Membrane chromatographic systems for high-throughput protein separations

Effect of porous structure of macroporous polymer supports on resolution in high-performance membrane chromatography of proteins

The effect of porous structures of 2-mm thick diethylamine functionalized monolithic polymethacrylate discs on their chromatographic behavior in ion-exchange mode has been studied. Discs with small pores did not perform well because they exhibited high back pressure and substantial peak broadening. Discs characterized with pores larger than 1,000 nm did not provide good separations either because the time required for some protein molecules to traverse the length across the pore to reach the wall for adsorption/desorption process that is essential for the separation may be longer than their residence time within the matrix. Optimum pore size is centered at about 700 nm. Excellent separations have been achieved with these discs even at very steep gradients and high flow-rates which allow to shorten the separation times substantially.

Characterization and application of new macroporous membrane ion exchangers

A new ready-to-use unit for high-performance membrane chromatography has been characterized. Its dynamic capacity, resolving power and protein recovery were measured at different flow-rates. The binding capacity was 0.5-2 mg/cm2 with a 95% recovery at 10 ml/min irrespective of the protein concentration up to 10 mg/ml. For very-high flow-rates (50 and 100 ml/min) the recovery was 90% and 70%. At these flow-rates, the maximum back-pressure was about 0.1 MPa and was independent of the filtration area. By increasing the filtration area, a proportional capacity increase was obtained, indicating an easy scale-up. High flow-rates had only a slight effect on resolution. This new adsorber was able to purify IgM from supernatant of cell culture of a human hybridoma in less than 8 min with a high degree of purity (95%).

Permeable packings and perfusion chromatography in protein separation

The use of permeable packings in perfusion chromatography for protein separation is reviewed. Mass transport mechanisms in large-pore materials include forced convection in addition to diffusive transport. The key concept in perfusion chromatography is the "augmented" diffusivity by convection which explains the improved efficiency of perfusive packings compared with conventional supports. An extended Van Deemter equation has to be applied when calculating the height equivalent to a theoretical plate (HETP) of chromatographic columns with flow-through particles. It is shown that the effect of forced convective flow in pores is to drive the separation performance between diffusion-controlled and equilibrium limits. A methodology to understand mass transfer mechanisms in permeable packings is proposed. Experimental results for protein separation by high-performance liquid chromatography in new packing media are discussed. Simulated moving bed technology is addressed.

Membrane adsorbers for selective removal of bacterial endotoxin

Surface-modified flat-sheet microfiltration membranes were functionalised with poly-L-lysine, polymyxin B, poly(ethyleneimine), L-histidine, histamine, alpha-amylase and DEAE as well as deoxycholate. Their suitability to remove endotoxin from both buffers and protein solutions was examined using bovine serum albumin, murine IgG1 and lysozyme as model proteins. In protein-free solutions reduction from 6000 EU/ml to <0.1 EU/ml was achieved with all applied ligands; only alpha-amylase as well as L-histidine and histamine, when immobilized via the non-ionic spacer bisoxirane, exhibited low clearance factors at neutral pH. The adsorption of endotoxin is mainly ruled by electrostatic interaction forces. Thus in multi-component systems, such as endotoxin-contaminated protein solutions, competing interactions take place: acidic proteins compete with endotoxin for binding sites at the membrane adsorbers, basic proteins compete with the ligands for endotoxin and act as endotoxin carriers. With properly chosen conditions the membrane adsorbers presented here show exceptional effectiveness also in the presence of proteins. They are generally superior to functionalised Sepharose chromatographic sorbents and allow fast processing. They may contribute to reduce the risks in the application of parenterals and diagnostics.

