH gradient from pH 7 to 10, the transporter region at pH 8.4 comprised one major and one or two minor zones. The pH interval 4-10 was also used and showed a single transporter zone. The glucose transporter tends to self-associate in detergent solution. Octyl glucoside-purified glucose transporter formed oligomers during incubation at 37 degrees C. Upon focusing, these oligomers appeared in a wide pH interval far below pH 8.4.

Electrophoretic phenotyping of erythrocyte enzymes

Erythrocyte acid phosphatase (EAP), esterase D (ESD) and phosphoglucomutase (PGM) phenotypes among the erythrocyte enzyme types of blood groups are surveyed and a modified cellulose acetate membrane isoelectric focusing (CAM-IEF) method for their exploration is described. The phenotyping procedures are usually classified as either equilibrium or non-equilibrium IEF. Equilibrium IEF, which is based on differences in pI values, includes three methods: (i) a narrow pH range of carrier ampholytes, (ii) a relatively narrow pH range of carrier ampholytes containing chemical separators and (iii) immobilized pH gradient gels. Among the three methods, immobilized pH gradients provides a better resolution of isozymes. Conversely, the disadvantages of immobilized pH gradients include longer focusing times and complex gel preparations. Moreover, immobilized pH gradients are unsuitable for stain analysis because of the insensitivity of PGM1 detection. A hybrid IEF system and a commercial immobilized pH gradient dry plate have overcome these problems. However, EAP typing is extremely expensive and ESD typing is not well distinguished by hybrid IEF. As each method has both merits and demerits, the most suitable technique should be selected based on the kind of erythrocyte enzyme types and sample conditions. On the other hand, non-equilibrium IEF is a rapid method because isozymes are detected on the basis of their charge differences under non-equilibrium conditions. Moreover, the appropriate addition separators increases the charge difference and provides a good resolution within a shorter time. Addition of more separators produces a narrow pH range in the gel and takes a substantially longer time to reach the optimum pH range for charge difference.(ABSTRACT TRUNCATED AT 250 WORDS)

Physico-chemical properties of amphoteric, isoelectric, macroreticulate buffers

We report here the properties of a new family of resins possessing an amphoteric character and able to strongly buffer at their pI values. They have been adopted as carriers for growth of cells in tissue culture and for hydroponics (Righetti et al. 1991; J. Biotechnol. 17, 169-176) but it is to be expected that such resins could have interesting chromatographic applications. It has been found that such beads [made by incorporating a pK 6.2 weak acrylamido base and a pK 4.6 weak acrylamido acid in a 2:1 ratio (thus with a pI of 6.2) into a neutral polyacrylamide backbone], independently from their initial conditioning (acid- or base-washed), spontaneously seek their equilibrium position (pI value) upon washing off excess titrant. Thus, upon potentiometric titration, they are seen to buffer in both directions of the pH scale (contrary to the behaviour of a pure carboxyl or a pure amino surface, which will exhibit only unidirectional buffering power). From the behaviour of these amphoteric beads when polymerized in the absence or in the presence of salts (0.2 M NaCl), it is hypothesized that, for exerting buffering power, both the buffering ion and its counterion must be incorporated non-randomly in the chain, but as a couple or in close proximity. Upon random incorporation of the two ions, buffering power is lost.

Improved method for identification of proteins using two-dimensional electrophoresis with immobilized pH gradient isoelectric focusing

Recent advances in protein sequence analysis now permit the determination of partial N-terminal and internal primary structure from low picomole quantities of protein. The major remaining hurdles to sequence analysis of small amounts of protein are the identification, isolation, and handling of microgram and submicrogram quantities of protein. The technique of two-dimensional electrophoresis using immobilized pH gradient isoelectric focusing circumvents many of these problems. However, poor correlation between the first and second dimension have prevented use of this technique for the identification of some proteins which can only be assayed prior to the denaturing conditions used in the second dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis procedure. An improved method is presented which allows correlation of the native biological activity (first dimension) to a silver stained protein (second dimension) with a high degree of confidence.

Purification of prosomatostatin-converting enzymes

The enzymes responsible for performing cleavage of propeptides at basic amino acids have proven difficult to characterize. Using the processing of anglerfish islet prosomatostatin (PSS) as a model system, we are pursuing the characterization of both a single basic amino acid-specific and a dibasic amino acid-specific converting enzyme. We describe here the model system and protein isolation methods that have allowed significant progress toward complete characterization of the somatostatin-generating propeptide converting enzymes (PCEs).

