IMPROVED METHOD FOR PURIFICATION OF RECOMBINANT COPPER/ZINC (CU-ZN) SUPEROXIDE DISMUTASE FROM BACTERIA OR EUCARYOTIC CELLS

Abstract: The subject invention provides a method for recovering a solution containing purified, enzymatically active Cu-Zn superoxide dismutase or a polypeptide analog thereof having substantially the same amino acid sequence as, and the biological activity of, naturally-occurring Cu-Zn superoxide dismutase from a composition which comprises cells containing Cu-Zn superoxide dismutase or a polypeptide analog thereof. The invention also provides a method of increasing the yield of recovered solutions having an increased concentration of b isoform of an enzymatically-active polypeptide analog of Cu-Zn superoxide dismutase from a composition which comprises cells containing a, b and c isoforms of the polypeptide analog.

Description: _χ_ IMPROVED METHOD FOR PURIFICATION OF RECOMBINAMT COPPER/ZINC (Cϋ-ZN) SUPEROXIDE DISMUTASE FROM BACTERIA OR EϋCARYOTIC CELLS Background of the Invention Throughout this application, various publications are referenced by Arabic numerals within parentheses. Full citations for these references may be found at the end of the specification immediately preceding the claims. The disclosures of these publications in their entireties are hereby incorporated by reference in this application in order to more fully describe the state of the art to which this invention pertains. This invention provides an improved method for purification of recombinant copper/zinc (Cu-Zn) superoxide dismutase from bacteria or eucaryotic cells. The clinical potential of the enzyme superoxide dismutase (SOD) is enormous. The scientific literature suggests that SOD could be useful in a wide range of clinical applications. These include prevention of oncogenesis and tumor growth and reduction of cytotoxic and cardiotoxic effects of anticancer drugs, anti-in¬ flammatory protection of ische ic tissues and protec¬ tion of spermatozoa. In addition, there is great in¬ terest in studying the effect of SOD on the aging pro¬ cess. There is also evidence that SOD can be used to prevent the damage sometimes caused to newborn babies by hyperbaric oxygen treatment. In particular, there is increasing evidence that "re¬ perfusion injury" caused by oxygen free radicals (prob¬ ably generated from xanthine oxidase) accounts for much of the damage caused by a period of ischemia in many organs of the body such as the spinal cord, intestine, skin, heart, lung, pancreas or kidney (11) . In vitro and animal experiments using human recombinant copper/zinc superoxide dismutase (hCu-Zn SOD analog) , produced as described in coassigned U.S. Patent No. 0 4,742,004, issued May 2, 1988, and in corresponding European patent application publication no. 0173280, published May 5, 1986, have demonstrated the efficacy of SOD in reducing reperfusion injury e.g., in the spinal cord of dogs (12), in the hearts of rabbits 5 (13) , and in the hearts of dogs (14) . The superoxide anion (0 * 2 ) produced on reperfusion of ischemic tissue and also in certain inflammatory condi¬ tions is highly toxic to macromolecules, e.g., 0 * may 0 react with lipid hydroperoxides to form alkoxy radicals in phospholipid membranes. The superoxide anion (0 * -) is removed in normal biolog¬ ic tissue by the dismutation reaction: 25 20- 2 + 2H + > H 2 0 2 + 0 2 The above reaction can proceed spontaneously or it can be catalyzed by SOD which increases the rate of inter- on . 9 .cellular dismutation by a factor of 10 . SOD is a ubiquitous mammalian enzyme. In the presence of normal intercellular concentrations of catalase and peroxi- dase, SOD is responsible for the "scavenging" of oxy¬ gen-free radicals, thereby serving as a normal biolog- 35 ical defense against the formation and accumulation of reduced oxygen intermediates. Copper/zinc (Cu-Zn) SOD has been demonstrated in virtu¬ ally all eucaryotic organisms. Human Cu-Zn SOD-1 is a dimeric metallo-protein composed of identical non-co- valently linked subunits, each having a molecular weight of 16,000 daltons and containing one atom of copper and one of zinc. Each subunit is composed of 153 amino acids whose sequence has been established. Endogenous SOD is present in tissues in limited amounts and when high levels of superoxide anion are produced, the amount of SOD present is not sufficient. Thus there is a need for clinical administration of exo¬ genous SOD. However, exploration of the therapeutic