Mass transfer limitations in protein separations using ion-exchange membranes

Sorption of bovine serum albumin to commercial 150-micron pore size membranes was measured in batch and flow experiments. For residence times of 2-40 min, early and broad breakthrough curves and broad asymmetric elution peaks were observed that depended strongly on flow-rate. System dispersion could not explain the flow-rate dependence. Breakthrough and elution curves were analyzed using new models that included Langmuir sorption, convection and diffusion. From the analysis, film mass transfer resistance was found to be the rate-limiting factor. The maximum allowable pore size that eliminates this limitation was calculated for different molecular weight solutes.

Application of compact porous tubes for preparative isolation of clotting factor VIII from human plasma

Membranes as well as compact porous disks are successfully used for fast analytical separations of biopolymers. So far, technical difficulties have prevented the proper scaling-up of the processes and the use of membranes and compact disks for preparative separations in a large scale. In this paper, the use of a compact porous tube for fast preparative separations of proteins is shown as a possible solution to these problems. The units have yielded good results, in terms of performance and speed of separation. The application of compact porous tubes for the preparative isolation of clotting factor VIII from human plasma shows that this method can even be used for the separation of very sensitive biopolymers. As far as yield and purity of the isolated proteins are concerned, the method was comparable to preparative column chromatography. The period of time required for separation was five times shorter than with corresponding column chromatographic methods. Compact porous disks made of the same support material can also be used for in-process analysis in order to control the separation. The quick response, which is obtained from these units within 5 to 60 s, allows close monitoring of the purification process.

New formats of polymeric stationary phases for HPLC separations: molded macroporous disks and rods

A molding process has been used for the preparation of separation media in different shapes such as rods and flat membrane-like disks. The polymerization is carried out using a mixture of monomers, porogenic solvent and free-radical initiator under conditions that afford macroporous materials with through-pores or channels large enough to provide the high flow characteristics required for applications in chromatography. In contrast to classical suspension polymerization, the solubility of monomers in water does not restrict their use. The versatility of the preparation technique is demonstrated in polymerizations involving both hydrophobic and hydrophilic monomers such as styrene, chloromethylstyrene, glycidyl methacrylate, alkyl methacrylates and acrylamide. Techniques have been developed that allow fine control of the porous properties of the polymers. These, in turn, determine the hydrodynamic properties of the separation devices that contain the molded media. Since all the mobile phase must flow through the separation medium, the mass transport within the molded media is accelerated considerably by convection. Therefore, the separations can be performed at much higher flow rates than in packed columns. This is particularly important for separations of large molecules such as proteins for which diffusion is a serious problem that significantly slows down the separation processes. The molded separation media have been used for the separation of biological compounds using gentle chromatographic modes such as hydrophobic interaction, ion-exchange and affinity chromatography during which the biological activity of the separated compounds is completely retained.

Preparative membrane adsorber chromatography for the isolation of cow milk components

Preparative membrane adsorber (MA) chromatography was used to process milk fractions such as the whey and the permeate commonly obtained during lactose production in modern dairies. In MA systems the fluid-dynamic and mass-transfer properties are superior to conventional HPLC or fast protein liquid chromatography (FPLC) columns. Since the flow resistance caused by the MA stacks is quite low, high throughputs can be realized without loss in resolution. Feed sizes were varied from the laboratory scale (several ml) up to batches of 10 1 during the investigations. MAs based on modified cellulose filtration membranes (average single layer thickness 200 microns, average pore size 5.0 microns) were used for the small-scale experiments. The MAs are functionalized by covalent linkage of strong and weak ion exchanger groups to their surface. Three commercially available types were used [strong ion exchanger: MA Q15 (3 layers of 5 cm2) and MA Q100 (5 layers of 20 cm2); weak ion exchanger: MA D15 (3 layers of 5 cm2); all Sartorious, Germany]. For the large-scale work a dead-end filtration unit containing up to 1300 cm2 of MA-area was used. Here MAs based on a synthetic co-polymer, that were prepared from cut-out sheets, were inserted. Chromatographic conditions were transferable from the cellulose- to the polymer-based MA carrying the same functional groups. The influence of the flow-rate and the pH of the mobile phase on the separation was investigated. The flow-rate could be raised to the limit of the respective chromatographic systems and/or MA modules without loss in resolution. The use of the strong anion exchanger MA together with a mobile phase pH of 6.0 and a fine-tuned gradient allowed the separation of BSA, alpha-lactalbumin and the genetic variants of beta-lactoglobulin, even though no baseline separation was possible in the latter case. The use of coupled modules rather than a single one is shown to improve the separation considerably.