Isoelectric focusing of human parotid salivary proteins in hybrid carrier ampholyte-immobilized pH gradient polyacrylamide gels

Isoelectric focusing of human salivary proteins with carrier ampholyte-isoelectric focusing systems requires prior desalting and concentration of samples, a procedure which is time-consuming and requires relatively large volumes of samples. By contrast, immobilized pH gradient gels are more tolerant to salt loads. Thus pretreatment of samples consists only of centrifugation prior to isoelectric focusing. If larger loads (greater than 50 micrograms) are required, the samples may be concentrated by lyophilization and reconstitution in a smaller volume of water or by dialysis against 30% w/v polyethylene glycol. Immobilized pH gradient polyacrylamide gels (incorporating a hybrid carrier ampholyte system) of two pH ranges (pH 4-9 and pH 3.5-5.0) have been used to separate the proteins in human parotid saliva. The effects of urea on focused patterns were studied; in pH 4-9 gels it gave improved resolution of protein bands, whereas in pH 3.5-5.0 gels it prevented protein precipitation. The salivary proteins were then visualized by staining with Coomassie Brilliant Blue G-250 or a silver procedure. Using the latter, 25-30 well-resolved bands were formed on a pH 4-9 gel loaded with 20 micrograms of proteins. The method offers considerable advantages compared with carrier ampholyte-isoelectric focusing.

Human skeletal muscle myosin light chains analyzed by immobilized pH gradients during ontogenesis: identification of new phosphorylatable isoforms of light chain 2

Previous studies using two-dimensional gel electrophoresis have described adult and fetal isoforms of skeletal muscle myosin light chains (MLC). They have also revealed an embryo-specific light chain (LC1emb), apparently absent in most adult skeletal muscles. In order to characterize more thoroughly the MLC family, we have analyzed the MLCs from human skeletal muscle at different developmental stages using a two-dimensional electrophoresis technique with an immobilized pH gradient in the first dimension. The high resolution of this novel technique, resolving components which in isoelectric points are less than or equal to 0.01 pH, combined with sensitive silver staining, has allowed us to identify four phosphorylatable isoforms of MLC2: two slow-myosin light chains (MLC2Sa and b), two fast myosin light chains (MLC2Fa and b), and their phosphorylated counterparts: MLC2SaP and bP, MLC2FaP and bP. The following major modifications during development were observed: (i) The embryonic LC (LC1emb) persists up to at least 26 weeks of fetal life. (ii) The polymorphism of LC2 is already evident at 10 weeks of development but only the nonphosphorylated forms of LC2S and LC2F seem to be present. The LC2Fa form is predominant. As early as 26 weeks of fetal life, the 4 phosphorylated forms are detected. In the adult, LC2Fb is a minor component. (iii) LC3F (fast) is already expressed at an early embryonic stage (10 weeks).

Enantiomer resolution in immobilized pH gradient gels via inclusion of a chiral discriminator

The separation of enantiomeric forms of dansylated amino acids by isoelectric focusing in immobilized pH gradients (IPG) is demonstrated for the first time. Separations occur in a pH 3.0-4.0 IPG interval, in presence of 7Murea, 10% methanol and 60 mM beta-cyclodextrin (CD) as chiral discriminator. It is found that the inclusion complex formed between the D-form and CD has a lower pI than the uncomplexed form (delta pI = 0.05 for DL-Phe and delta pI = 0.025 for DL-Trp); from this, it is calculated that the pK of the tertiary amino group in the dansyl moiety is lowered by 0.1 pH unit in the former case (D-Phe) and by 0.05 in the case of D-Trp (both values referring to 60 mM CD gels). For some racemates (e.g., DL-Phe) the separation mechanism is still operative with CD concentrations as low as 20 mM. In our system 60 mM CD appears to be the solubility limit of CD. As the complex is stable in the electric field for at least 15 h, this separation mechanism could be exploited for purifying large quantities of pure D and L forms from racemates in multicompartment electrolyzers with isoelectric Immobiline membranes.

HydroLink gel electrophoresis (HLGE). I. Matrix characterization

A new gelatinous matrix is reported, having intermediate properties between those of polyacrylamide and agarose gels. The matrix has the unique property of being amphiphilic, i.e. of swelling in both plain water and polar organic solvents, and seems particularly well suited for electrophoresis of DNA. The compatibility with organic solvents includes 50% dimethyl sulphoxide, 50% tetramethyl urea, 50% acetonitrile and 50% tetrahydrofuran, the latter having a dielectric constant of 20. The matrix is hypothesized to consist of brush-like pillars, having a hydrophobic core and a hydrophilic coating. The latter is formed by short chains protruding in the surrounding liquid and able to coordinate large amounts of water.