potential of human SOD-1 (EC 1.15.1.1) has been limited mainly due to its scarce availability. To overcome this problem we inserted an SOD clone containing the entire coding region of human SOD-1 described by Groner and his colleagues (15) into efficient bacterial expression vectors which express hSOD analog at high levels; this has been described in coassigned U.S. Patent No. 4,742,004 described above. Using this method, we have produced recombinant hSOD analog which differs from authentic human SOD (from blood) in that the amino terminus alanine is not acetylated. The amino acid sequence of the bacterial-produced SOD analog does not contain a ethionine residue at its N- terminus. The recombinant hSOD analog thus produced is very pure. However, it was desirable to produce even higher levels of purity. An improved chromatography method which produces an even higher degree of purification. surprisingly achieved without loss of yield, is described in this application. A novel "exchange" procedure which increases the yield of pure recombinant SOD produced, without loss of purity, is also described. European patent application publication no. 0180964, published May 14, 1986 and assigned to Ube Industries Limited, discloses the production of human Cu-Zn SOD in Escherichia coli and partial purification of the hSOD produced. This method was done on small scale only (20 g of wet cells) . No details of purification achieved are given apart from specific activity. There is no disclosure of the extent of contamination by Escherichia coli proteins and endotoxins. European patent application publication no. 0138111, published April 24, 1985 and assigned to Chiron Corporation, discloses the production of hSOD in both Escherichia coli and yeast, but no methods of produc¬ tion of hSOD from- crude cell lysates are disclosed. European patent application publication no. 0164556, also assigned to Chiron, discloses the production of a yeast expression plasmid for hSOD, but again no methods of purification of hSOD are disclosed. Hallewell et al. from Chiron Corporation (16) disclose production of recombinant SOD but merely state that the recombinant SOD was purified to homogeneity by conventional means after lysing the yeast cells with glass beads and pelleting the cell debris. Hallewell et al. from Chiron Corporation (17) disclose a method of purifying recombinant SOD from yeast cells. This method gives no detail of the yield of hSOD produced, nor its degree of purification; it is also a purification method of hSOD analog from yeast cells and not Escherichia coli cells. Takahara et al. (18) disclose the secretion by Escherichia coli cells of hSOD into the periplasmic space. This is a very small-scale procedure (16 ml medium) , and the SOD produced is not purified; the Escherichia coli cells are simply subjected to osmotic shock, and subcellular fractions are analyzed by SDS- PAGE. Summary of the Invention The subject invention provides a method for recovering a solution containing purified, enzymatically active Cu-Zn superoxide dismutase or a polypeptide analog thereof having substantially the same amino acid se¬ quence as, and the biological activity of, naturally- occurring Cu-Zn superoxide dismutase from a composition which comprises cells containing Cu-Zn superoxide dismutase or a polypeptide analog thereof comprising: (a) treating the composition so as to separate soluble proteins present in the cells from whole cells, cellular debris and insoluble proteins so as to obtain a solu¬ tion containing such soluble proteins; (b) treating the resulting solution containing the soluble proteins with a second solu¬ tion containing a salt at a concentration such that the soluble proteins other than Cu-Zn superoxide dismutase or the polypeptide analog thereof present in the solution containing the soluble proteins are rendered capable of binding to an appropriate hydrophobic substance; (c) contacting the then-resulting solution containing the soluble proteins with an appropriate hydrophobic substance so as to bind the soluble proteins other than Cu-Zn superoxide dismutase or the polypeptide analog thereof present in such solution to the hydrophobic substance and thus sepa¬ rate such other proteins from the Cu-Zn superoxide dismutase or polypeptide analog thereof; and (d) recovering the resulting solution contain¬ ing purified, enzymatically active Cu-Zn superoxide dismutase or polypeptide analog thereof. Also provided is a method of