Classifications

• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07K—PEPTIDES
  • C07K14/00—Peptides having more than 20 amino acids; Gastrins; Somatostatins; Melanotropins; Derivatives thereof
  • C07K14/435—Peptides having more than 20 amino acids; Gastrins; Somatostatins; Melanotropins; Derivatives thereof from animals; from humans
  • C07K14/76—Albumins
  • C07K14/765—Serum albumin, e.g. HSA
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61P—SPECIFIC THERAPEUTIC ACTIVITY OF CHEMICAL COMPOUNDS OR MEDICINAL PREPARATIONS
  • A61P7/00—Drugs for disorders of the blood or the extracellular fluid

Definitions

• the present invention relates to the field of protein purification, and especially to the pu- rification of human serum albumin.
• the method utilises a series of chromatography steps, which results in an efficient purification suitable for use in large-scale operations.
• HSA Human serum albumin
• EP 0 612 761 discloses a method of producing recombinant human serum albumin of high purity, which does not contain free non-antigenic contaminants.
• the method util- ises hydrophobic interaction chromatography (HIC) under specified conditions combined with other steps such as ion exchange chromatography, treatment with boric acid or a salt thereof followed by ultrafiltration, and heat treatment.
• HIC hydrophobic interaction chromatography
• EP 0 570 916 also discloses a process for producing recombinant human serum albumin by gene manipulation techniques, wherein purification is by a combination of steps in which a culture supernatant is subjected to ultrafiltration, heat treatment, acid treatment and another ultrafiltration, followed by subsequent treatments with a cation exchanger, a hydrophobic chromatography carrier and an anion exchanger and by salting out.
• this purification scheme is too complex, time-consuming and accordingly too expensive to provide an efficient procedure for use in large-scale operation.
• EP 0 699 687 discloses a method of purification of rHSA wherein culture medium is heat treated to inactivate proteases and then contacted with a fluidised bed of cation exchange particles. The eluent can subsequently be subject to ultrafiltration, HIC and anion exchange chromatography.
• use of a fluidised bed will require equipment different to the conventional packed bed chromatographic step. Accordingly, there is still a need of more efficient and economically attractive procedures for purification of rHSA from a culture broth.
• One object of the present invention is to provide a method for purification of recombi- nant HSA that is easily adapted to large-scale operation. This is achieved by a method, which comprises subjecting a cell culture supernatant (CCS) comprising rHSA to the following steps:
• the method according to the invention comprises fewer process steps than the above- discussed methods.
• the bimodal high salt tolerant cation exchange matrix used in step (a) allows use of a cell culture supernatant without any further dilution, which is an advantageous feature since it greatly reduces the total volume and hence the costs as compared to the prior art methods.
• Another object of the invention is to improve the adsorption capacity of the chroma- tographic steps used in purification of rHSA. This can be achieved by the method described above wherein a cation exchanger, comprising ligands known as high salt lig- ands (HSL), is used in step (a).
• Said ligands are bimodal in the sense that they comprise at least two sites that interact with the substance to be isolated, one providing a charged interaction and one providing an interaction based on hydrogen bonds and/or a hydro- phobic interaction.
• Another object of the invention is to further decrease the colour content, and more spe- cifically to further increase the purity, of the final product. This can be achieved by use of the method described above, wherein a weak anion exchanger is used.
• Figure 1 A-D illustrates possible matrix materials suitable for use in the different steps of cation exchange, hydrophobic interaction and anion exchange chromatography according to the invention.
• Figure 2 shows the results of cation exchange chromatography according to step (a) of the method according to the invention of an undiluted cell culture supernatant (CCS).
• Fraction 2B contains the rHSA.
• Figure 3 shows the results of HIC of fraction 2B from Fig 2. The rHSA is eluted in fraction 3A.
• Figure 4 shows the results of anion exchange chromatography of fraction 3 A from Fig 3.
• the purified rHSA is eluted in fraction 4B.
• Figure 5 shows electrophoresis analyses of the main fractions obtained using the three step purification protocol according to the present invention.