Of matrices and men

Matrices used in modern electrokinetic techniques are surveyed. They are essentially three: cellulose acetate, agarose and polyacrylamide gels. The use of cellulose acetate is confined mostly to analyses in clinical chemistry labs. The properties of agarose are discussed, in particular its capacity of forming large-pore structures via supercoiling, i.e. formation of suprafibers with average radii of approximately 20-25 nm. Several modified agaroses are reviewed, in particular the SeaPlaque, SeaPrep, NuSieve, NuFix, Seakem and Isogel brands and a composite agarose-polyacrylamide matrix, quite popular in the seventies for DNA and RNA separations. The field of polyacrylamide gels seems to be bursting, with the large number of crosslinkers described, imparting special properties to such matrices. The properties of new, modified acrylamide monomers, little known in the field of electrophoresis, are evaluated; in particular: trisacryl gels, hydroxyalkyl methacrylate gels and acryloylmorpholine-bisacrylylpiperazine gels, the latter formed by amphiphilic monomers, highly resistant to alkaline hydrolysis. The properties and formulas of a host of acidic and basic acrylamido derivatives (11 of them) used as buffers and titrants for isoelectric focusing in immobilized pH gradients are reviewed here for the first time. The review culminates with a glimpse at a new generation of amphiphatic matrices, such as HydroLink and 'shielded hydrophobic phase' gels, which appear to be the latest developments in the fields of electrophoresis and chromatography, respectively.

Isolation of human monoclonal antibody isoproteins by preparative isoelectric focusing in immobilized pH gradients

A method for preparative isolation of human monoclonal antibody isoproteins is described in the present paper. A human monoclonal antibody directed against the transmembrane protein gp 41 from the human immunodeficiency virus (HIV-1) was used in this study. The antibody belongs to the IgG1 subtype and exhibits antibody dependent cellular cytotoxicity. The resolving power of conventional preparative protein separation techniques such as ion-exchange chromatography, chromatofocusing and lectin affinity chromatography is too poor for a complete separation of isoproteins. The more sophisticated technique of chromatofocusing on FPLC-based material (Mono P, Pharmacia) did not satisfy our expectation. With semipreparative IEF in immobilized pH gradients we were able to prepare the different isoproteins of a human monoclonal antibody in milligram amounts. No significant difference between the single isoproteins with respect to specificity and avidity to the recombinant antigen (rec gp 160) was detected. Therefore, we assume that the separation conditions did not influence the immunochemical nature of the antibody and significant denaturation and/or precipitation of the IgG did not occur. Furthermore the method affords preparative separation with resolution equivalent to analytical runs. Experiments for scale up and further characterization of isoproteins (carbohydrate composition, amino acid analysis, half life times etc.) are in progress.

Isoelectric focusing studies of bacteriorhodopsin

Purified bacteriorhodopsin (BR) samples show a minimum of four isoelectric forms in immobilized pH gradient isoelectric focusing gels. The bands occur as doublets with isoelectric points (pI) centered at 5.20 (principal species) and 5.60. In typical preparations additional bands may be observed at 4.90, 5.07 and 5.50. Purple membrane (PM) was proteolyzed with papain to calibrate the pI shift produced by changing the number of charges on the protein. Asp-242 is removed during the first cleavage between residues 239 and 240 resulting in the loss of a single negative charge and a shift of the principal doublet by +0.35 pH units to pI 5.55. The second papain cleavage occurs between residues 231 and 232 which removes Glu-232, -234 and -237 and shifts the pI by +0.60 pH units to pI 6.10. The +0.60 pH shift upon the second papain cleavage is consistent with the loss of two negative charges and is supported by prior evidence that at least one of the three glutamate residues lost during the second proteolysis step is protonated and neutral in the intact protein. The native and proteolyzed products of BR retain the characteristic 550 nm absorption maxima for solubilized BR. A model for the structural origin of the pI heterogeneity of BR species in proteolyzed PM is presented.