Modified poly(glycidyl methacrylate-co-ethylene dimethacrylate) continuous rod columns for preparative-scale ion-exchange chromatography of proteins

A continuous rod of porous poly(glycidyl methacrylate-co-ethylene dimethacrylate) has been prepared by a free radical polymerization within the confines of a 300 x 8 mm I.D. chromatographic column. The epoxide groups of the rod have been modified by a reaction with diethylamine that affords ionizable functionalities required for the ion-exchange chromatographic mode. The properties of this rod column have been characterized and the column has been used successfully for the chromatographic separation of proteins. The column exhibits a dynamic capacity that exceeds 300 mg at a flow velocity of 200 cm/min. An excellent selectivity allows the separation of up to 300 mg of a protein mixture in a single run.

Application of thiophilic membranes for the purification of monoclonal antibodies from cell culture media

The application of thiophilic membranes for the purification of monoclonal antibodies from hybridoma culture media was studied. Affinity filtrations were performed with membrane stacks and also in a cross flow module with a spiral filtration channel. Purification factors up to five and concentration factors of about eight could be achieved. The flux behaviour was analysed and interpreted according to existing models of filtration. The results were confirmed by scanning electron microscopy. The binding capacity of the membranes differed considerably with the mode of operation. The main component responsible for membrane fouling was identified by sodium dodecyl sulfate polyacrylamide gel electrophoresis and amino acid sequence analysis as bovine serum albumin or its fragments.

Chromatography in the downstream processing of biotechnological products

Chromatography techniques are essential for the isolation and purification of most of the high value products of modern biotechnology. The economically sensible and technically satisfactory downstream processing of a therapeutic protein, usually involves a number of chromatographic steps. Its development and optimization require considerable knowledge of the various physico-chemical and engineering aspects of biochemical chromatography. This review addresses the various modes of chromatography and the design of chromatographic separation processes from a biotechnologist's point of view. Strategies for optimizing the structure of the downstream process are outlined and scaling up consideration are discussed. The importance of the different chromatographic methods in research and development is estimated in an analysis of protein purification schemes recently published in the literature. Finally, examples of the application of chromatographic procedures for process scale product purification in the biotechnological industry are given.

Application of high-performance membrane chromatography for separation of annexins from the plasma membranes of liver and isolation of monospecific polyclonal antibodies

The separation of annexins, calcium-binding plasma membrane-associated proteins from rat liver and Morris hepatoma 7777 by high-performance membrane chromatography (HPMC) is described. The annexins with low molecular masses, CBP 33 and CBP 35, and the annexin with a high molecular mass, CBP 65/67, can be separated within 10 min from one another by anion-exchange HPMC under non-denaturing conditions. The separation devices used consist of compact, porous disks (QuickDisk) on the one hand and of bundled membranes made of cellulose fibers (MemSep) on the other. Both have been found to be equally well suited for this separation. The annexins obtained in this way are subsequently bound to epoxy-activated porons disks and used for the separation of monospecific polyclonal antibodies against the annexin CBP 65/67.

Sorption kinetics and breakthrough curves for pepsin and chymosin using pepstatin A affinity membranes

Isotherms and kinetic parameters for pepsin and chymosin sorption to immobilized pepstatin A were measured in batch experiments. The measured single-solute parameters were used in an affinity-membrane model which included competitive sorption kinetics, axial diffusion and dead volume mixing. The predictions made using the affinity-membrane model matched the experimental breakthrough curves, whereas predictions made using local-equilibrium theory were a distinct mismatch. The performance of affinity-membrane separations was dominated by slow sorption kinetics.