• One aspect of the present invention is a method of purifying recombinant human serum albumin (rHSA) from a solution, which method comprises subjecting a cell culture su- pernatant (CCS) comprising rHSA to the following chromatographic steps (a) cation exchange on a bimodal high salt tolerant matrix; (b) hydrophobic interaction chromatography (HIC); and
• the conductivity of the CCS is above about 10 mS/cm, such as above about 15 and preferably above about 20, e.g. about 25- 50, or more specifically 25-30 mS/cm, when applied to step (a), which is possible due to the kind of cation exchanger used, which comprises ligands of the type known as high salt ligands (HSL).
• HSL high salt ligands
• the CCS can be any fermentation broth from which cells preferably have been removed e.g. by centrifugation.
• the origin of the rHSA isolated according to the pre- sent invention can be any suitable host, such as microbial cells, such as Escherishia coli, Bacillus subtilis etc, yeast cells, such as Saccharomyces cerevisiae, Pichia pastoris etc, or animal cell lines.
• the host cell is Pichia pastoris.
• step (a) utilises a cation exchanger, which comprises ligands of the type known as high salt ligands (HSL).
• HSL high salt ligands
• high salt refers to the above-mentioned property of high salt concentration tolerance that is characteristic for this class of ion exchangers. This property is provided by the nature of the ligands, which is bimodal in the sense that each ligand comprises two groups capable of interacting with the substance to be isolated, in the present case rHSA.
• the primary binding mode is provided by a charged binding group, i.e.
• a second binding mode is provided by a secondary binding group, which provides an ad- ditional interaction with the substance to be isolated.
• the secondary binding group provides a hydrogen bonding or a hydrophobic interaction, but other interactions can be envisaged, as will be discussed in more detail below.
• the term "bimodal" is used herein to define that two or more binding modes are involved, and it is therefore not limited to only two binding modes.
• cation exchangers of HSL-type are utilised, and such cation exchangers have been disclosed in detail, see e.g. PCT/EPO 1/08203 (Amersham Pharmacia Biotech
• the charged binding group present on a cationic high salt ligand can be selected from the group comprised of sulphonate (-S0 3 7-S0 3 H), sulphate (-OS0 3 7-OS0 3 H), car- boxylate (-COOV-COOH), phosphate (-OP0 3 2 7-OP0 3 H7-OP0 3 H 2 and phosphonate (- P0 3 2 7-P0 3 ⁇ /-P0 3 H 2 ).
• the HSL-type of cation exchanger is a weak cation-exchanger, i.e. cation-exchangers that have a pKa above 3.
• they are strong cation exchangers that have a pKa below 3.
• Typical examples of such weak cation exchangers are carboxylate (-C007-COOH), phosphate (-OP0 3 2 7-OP0 3 H7-OP0 3 H 2 and phosphonate (-P0 3 2 7-P0 3 H " /-P0 3 H 2 ).
• the secondary binding group comprises at least one hydrogen-bonding atom, which is located at a distance of 1-7 atoms from the cation-exchanging group.
• a hydrogen- bonding atom is an atom that is capable of participating in hydrogen bonding (except hydrogen), see Karger et al., An Introduction into Separation Science, John Wiley & Sons (1973) page 42.
• the hydrogen-bonding atom can be selected from the group that consists of heteroatoms, such as oxygens (carbonyl oxygen, ether oxygen, ester oxygen, hydroxy oxygen, sulphone oxygen, sulphone amide oxygen, sulfoxide oxygen, oxygen in aromatic rings etc), nitrogens (amide nitrogen, nitrogen in aromatic rings etc), sulphurs (thioether sulphur, sulphur in aromatic rings etc); and sp- and sp 2 -hybridised carbons; and halo groups, such as fluoro, chloro, bromo or iodo, preferably fluoro.
• the second binding group typically contains no charged atom or atom that is chargeable by a pH change.
• the stability of the cation exchange ligands used in step (a) can in general terms be defined as the capacity to resist 0.1 or 1 M NaOH in water for at least 40 hours.
• suitable chemical ligand structures of the cation exchangers useful in step (a) of the present method see above-mentioned PCT/EPO 1/08203.
• step (a) utilises the HSL cation exchange ligand illustrated in FiglA of the present specification.