Systematic analysis of the total proteins of a mammalian organism: principles, problems and implications for sequencing the human genome

High-resolution two-dimensional electrophoresis (2-DE) has reached a technological level that allows us to resolve most of the numerous unknown protein species of a mammalian organism if appropriate strategies are used. We will discuss the problems of classification and characterization of proteins and propose a systematic approach to the analysis of the total protein complex. Both a comprehensive as well as a pragmatic approach towards systematic analysis have been considered. A "complex protein database" is suggested and considered with regard to various uses. A systematic analysis of the mouse proteins has been started and some of the preliminary results are summarized here. In particular, genetic properties of the proteins were investigated and are presented in order to demonstrate the significance of a systematic analysis of proteins for research and practical application (e.g. mutagenicity testing). A concept is presented for sequencing the coding DNA of mouse and man, starting with a systematic analysis of mouse proteins and then using two recently developed methods - microsequencing of proteins from spots of 2-DE protein patterns, and utilization of the relatively short N-terminal sequences obtained - to produce the corresponding cDNA's of these proteins.

Analysis of human apolipoproteins C by isoelectric focusing in immobilized pH gradients

Apolipoproteins C are involved in many ways in the metabolism of plasma lipoproteins. Apolipoproteins C from the delipidated VLDL of 35 controls and 165 normo- and hyperlipoproteinemic patients were analyzed by isoelectric focusing on an immobilized pH gradient, pH 4.0-5.0, with 7 M urea, which raised the apparent pH range to 4.8-5.7. This method is an improvement over conventional isoelectric focusing with carrier ampholytes with regard to both resolution and reproducibility. Due to the high resolution (0.1 pH units per cm) additional apolipoprotein C-III bands: C-III0 A1, C-III0 A2, C-III1 C and C-III2 C (the designations A, anodic, and C, cathodic, refer to direction of migration on IEF in relation to the main band) are described for the first time. The possible artifactual nature of these protein bands could be excluded. Cleavage with neuraminidase and peptidases, immunological detection and/or two-dimensional electrophoresis were used to obtain more information. The additional bands seem, in part, to be hydrolysis products of carboxypeptidase A (C-III1 C, C-III2 C). The appearance of C-III1 C and C-III2C was dependent upon the serum triglyceride concentration. The percent distribution of C apolipoproteins in very low density lipoproteins (VLDL) from control serum agreed with previously published data. Apolipoproteins C can also be focused in immobilized pH gradients from VLDL and serum without delipidation.

Immobilized pH gradients in ultrathin-layer (100 micron) gels: avoiding capillarity by a 'squeezing-sealing mold' technique

Several procedures for casting 100 microns ultrathin immobilized pH gradients are described. When acrylate/glass molding cassettes (Pascali et al., Electrophoresis 1987, 8, 371-373) are used two main problems are encountered: (i) a tendency of polymerization solutions, at the beginning of delivery, to spread across the glass surfaces with troublesome effects on the gradient stratification, and (ii) the raising of steep menisci at both extremities of the pH intervals, originating from capillarity phenomena and resulting in nonuniform gradients with bowed electrophoretic patterns. The first shortcoming was acceptably solved by increasing the density of sucrose gradients, and pouring them into prewarmed molding cassettes. The detrimental effect of menisci could be overcome by using a 'squeezing-sealing mold' technique. A molding cassette was endowed with a continuous, squared spacer frame, the upper side being open by inserting a wedged clip. A slight excess of polymerization solution was first dispensed into the cassette and squeezed away on removal of the clip. By completely excluding air from the molding cassette, uniform and well reproducible ultrathin gels could be cast. A major advantage of ultrathin immobilized pH gradient gels is the drastically shorter focusing time.

Characterization of genetic variants of human serum transferrin by isoelectric focusing: comparison between conventional and immobilized pH gradients, and application to a protocol for paternity testing

The polymorphism of transferrin (Tf) is currently being studied by isoelectric focusing in carrier ampholyte-generated pH gradients, carrier ampholyte-separator pH gradients or in immobilized pH gradients. Details for obtaining reproducible results with each of the three procedures are outlined. The effectiveness of pretreatment of serum samples with ferrous/ferric salts is discussed, and incubation times optimized after spectrophotometric measurement of the monoferric Tf conversion. Most of the presently available commercial batches of carrier ampholytes do not reliably discriminate the six common TfC subtypes. Resolution of C1, C3 and C2 was achieved by adding 20 to 90 mM HEPES slab gels prepared with various carrier ampholytes. Isoelectric focusing in carrier ampholyte-separator pH gradients cannot be recommended as a standard typing procedure because the results strongly depend on the batch of carrier ampholytes. Tf subtype resolution was only achieved by using isoelectric focusing in immobilized pH gradients with pH slopes reliably reproducible from one experiment to another. Two major shortcomings of immobilized pH gradients are a marked tendency to protein precipitation at the application site and an interaction of proteins with the charged matrix. A protocol for Tf subtyping in immobilized pH gradients is described, based on prior desialylation of samples instead of pretreatment with iron. Sample entry into the matrix was optimized by addition of 5 mM Tris to the gels, and initially running them at low voltage. Recommendations are provided for the application of Tf typing for paternity testing.