Reversed-phase chromatography of small molecules and peptides on a continuous rod of macroporous poly(styrene-co-divinylbenzene)

A continuous "molded" rod of porous poly(styrene-co-divinylbenzene) prepared by a bulk free radical polymerization within the confines of a chromatographic column has been used successfully for the reversed-phase HPLC of alkylbenzenes and peptides. An excellent rapid separation of bradykinin and [D-Phe7]-bradykinin with a molecular mass of about 1000 was also achieved.

A glutathione S-transferase (GST) isozyme from broccoli with significant sequence homology to the mammalian theta-class of GSTs

A novel glutathione S-transferase (GST) was purified from broccoli (Brassica oleracea var. italica). Partial amino-acid sequencing indicated that the protein shared significant homology with several different plant GSTs from maize, silene, Dianthus, Nicotiana and Triticum, but little homology to yeast (Issatchenkia) GST. One region of the polypeptide near the N-terminal also shared significant homology to a region of rat 5-5, rat 12-12 and human theta-GST (collectively referred to as the theta-GST-class) but little structural homology to the common mammalian cytosolic GSTs (alpha-, mu- or pi-classes). The broccoli GST was retained on a novel membrane based glutathione affinity matrix and displayed activity towards 1-chloro-2,4-dinitro-benzene (CDNB), a general GST substrate, as well as 4-nitrophenethyl bromide, a marker substrate for the theta-class of GSTs. The characteristics of the broccoli GST potentially define it as a member of the theta-class. This is consistent with the view that the theta-class may have arisen prior to the divergence of animals and plants while the mammalian mu-, pi- and alpha-classes evolved after the two kingdoms were established.

Purification of human tumour necrosis factor by membrane chromatography

The recombinant human tumour necrosis factor alpha from an extract of Escherichia coli was enriched to homogeneity according to specific activity and sodium dodecyl sulphate-polyacrylamide gel electrophoresis by purification using anion-exchange HPLC and hydrophobic interaction HPLC. Parallel experiments with the same separation methods, but carried out with membrane chromatography on compact discs, gave similar results in terms of yield and purity of the product. The active form of the protein is a trimer. The second isolation step, hydrophobic interaction chromatography, causes dissociation of the trimer into monomers and a partial loss of the biological activity of the protein. The phenomenon occurs on both the column and the disc. This in turn indicates strongly that the dissociation of the protein is a consequence of interaction between the samples and the hydrophobic ligand, and is not caused by non-specific interaction with the matrix.

Characterization and application of strong ion-exchange membrane adsorbers as stationary phases in high-performance liquid chromatography of proteins

Filtration membranes carrying strong cation- or anion-exchange groups on their surface were evaluated for their potential as membrane adsorber stationary phases in the high-performance liquid chromatography of proteins. The membranes are commercially available and can be obtained inserted into ready-to-use filter holders. Owing to their thinness (170-190 microns), the pressure drop of the membranes is extremely low. Flow-rates of up to 65 ml min-1 per unit became thus possible. The low pressure drop of a single membrane layer also permitted an effortless scaling up, as a stack of several membranes or filter units could be used, if necessary. Sample distribution, protein binding capacity, elution conditions, separation efficiency and recovery were investigated as a function of the flow-rate. The time required for the separation of certain protein mixtures could be reduced to less than 1 min. Appropriate conditions were defined for the separation of human serum and for the isolation of subtilisin Carlsberg and beta-galactosidase from cell culture supernatants.

Preparative chromatography in biotechnology

The field of preparative chromatography has experienced several important advances recently in both the structure of chromatographic materials and modes of chromatographic operation. This review presents recent applications of preparative chromatography in biotechnology, as well as novel stationary phase materials and engineering approaches to preparative chromatographic bioseparations.