• the cation-exchanger used in step (a) of the present method is capable of (a) binding rHSA by cation-exchange in an aqueous reference liquid at an ionic strength corresponding to 0.3 M NaCl and, (b) permitting a break through capacity for the substance > 200 %, such as > 300% or > 500% or > 1000 %, of the break through capacity of the substance for a reference cation-exchanger containing sulphopropyl groups -CH 2 CH 2 CH 2 S0 3 " at the ionic strengths shown above.
• the level of cation-exchange ligands in the cation-exchangers used in the inventive method is usually selected in the interval of 1-4000 ⁇ mol/ml matrix, such as 2-500 ⁇ mol/ml matrix, with preference for 5-300 ⁇ mol/ml matrix. Possible and preferred ranges are, among others, determined by the nature of the matrix, ligand etc. Thus, the level of cation-exchange ligands is usually within the range of 10-300 for agarose-based matrices. For dextran- based matrices, the interval is typically 10-600 ⁇ mol/ml matrix.
• PCT/EPO 1/08203 also comprises an extensive discussion of matrix materials useful with cation exchangers of HSL-type.
• such matrices can be based on organic or inorganic material. It is preferably hydrophilic and in the form of a polymer, which is insoluble and more or less swellable in water. Hydrophobic polymers that have been derivatized to become hydrophilic are included in this definition. Suitable polymers are polyhydroxy polymers, e.g. based on polysaccharides, such as agarose, dextran, cellulose, starch, pullulan, etc.
• polymers such as poly- acrylic amide, polymethacrylic amide, poly (hydroxyalkylvinyl ethers), poly(hydroxyalkylacrylates) and polymethacrylates (e.g. polyglycidylmethacrylate), polyvinylalcohols and polymers based on styrenes and divinylbenzenes, and co-polymers in which two or more of the monomers corresponding to the above-mentioned polymers are included.
• Polymers, which are soluble in water may be derivatized to become insoluble, e.g. by cross-linking and by coupling to an insoluble body via adsorption or co- valent binding.
• Hydrophilic groups can be introduced on hydrophobic polymers (e.g. on co-polymers of monovinyl and divinylbenzenes) by polymerisation of monomers exhib- iting groups which can be converted to OH, or by hydrophilisation of the final polymer, e.g. by adsorption of suitable compounds, such as hydrophilic polymers.
• An illustrative example of a suitable matrix is the commercially available beaded Sepharose, which is agarose-based and available from Amersham Biosciences, Uppsala, Sweden.
• Suitable inorganic materials to be used as support matrices are silica, zirconium oxide, graphite, tantalum oxide etc.
• PCT/EP01/08203 comprises an extensive disclosure of the preparation of cation exchangers of HSL-type. Also, a review of methods of immobilising li- gand-forming compounds to surfaces is given in Hermanson, G. T., Mallia, A. K. & Smith, P. K., (Eds.), Immobilisation Affinity Ligand Techniques, Academic Press, INC, 1992.
• HSL-type of ion exchangers it is possible to run adsorption to the column, i.e. binding of rHSA to the ligand, at elevated ionic strengths compared to what has normally been done for conventional cation- exchangers, for instance the reference sulphopropyl cation-exchanger discussed above.
• the exact ionic strength to be used during binding depends on the nature of the protein and the type and concentration of the ligand on the matrix.
• Useful ionic strengths often correspond to NaCl concentrations (pure water) > 0.1 M, such as > 0.3 M or even > 0.5 M. Deso ⁇ tion can be performed e.g.
• Typical salts to be used for changing the ionic strength are selected among soluble ammonium or metal salts of phosphates, sulphates, etc, in particular alkali metal and/or alkaline earth metal salts.
• the same salts can also be used in the adso ⁇ tion steps, but then often in lower concentrations.
• the amount of cation exchange matrix used in step (a) is about half the amount of HIC matrix used in step (b). Accordingly, one advantage of the present invention is the outstanding binding capacity of the bimodal cation exchange matrix that allows a reduction in volume and therefore operational costs as compared to conventional cation exchangers.
• a high salt concentra- tion such as a conductivity of about 25-30 mS/cm
• the adso ⁇ tion of commercially available SP Sepharose for rHSA is about 2-4 mg/mL packed gel, while that of the high salt ligand prototype (Fig 1 A) has been shown to be at least 50 mg/mL. Accordingly, the use of the bimodal high salt ligand (HSL) clearly simplifies and improves the purification process significantly and reduces the cost of operation on large scale.