Use of electrofocusing for the analysis and purification of turkey erythrocytes beta 1-adrenoceptors

We show that following one cycle of alprenolol affinity chromatography of turkey erythrocyte beta 1-adrenoceptors, electrofocusing on polyacrylamide gels in digitonin, followed by electroelution, results in complete receptor purification. The overall yield from the electrofocusing-electroelution step of turkey erythrocyte beta-adrenoceptor is 75 +/- 3%. In addition, we are able to demonstrate that receptor-binding assays can be performed directly on the polyacrylamide gel, using 125I-cyanopindolol. This method can be employed for minute quantities of receptor which is an advantage when one wishes to characterize rapidly the beta-adrenoceptor in its native state from tissues that may be available only in limited amounts. We also report, for comparison, on the behavior of the turkey erythrocyte beta 1-adrenoceptor on immobiline polyacrylamide gels and the ability to purify only partially the receptor on these gels.

Focusing of pepsin in strongly acidic immobilized pH gradients

A new acrylamido buffer has been synthesized, for use in isoelectric focusing in immobilized pH gradients. This compound (2-acrylamido glycolic acid) has a pK = 3.1 (at 25 degrees C, 20 mM concentration during titration) and is used, by titration with the pK 9.3 Immobiline, to produce a linear pH gradient in the pH 2.5-3.5 interval. Pepsin (from pig stomach) focused in this acidic pH gradient is resolved into four components, two major (with pI values 2.76 and 2.78) and two minor (having pI values 2.89 and 2.90). This is the first time that such strongly acidic proteins could be focused in an immobilized pH gradient. Even in conventional isoelectric focusing in amphoteric buffers it has been impossible to focus reproducibly very-low-pI macromolecules.

Isoelectric focusing as the crow flies

The evolution of isoelectric focusing is traced back over the years, from a somewhat shaky origin to present-day immobilized pH gradients. Four generations of methodology are classified and discussed: (A) Kolin's approach, consisting of a two-step technique, generation of a pH gradient by diffusion followed by a rapid electrokinetic protein separation; (B) Svensson-Rilbe's approach, consisting of creating a pH gradient in an electric field by utilizing as buffers a multitude of carrier ampholytes, i.e. of amphoteric species possessing good buffering capacity and conductivity at their pI; (C) immobilized pH gradients, by which non-amphoteric buffers and titrants (acrylamido weak acids and bases), titrated around their pK values, are grafted (insolubilized) onto a polyacrylamide gel matrix and (D) mixed-bed carrier ampholyte-Immobiline gel, by which a soluble, carrier ampholyte generated pH gradient coexists in the same matrix with an insoluble, Immobiline generated, pH gradient.

2-D analysis of human skeletal muscle myosin light chains with immobilized pH gradients

Myosin light chains (LC) are a low molecular mass fraction non-covalently bound to the heavy chains. They are present in the myosin molecules and exhibit various degrees of polymorphism among the different species. By utilizing a highly-resolving 2-D technique, in narrow immobilized pH gradients, we have compared the LC forms of skeletal muscle in human and rabbit. Our findings: (1) both forms, LC1 and LC3, migrate in the two species with rather similar electrophoretic constants (both in terms of pI and Mr); (2) the LC2 forms of rabbit and humans exhibit the same Mr but quite different pI values, the rabbit forms being more acidic; (3) the chain LC2Sb is resolved into two spots in both rabbit and humans. In the former, the two bands have equal intensity, while in the latter the high pI component is clearly the most abundant.

Use of polybuffer as carrier ampholytes in mixed-bed Immobiline gels for isoelectric focusing

In mixed-bed, carrier ampholyte-Immobiline gels, a primary, insolubilized pH gradient is admixed with a secondary, soluble pH gradient generated by amphoteric buffers. The latter are the standard carrier ampholytes (e.g. Ampholine, Pharmalyte, Biolyte, Servalyte), used in conventional isoelectric focusing, admixed to Immobiline gels in levels of approximately 0.5-1%. It is here shown that polybuffers 96 (covering the pH 6-9 range) and 74 (covering the pH 4-7 interval) used as eluents in chromatofocusing, can effectively substitute the standard carrier ampholytes with considerable savings (they are 1/16th as expensive as the latter chemicals).

