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

Properties of poly-aminophenylboronate coatings in capillary electrophoresis for the selective separation of diastereoisomers and glycoproteins

The polymerisation of 3-aminophenylboronic acid (APBA) in aqueous environment has been used for the open tubular modification of capillary electrophoresis (CE) capillaries. Being poly-APBA endowed with boronic acid, aromatic rings and secondary amines groups, it posses a variety of functional groups affecting selectivity. Diastereoisomers (e.g. ascorbic and isoascorbic acid) and proteins (e.g. haemoglobins) were successfully separated onto poly-APBA column, by means of a combination of electrophoresis and open tubular electrochromatography. The mechanism of selection was investigated: results indicate an interplay between enhancing or silencing the contribution of the protonable functionalities (amino groups, boronic acid). The properties of APBA polymer coating make it attractive for CE separation and for further application in affinity separations and chip technologies.

Isoelectric focusing as a tool for the investigation of post-translational processing and chemical modifications of proteins

It has been demonstrated that good agreement may be observed between computed and experimental isoelectric point (pI) values when proteins of known sequence are focused under denaturing conditions on immobilized pH gradient IPG slabs, at least in the pH range 4-7.5. Hence, discrepancies between expected and found in this experimental set-up may be reliably ascribed to some kind of post-transcriptional processing, or chemical modification, having taken place in the sample. This evaluation is made easier when the comparison is set between the pI of a parent molecule and that (or those) of one to several of its derivatives as resolved in a single experiment (for instance, as a spot row in two-dimensional maps); no previous knowledge is required in these cases about the amino acid composition of the primary structure. The effects on protein surface charge are discussed in this review mainly for two biologically relevant processes, glycosylation and phosphorylation. Then, the pI shifts are analysed for some protein modifications that may occur naturally but can also be artefactually elicited, such as NH2 terminus blocking, deamidation and thiol redox reactions. Finally, carboxymethylation and carbamylation are used to exemplify chemical treatments often applied in connection with electrophoretic techniques and involving charged residues. Procedures to be applied in order to verify whether a given modification has occurred, and often relying on the focusing of a treated specimen, are detailed in each section. Numerical examples on model proteins are also discussed. As an important field of application of the above concepts may be genetic engineering, an exhaustive bibliographic list dealing with pI evaluation and structural assessment on recombinant proteins is included.

Image analysis techniques for automatic evaluation of two-dimensional electrophoresis

Techniques for automatic analysis of two-dimensional electrophoresis gels by computer-aided image analysis are described. Original gels or photographic films are scanned using a laser scanner and the files are transferred to a microcomputer. The program package first performs a compression and preevaluation of the files. Spot identification and quantification is performed by the chain code algorithm after appropriate zooming and cutting. Labeling facilitates spot identification and quantification in numerical and graphical (pseudocolor) representation on peripheral devices for camera ready output. Interpolation between measured basepoints is performed by cubic spline algorithms which are automatically switched on and off, depending on the need by the program. High speed analysis and graphic representation is achieved using fast Assembler language routines rather than high level languages. One-dimensional gels can be analyzed using the same software. Spot matching between parallel two-dimensional gels has not yet been implemented.

Bovine beta-lactoglobulin H: isolation by preparative isoelectric focusing in immobilized pH gradients and preliminary characterization

In spite of the fact that beta-lactoglobulin (beta-lg) was first discovered in bovine milk more than fifty years ago, and that it represents the main whey protein component in all the milks in which it has been found, its biological role and genetic evolution still remain rather uncertain. From comparative studies of the primary and tertiary structures of beta-lg and of other proteins of a similar size, the existence of a new superfamily of proteins with the function of transporter of hydrophobic molecules has been conjectured. The elucidation of the structure of beta-lg either from different species or from different genetic variants of the same species should give useful information on the evolution and function of this protein family. With this aim in mind we have now undertaken the isolation and characterization of a recently discovered, new genetic variant of bovine beta-lg. A two-step purification procedure involving preparative HPLC gel filtration and preparative IEF-IPG has been successfully carried out; it affords a good recovery of the new beta-lg in highly purified form.

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.