• One embodiment of the present method comprises heat treatment of the CCS before step (a).
• the heating can be performed directly i.e. while the host cells are still present or after removal thereof, such as by centrifugation, ultrafiltration or any other suitable method.
• the heating can be performed at 50-100°C during a period of time of from 1 minute up to several hours, preferably at 60-75°C for 20 minutes to 3 hours and most preferably at about 68°C for about 30 minutes.
• the heating is conveniently performed in a water bath equipped with thermostat.
• a stabiliser is added before the heating, such as sodium caprylate at a pH of about 6.0.
• Other stabilisers can be used, such as acetyltryptophan, organic carboxylic acids etc.
• the pH of the CCS is preferably adjusted to a lower value suitable for the subsequent adso ⁇ tion on a cation exchanger, such as pH 4.5.
• a reducing agent such as cysteine
• Other examples of useful reducing agents are mercaptoethanol, reduced glutathione etc.
• the pu ⁇ ose of this is to fa- cilitate the removal of coloured substances during step (b).
• This heat treatment is in general performed as described above, even though in the preferred embodiment, a slightly lower temperature such as about 60°C for a slightly longer period of time such as about 60 minutes is used.
• step (b) of the present method utilises hydrophobic interaction chromatography (HIC).
• HIC hydrophobic interaction chromatography
• the main pu ⁇ ose of step (b) is to remove proteolytic degradation products of rHSA, which products are usually of a size of about 10-50 kDa.
• HIC is a well-known principle in the art of chromatography, and provides a versatile tool for separation based on differences in surface hydrophobicity. Many biomolecules that are considered to be hydrophilic have been shown to still expose sufficient hydrophobic groups to allow interaction with the hydrophobic ligands attached to the chroma- tographic matrix. HIC has already been suggested for the purification of rHSA, see e.g. EP 0 699 687.
• HIC Compared with another well known separation principle, namely reverse phase chromatography, HIC utilises a much lower density of ligand on the matrix. This feature promotes a higher degree of selectivity, while allowing mild elution conditions to help preserve the biological activity of the protein of interest.
• the HIC step is used to adsorb the above-mentioned proteolytic degradation products of rHSA, while the full length rHSA is eluted in the unbound fraction.
• the hydrophobic interaction between the rHSA and the immobilised ligand on the matrix is enhanced by a small increase in the ionic strength of the buffers used.
• the matrix used in step (b) can be based on an organic or inorganic material.
• organic materials it can e.g. be a native polymer, such as agarose, dextran, cellulose, starch etc, or a synthetic polymer, such as divinylbenzene, styrene etc.
• silica is a well-known and commonly used material.
• the matrix is cross-linked agarose, which is commercially available from a number of companies, such as SepharoseTM from Amersham Biosciences (Uppsala, Sweden).
• the HIC matrix used in step (b) exhibits one or more hydrophobic ligands capable of interaction with rHSA, selected from the group that consists of phenyl, butyl, such as n-butyl, octyl, such as n-octyl, etc, preferably on an agarose matrix.
• hydrophobic ligands such as ethers, isopropyl or phenyl are present on a divinyl- benzene matrix, such as SourceTM from Amersham Biosciences (Uppsala, Sweden).
• phenyl ligands on a crosslinked agarose matrix are used for step (b).
• the matrix is preferably comprised of porous beads, which can have a water content of above about 90%, preferably about 94%.
• the average particle size can e.g. be between 10 and 150 ⁇ m as measured on a wet bead, preferably below 100, such as about 90 ⁇ m.
• the ligand density on the matrix can for example be between 20 and 60, such as about 40 ⁇ mol/ml gel.
• the matrix used is Phenyl SepharoseTM 6 Fast Flow from Amersham Biosciences (Uppsala, Sweden).
• the denotation Fast Flow is used for a matrix the cross-linking of which has been optimised to give process adapted flow characteristics with typical flow rates of 300-400 cm/h through a 15 cm bed height at a pressure of 1 bar.
• This step can be performed at a pH of about 4-8, such as 6.5-7, and at a salt concentration of about 0.01 to 0.5 M, such as 0.05 to 0.2 M.
• step (c) of the present method utilises an anion exchanger, preferably a weak anion exchanger, for removing minor impurities from step (b), and especially to remove undesired compounds, such as low molecular weight pigments.