Stable storage conditions of Immobiline chemicals for isoelectric focusing

Most of the problems connected with the use of the Immobiline chemicals (a set of six, non-amphoteric, acrylamido buffers having pK values in the pH 3.5-9.5 interval) can be attributed to the alkaline species (with pK values 6.2, 7.0, 8.5 and 9.3). These compounds, to varying degrees are subjected to two degradation pathways: (a) hydrolysis of the amido bond, producing free acrylic acid and a diamine, the latter unable to be incorporated into the polyacrylamide matrix; (b) spontaneous auto-polymerization, producing a number of oligomers up to n-mers, able to aggregate and precipitate large proteins. Storage of their water solutions as frozen aliquots, a method widely employed, only partially alleviates the problem. Addition of trace-amounts of inhibitors, as lately adopted by the manufacturer, could only reduce the problem of auto-polymerization, but not block the hydrolysis of the amido bond. A new solution has been found, which abolishes both phenomena: storage in n-propanol. As demonstrated by gas chromatography, HPLC analyses and two-dimensional separations of complex samples, storage in organic solvent completely abolishes both hydrolysis and auto-polymerization and allows production of highly reproducible focusing patterns.

Determination of neutral haemoglobin variants by immobilized pH gradient, reversed-phase high-performance liquid chromatography and fast-atom bombardment mass spectrometry: the case of a Hb Torino alpha 43 (CE1) Phe----Val

A neutral haemoglobin variant was identified by the combined use of different analytical methods. Isoelectric focusing on immobilized pH gradients (under denaturing conditions) allowed to detect and alpha chain variant. HPLC of tryptic digest showed that its amino-acid variation resided on T alpha 6 peptide (position 41-56). In the fast-atom bombardment (FAB) positive mass spectrum of the tryptic digest, the protonated molecular ion of the T alpha 6 peptide occurred 48 mass units lower than the normal T alpha 6 fragment, corresponding to a Phe----Val substitution. To a partial sequence determination (aminopeptidase digestion of the T alpha 6 peptide, followed by amino acid determination and FAB-MS analysis of the digestion-generated mixture) the substitution appeared to be on Phe43 (CE1). This variant is already known as Hb Torino. The procedure here described proved to be fast and simple, and feasible whenever neutral variants are supposed to occur.

Effect of salt on the performance of immobilized pH gradient isoelectric focusing gels

The effect of salt and buffer ions in the sample or in an immobilized pH gradient (IPG) on sample entry into the gel and on the final focused pattern are presented. During the initial phase of electrofocusing, ions present in the gel, either as counter ions to the immobilized charge groups of the IPG gel or added to the gel matrix during the rehydration process, are transported toward the electrodes. For ions present at a concentration exceeding approximately 1 mM the transport can be followed by the refractile line marking the trailing edge of an ion-containing zone. Gradual sample entry may be achieved by applying the sample at a site (near the anode or cathode) opposite to that from which the sharpest refractile line, marking the ion present in the highest concentration, approaches the sample. Additionally, lateral band spreading of the sample is avoided. Thus, sample applied at the cathode for IPG gels rehydrated with 1-2 mM Tris base, or at the anode for gels rehydrated with 1-2 mM acetic acid or sodium acetate, enters the gel matrix gradually without lateral band spreading. In contrast, sample applied at the anode, for Tris-containing gels, or at the cathode, for acetate-containing gels, enters rapidly in a sharp zone when the refractile line reaches the sample zone. This results in a high local protein concentration in the zone immediately behind the boundary with lateral band spreading.(ABSTRACT TRUNCATED AT 250 WORDS)

Different patterns of human serum transferrin on isoelectric focusing using synthetic carrier ampholytes or immobilized pH gradients

Isoelectric focusing (IEF) of human serum transferrin allows splitting of the protein pattern into three forms corresponding to the diferric, monoferric and apoform. A detailed analysis of this pattern, performed on transferrin at different degrees of iron saturation, demonstrated that free Ampholine carrier ampholytes (CA) alter the expected results, always giving a complex pattern with multiple bands. In particular, the monoferric form appears to be the predominant one, regardless of the starting saturation of transferrin. In contrast to IEF-CA, the new technique of IEF in immobilized pH gradients (IPG), shows a much simpler pattern with the same samples. Moreover, the different transferrin forms are focused at the same pI values as in IEF-CA but the pattern appears to correspond to the expected distribution. IPG analysis gives a pattern similar to IEF-CA when free Ampholine CA are added either to the samples and/or as electrode solutions. A chelating action of Ampholine CA on Fe+3 might be responsible for these effects, while Immobilines, due to their different chemical nature or to the different focusing procedure, are not able to interact with iron.