• the invention is not limited to use of any specific anion exchanger material, as long as it exhibits a sufficient amount of ligands capable of binding compounds of negative charge that are undesirable in the final rHSA product.
• the anion exchange chromatographic step can e.g. be performed at a pH of about 5.0-8.0 and a salt concentration of 0.01 to 0.2 M for removal of the impurities.
• the matrix material it may be of any organic or inorganic material, as discussed above.
• the matrix is comprised of porous beads of cross-linked agarose, such as SepharoseTM from Amersham Biosciences (Uppsala, Sweden).
• the ligands attached thereto are weak anion exchangers, the binding group of which is for example a primary or secondary amine.
• Such a binding group can be attached to the matrix e.g. via an alkyl chain with an ether group closest to the matrix.
• the literature describes many ways of attaching a binding group to a matrix via spacers, arms etc, and as mentioned above the present invention is not limited to any specific structure.
• an agarose bead having ligands according to the following formula -0-CH 2 CH 2 -N + (C 2 H 5 ) 2 H e.g. DEAE SepharoseTM from Amersham Biosciences.
• This embodiment will result in rHSA in both the bound and the unbound fraction eluted from the column which, for some applications, is satisfactory.
• an alternative embodiment which is more advantageous as far as purity of the final product is concerned is using an agarose bead having ligands that comprise two ester groups, and preferably also two hydroxyl groups.
• the binding group is then pref- erably primary amine.
• the advantage with this embodiment is that the rHSA will be present only in the fraction that is moderately bound to the matrix, resulting in a much- improved purity and operational convenience.
• An illustrative general formula for this last mentioned embodiment is presented in Fig 1, however it is to be understood that the present invention also comprises a method wherein similar structures are used in step (c).
• the present invention encompasses use also of matrices similar to the above, which are based on the same general ligand structure.
• the denotation "Gel" in the formula of Fig 1 A is understood to include any matrix, as discussed above in relation to step (b).
• a matrix comprising the above-described ligand is ButylSepharoseTM (Amersham Biosciences, Uppsala, Sweden).
• ButylSepharoseTM Amersham Biosciences, Uppsala, Sweden.
• a ligand density of 160 ⁇ moles/ml was used. Accordingly, it appears that especially advantageous results can be achieved by optimising the ligand density of the matrix.
• the first mentioned kind of anion exchangers are used, i.e.
• the preferred embodiment of the present method utilises a secondary amine as the anion-exchanging group during step (c) and a ligand density of at least about 100 ⁇ moles/ml.
• the ligand density of the anion exchanger is in the range of
• the purified rHSA can then be recovered only from the bound fraction from step (c), as compared to e.g. DEAE when it may be present in both bound and unbound fractions.
• Figure 1 A-D illustrates possible ligand structures suitable for use in the present method.
• Fig 1 A shows a cation exchanger of high salt ligand (HSL) type
• IB shows a hydrophobic interaction chromatography matrix namely Phenyl Sepharose
• Figure 2 shows the results from step (a), i.e. cation exchange chromatography of 147 mL undiluted cell culture supernatant (CCS) on a 20 mL column comprising a cation exchange matrix with the ligand structure illustrated in Fig 1 A.
• Fraction IB contains the rHSA, which is clearly separated from the impurities represented by fraction 2A.
• Figure 3 shows the results from step (b), i.e. hydrophobic interaction chromatography (HIC) of fraction 2B from Fig 2 on a 40 ml column comprising Phenyl Sepharose as il- lustrated in fig IB.
• Fraction A represents the rHSA.
• Figure 4 shows the results from step (c), i.e. anion chromatography of fraction 3 A from Fig 3 on a 40 ml column comprising the modified butyl-Sepharose as described in the experimental part below.
• the purified rHSA is in fraction 4B.
• Figure 5 shows native PAGE (8-25%) and SDS-PAGE (10-15%) analyses of the main fractions obtained using the three step method according to the invention.
• native PAGE visualised by silver staining 5 A
• SDS-PAGE visualised by silver staining 5B
• SDS-PAGE visualised by silver staining (5B) about 2 ⁇ g of protein per spot was applied.
• SDS-PAGE visualised by Coomassie staining (5C) about 10 ⁇ g of protein per spot was applied.
• control The arrows show the positions of rHSA.
• the cell culture supernatant (CCS) containing rHSA was prepared by fermenting genetically-modified P. pastoris cells for 2 weeks or more, followed by separation of the cells by filtration.
• the CCS which was green in colour, was divided into aliquots of about 200 ml and stored at -20°C until use.
• the quality of the CCS was determined by gel filtration on an analytical column of SuperdexTM 200 HR 10/30 (Amersham Biosciences, Uppsala, Sweden). This analysis gave the relative amounts of high molecular weight (HMW) and low molecular weight (LMW) impurities in the CCS as well as the approximate content of the monomeric form of rHSA.
• HMW high molecular weight
• LMW low molecular weight
• Sodium caprylate (octanoic acid, Na salt) and L-cysteine were bought from SIGMA Chemical Co. Chromatographically purified HSA from human plasma was kindly provided by I. Andersson at the plasma processing unit of Amersham Biosciences, Uppsala, Sweden. The concentration of protein in various samples was determined using the Bio- Rad Protein Assay kit (known as the Bradford method). Bovine serum albumin (BSA) was used to construct the standard curve. UN/Vis abso ⁇ tion measurements were made using a Shimadzu UV-160A recording spectrophotometer (Shimadzu Co ⁇ oration, Japan). All other chemicals used were of analytical or reagent grade.
• BSA Bovine serum albumin
• Analytical electrophoresis was performed using a Phastgel electrophoresis system and appropriate PhastGel media and buffer Strips (all from Amersham Biosciences, Uppsala, Sweden).
• the electrophoretic analyses were performed using native-PAGE (8-25%) or SDS-PAGE (non-reduced, 10-15%) gels according to the Manufacturer's recommendations.
• the amount of sample applied per spot was as follows: about 3.3 ⁇ g for native samples and 2 ⁇ g for the SDS-treated samples, both of which were stained with Silver Staining Kit (Amersham Biosciences, Uppsala, Sweden); 10 ⁇ g for the SDS-treated samples that were stained with Coomassie Brilliant Blue (CBB).
• CBB Coomassie Brilliant Blue
• step (b) Phenyl SepharoseTM 6 Fast Flow (high sub), a regular product of Amersham Biosciences, Uppsala, Sweden.
• step (c) either commercially available DEAE SepharoseTM Fast Flow (Amersham Biosciences, Uppsala, Sweden) or a modified matrix was used: Butyl SepharoseTM 6 Fast Flow (Amersham Biosciences, Uppsala, Sweden) was produced with an increased ligand density (batch U238025: 160 ⁇ mol/ml) as compared to the commercial product (20-40 ⁇ mol/ml gel).
• This modified matrix will herein be denoted "modified Butyl-Sepharose”.
• step (a) utilised a prototype matrix of HSL-type, cation-exchanger, see Fig 1 A.
• This medium was packed in a XK26/20 glass column as a thick suspension in 20% ethanol to obtain a bed volume of 40 ml. A linear flow rate of 300 cm/h was used. The packed column was washed with about 2 bed volumes of de-ionised water to elute most of the ethanol and then equilibrated with the appropriate buffer solution prior to sample application. The amount of buffer required for each of the chromatographic steps using the various media is shown in Table 1 below.
• Buffer A 25 mM sodium acetate, pH 4.5 Mix 25 mL of 1 M sodium acetate and 40 mL of 1 M acetic acid and dilute to 1 L with de-ionised water. Conductivity: about 2mS/cm at room temperature (RT).
• Buffer B 50 mM sodium phosphate, 0.1 M NaCl, 10 mM sodium caprylate, pH 7.0 Mix 155 mL 0.2 M Na 2 HP0 4 + 95 mL of 0.2 M NaH 2 P0 4 + 5.8 g of NaCl + 1.66 g sodium caprylate and dilute to 1 L with de-ionised water. Conductivity: about 16 mS/cm at RT.
• Buffer C 50 mM sodium phosphate, 0.1 M NaCl, pH 6.0 Mix 212 mL 0.2 M NaH 2 P0 4 + 38 mL of 0.2 M Na 2 HP0 4 + 5.8 g of NaCl and dilute to 1 L with de-ionised water. Conductivity: 14 mS/cm at RT.