Immobilized pH gradients: effect of salts, added carrier ampholytes and voltage gradients on protein patterns

Salts formed from strong acids and bases (e.g. NaCl, Na2SO4, Na2HPO4), present in a protein sample applied to an immobilized pH gradient (IPG) gel, induce protein modification (oxidation of iron moiety in hemoglobin) already at low levels (5 mM) and irreversible denaturation (precipitation) at higher levels (greater than 50 mM). This effect is due to production of strongly alkaline cationic and strongly acidic anionic boundaries formed by the splitting of the salt's ion constituents, as the protein zone is not and can not be buffered by the surrounding gel until it physically migrates into the gel matrix. Substitution of "strong" salts in the sample zone with salts formed by weak acids and bases, e.g.. Tris-acetate, Tris-glycinate, Good's buffers such as (N-[2-acetamido]-2-iminodiacetic acid (ADA), (2-[(2-amino-2-oxoethyl)-amino] ethanesulfonic acid (ACES), (3-[N-morpholino]propane sulfonic acid (MOPS), essentially abolishes both phenomena, oxidation and irreversible denaturation. Suppression of "strong" salt's effects is also achieved by adding, to the sample zone, carrier ampholytes in amounts proportional to the salt present (e.g. by maintaining a salt: carrier ampholytes molar ratio of at least 1:1). This suppression is due to the strong buffering power of the added carrier ampholytes, able to counteract drastic pH changes in the two moving boundaries. A reduction of these deleterious effects of strong salts is also achieved when the IPG run is performed at low voltage for a prolonged time (4 h at 500 V instead of only 1 h at 500 V, before switching to high-voltage settings). Guidelines are given for trouble-free IPG operations.

A horizontal apparatus for isoelectric protein purification in a segmented immobilized pH gradient

A modification of the previously described apparatus (Faupel et al. (1987) J. Biochem. Biophys. Methods 15, 147-162), for recycling isoelectric focusing in a segmented immobilized pH gradient, is here reported. The most important improvements are: (1) a horizontal, vs. the previously vertical assembly; (2) a reduction of the thickness of the central flow chamber to 6 mm, vs. the previous 3 cm length and (3) the introduction, at both gel extremities of each Immobiline segment, of polypropylene filters, thus efficiently blocking the gel in situ. The advantages are: (i) the spontaneous removal of air bubbles, which in the vertical apparatus tend to accumulate in the ceiling of the flow chamber and to obstruct the flow of electric current; (ii) a more efficient hydraulic flow with a reduced chance of heating the liquid stream in the flow chamber, due to its reduced length along the separation path and (iii) a reduced risk of gel detachment from the tube walls, due to osmotic swelling caused by focused protein zones in the gel phase and by the fixed Immobiline charges in the polyacrylamide matrix.

Isoelectric protein purification in segmented immobilized pH gradients. Effect of salts on rate of contaminants' removal

A method is described for keeping a constant salt background during protein purification in a segmented immobilized pH gradient. It is based on an external hydraulic flow replenishing the salt loss due to combined electric and diffusional mass transport (similar to the concept of Ribes' steady-state rheoelectrolysis). Such a minimum of ionic strength might be needed for proteins which tend to precipitate and aggregate at or in vicinity of the isoelectric point. However, it is found that any salt level in the sample feed (already at 1 mM concentration) deteriorates transport of non-isoelectric proteins, because of the much larger current fraction carried by the ions themselves as opposed to proteins. In addition, high salt levels in the sample reservoir might form cathodic and anodic ion boundaries, alkaline and acidic, respectively, which might hamper protein migration and even induce denaturation. Thus, when high salt backgrounds are needed in the sample feed, external pH control should be exerted, e.g. with a pH-stat. Three parameters influence protein transport in the segmented IPG chamber: (a) cross-sectional area of the Immobiline membranes; (b) delta pI between the isoelectric protein and the contaminants and (c) salt molarity in the sample reservoir. The first 2 show a positive, the last a negative correlation.