• Buffer D 50 mM sodium phosphate, 0.2 M NaCl, pH 6.0
• Buffer E Cleaning-in-place (CIP) solution 30% isopropanol dissolve in 1 M NaOH solution.
• the CCS was heat-treated primarily to inactivate proteolytic enzymes produced during fermentation of P. pastoris. This was performed as follows: The frozen sample of CCS was thawed and 10 mM Na-caprylate was dissolved. The pH was adjusted to 6.0 and it was heated for 30 minutes in a water bath (maintained at 68°C by thermostat). The sample was cooled to room temperature and its pH adjusted to 4.5.
• step (a) If a conventional cation exchange medium, such as SP Sepharose BB, was to be used for step (a), it would have been required to dilute the CCS 2-8 times, depending on the original conductivity of the solution, with de-ionised water to reach a conductivity of about 5-10 mS/cm (approximately 0.1 M salt concentration).
• the HSL-type matrix used according to the present invention is much more tolerant to increased salt concentrations, and therefore the heat-treated CCS can normally be applied to step (a) without any further dilution, as long as the conductivity thereof is less than about 30 mS/cm.
• the partially purified rHSA obtained after the cation exchange according to step (a) (i.e. the fraction bound to the HSL-type matrix) was also heat-treated prior to step (b) as follows: The pH of the sample was adjusted to 6.0 with 1 M NaOH and cysteine was dissolved therein to a concentration of 5mM to serve as a reducing agent. This solution was then heated for 60 minutes in a water bath maintained at 60°C. The main pu ⁇ ose of this operation is to facilitate the removal of coloured substances by the HIC matrix.
• the cation-exchange medium was packed in an XK 16/20 column (packed bed volume 20 mL) and washed with 2 column volumes (CV) of Buffer A for equilibration.
• the heat-treated CCS was applied to the column via a 150 mL SuperloopTM (Amersham Biosciences, Uppsala, Sweden) at a flow rate of 300 mL/h (150 cm/h).
• the amount of rHSA applied was about 1 g (i.e. 50 mg rHSA/ml of packed gel).
• the unbound material was eluted with 3 CV of Buffer A followed by elution of the bound rHSA with 5 CV of Buffer B.
• the two fractions were pooled separately and the pH of the bound fraction was adjusted to 6.0 with a 1 M NaOH solution. The solution was then heated as described above, cooled to room temperature and further purified on a HIC column as described below. An 1 mL aliquot from each pooled fraction was saved for analytical pu ⁇ oses (i.e. to determine protein content, the A 35 o/A 28 o ra ti° and electropho- retic analysis).
• the rHSA-containing fraction from the previous step was transfe ⁇ ed to a 150 mL Su- perloopTM and applied to an XK26/20 column packed with Phenyl SepharoseTM Fast
• the bound material (containing mainly the 45 kDa degraded form of rHSA) was eluted with 2 CV of de-ionised water.
• the two fractions obtained from the previous HIC step were pooled and 1 mL aliquots from each were saved for analytical determinations (see above).
• a column (XK26/20) was packed with DEAE SepharoseTM Fast Flow or the above described modified Butyl- Sepharose to obtain a packed bed volume of 40 mL.
• Each of the packed media was washed with 2 CV of de-ionised water and then with about 5 CV of Buffer C to equilibrate them.
• the unbound fraction obtained from the HIC step was transferred to a 150 mL Superloop and applied to one or the other of the above two columns.
• the unbound fraction was eluted with 6 CV of Buffer C (from the DEAE Sepharose Fast Flow col- umn) or with 2 CV from the modified Butyl-Sepharose column.
• the bound fraction was eluted with 2 CV of a 2 M solution of NaCl (for the DEAE column) or with 5 CV of Buffer D for the modified Butyl-Sepharose column.
• the flow rate was maintained at 90 cm/h throughout.
• step (c) Since the three step purification process is based on step-wise elution, it is easily adaptable to large-scale operation.
• Use of a high ligand-ButylSepharose for step (c) results in efficient removal of LMW impurities that elute as a group in the unbound fraction.
• Use of the high ligand-ButylSepharose also results in a better A 350 /A 28 o ratio than use of the DEAE-type of matrix for step (c).

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