Isoelectric protein purification by orthogonally coupled hydraulic and electric transports in a segmented immobilized pH gradient

A new method is described for preparative protein purification, based on isoelectric focusing on immobilized pH gradients. The principle is entirely new, as it is based on keeping the protein of interest isoelectric, in a flow-chamber, and focusing the impurities in the Immobiline gel. For this, a hydraulic flow is coupled orthogonally to an electric flow, sweeping away the non-isoelectric impurities from the recycling chamber. The sample flow-chamber is built in the centre of the apparatus, and is coupled to an upper and lower segment of an immobilized pH gradient. The protein to be purified is kept isoelectric in the flow-chamber and prevented from leaving it by arranging for the extremities of the immobilized pH gradient, forming the ceiling and the floor of this chamber, to have isoelectric points just higher (e.g. +0.05 pH units, on the cathodic side) and just lower (e.g. -0.05 pH units, on the anodic side) than the known pI of the species of interest. Macromolecules and small ions leave the flow chamber at a rate corresponding to a first order reaction kinetics (the plot of log C vs. time being linear). In general, for macromolecules, 12 h of recycling under current allow removal of 95% impurities. After 24 h of recycling, the protein of interest is more than 99.5% pure. The recoveries are very high (approaching 100%) as the sample under purification never enters the Immobiline gel and thus does not have to be extracted from a hydrophilic matrix, as typical of preparative gel electrophoresis.

Protein desalting by isoelectric focusing in a segmented immobilized pH gradient

We have recently described an apparatus for protein purification based on a segmented Immobiline gel, having one or more liquid interlayers in between. The principle is entirely new, as it is based on keeping the protein of interest isoelectric, in a flow chamber, and focusing the impurities in an Immobiline gel. For this, a hydraulic flow is coupled orthogonally to an electric flow, sweeping away the non-isoelectric impurities from the recycling chamber. We now demonstrate that the present apparatus can be efficiently used for protein desalting. Hemoglobin A samples, containing 50 mM NaCl or 50 mM ammonium acetate, could be efficiently desalted in 2 h of recycling, after which the total salt content had decreased to less than 0.005 mM (a salt decrement of more than 10,000 fold the initial input). However, with polyprotic buffers (sulphate, citrate, phosphate, oligoamines) the desalting process was much slower, typically of the order of 20 h, possibly due to interaction of these species with the surrounding Immobiline matrix. In this last case, outside pH control (e.g. with a pH-stat) is necessary during protein purification, as, due to the faster removal of the monovalent counterion, the solution in the recycling chamber can become rather acidic or alkaline. It is demonstrated that the 2 extremities of the Immobiline segments facing the sample recycling chamber act indeed as isoelectric membranes, having a good buffering capacity, preventing the protein macroion from leaving the chamber by continuously titrating it to its isoelectric point.

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.

Gel electrophoresis of intact subcellular particles

The review describes the application of gel electrophoresis to the characterization and separation of viruses, ribosomes, vesicles and other subcellular particles. The preparation of the sample, the choice of the buffer, the gel medium, the apparatus and the detection of the particle (staining and scanning) as well as the necessary theory are discussed. This includes the mathematical evaluation of experimental data on the basis of Ferguson plots using the extended Ogston theory. Simple methods and sophisticated computer simulation techniques are described and exemplified in application to the determination of particle size and charge, the pore size of the gel (unpublished data) and the two-dimensional agarose electrophoresis (unpublished). It is shown that the nature of the particle (e.g. spherical or rod-shaped, pliable or rigid texture) determines the shape of the non-linear Ferguson plot. In addition, the review gives a number of practical applications of gel electrophoresis, isoelectric focusing, titration curves and immuno-electrophoresis to subcellular particles. Pros and cons are evaluated. A comparison with other analytical procedures is made. The review is concluded by a futuristic outlook.

Resolution of G gamma and A gamma foetal haemoglobin tetramers in immobilized pH gradients

Intact tetramers of foetal haemoglobin (G gamma, A gamma and the mutant A gamma T) can be separated by isoelectric focusing in immobilized pH gradients over a very shallow pH interval (pH 7.35-7.55). The G gamma tetramer exhibits a lower pI (7.450) than the A gamma tetramer (pI 7.453); the delta pI between the two species is barely 0.003 of a pH unit, close to the theoretical resolution limit of the technique, delta pI = 0.001. Haem-free, denatured gamma chains exhibit a reversal in pI order, the A gamma chain being more acidic than the G gamma chains: this is attributed to preferential binding of detergent micelles to the more hydrophobic A gamma polypeptide. The advantage of the present technique is the simultaneous analysis of several samples (30-40 per gel slab) and the recovery of intact, haemoglobin tetramers for subsequent studies.