increasing the yield of recovered solutions having an increased concentration of b isoform of an enzymatically-active polypeptide analog of Cu-Zn superoxide dismutase from a composition which comprises cells containing a, b and c isoforms of the polypeptide analog which comprises: (a) treating the composition so as to separate soluble proteins present in the cells from whole cells, cellular debris and insoluble proteins so as to obtain a solu¬ tion containing such soluble proteins, including the a, b and c isoforms; (b) treating the resulting solution contain¬ ing the soluble proteins so as to produce three separate solutions, each of which has an increased concentration of one of either of the a, b or c isoform; (c) recovering the separate solution which has an increased concentration of the b isoform; (d) combining the separate solution which has an increased concentration of the a isoform with the separate solution which has an increased concentration of the c isoform; (e) treating the resulting combined solution so as to produce a solution which has an increased concentration of the b isoform; and (f) recovering the then-resulting solution which has an increased concentration of the b isoform. Brief Description of the Figures Figure 1. Plasmid pSOD^Tll -a high level expressor of Cu-Zn SOD analog protein The construction of plasmid has been fully described in co-assigned, copending European patent application publication no. 0173280, published on May 5, 1986 (corresponding to U.S. patent application Serial No. 644,105, filed August 27, 1984, issued May 2, 1988 as U.S. Patent No. 4,742,004), and pSODø j TIl has been deposited in the ATCC under Accession No. 53468. This plasmid contains the following elements: (a) the origin of replication of plasmid pBR322; (b) the Tet gene of plasmid pBR322 in counter¬ clockwise orientation; and (c) in clockwise orientation and in 5' to 3' order the λ L ° T promoter/operator region, the 0-lac- tamase promoter and ribosomal binding site, and the coding sequence for hSOD analog. Plasmid pSOD^Tll is a high level expressor of hSOD analog protein under the control of the strong leftward promoter of bacteriophage λ (P_) which is thermoin- ducibly controlled by the cI857 temperature sensitive repressor situated on the host chromosome. Figure 2. Preparative ion exchange chromatography of hSOD analog solution A. DEAE-Sepharose FF column (15 x 37 cm) This column may be used in the ion-exchange chromatog¬ raphy step of the method for purifying hSOD analogs. Application of hSOD analog solution to the DEAE-Sepha¬ rose FF column and elution conditions on the column are as described in the text (Example 3E) . D 1, DW2, D 3, DW4 = eluate from washing step (20 mM tris - 25 mM NaCl at pH = 7.8) First arrow step change in eluent to 20 mM tris - 80 mM NaCl at pH - 7.8 DMP (black peak) main hSOD analog peak (collected and saved) Second arrow = addition of high salt solution for regeneration of the column The grey peaks (DW4 and DEI) can be collected and treated as described in Example 4B. B. CM-Sepharose FF column (15 x 37 cm) The CM-Sepharose FF column may also be used in the ion- exchange chromatography step of the method for purify¬ ing hSOD analog. Application of the hSOD analog solu¬ tion to the CM-Sepharose FF columns and solution condi¬ tions on the column are as described in the text (Exam¬ ple 3F) . CW1, CW2 eluate from washing steps (40 mM sodium acetate pH 4.8) First arrow step change in eluent to 85 mM sodium acetate pH 4.8 CMP (black peak) main hSOD analog peak which is collected and saved Second arrow = addition of high salt solution (0.5 M NaCl) The grey peaks C 2, CEl and CE2 can be collected and treated as described in Example 4B. Figure 3. Analytical fast performance anion exchange chromatography on Mono Q (HR 5/5) column of isolated isoforms of hSOD analog and mixtures thereof A: isolated isoform a of hSOD analog (2.5 mg/ml) B: isolated isoform c of hSOD analog (2.5 mg/ml) C: mixture (1:1) of isolated isoforms a (5 mg/ml) and c (5 mg/ml) without incubation. D, E and F: the same mixture of isolated isoforms a and c as shown in panel C but incubated at 37"C for 30, 60 and 240 minutes respectively. The samples were dissolved in buffer A (25 mM bis-Tris- HC1, pH 7.0) and 100 μl of the solution was injected onto an analytical fast performance Mono Q (HR 5/5) column. The elution was performed with gradient of 100% buffer A to 100% buffer B (same as buffer A but containing 0.25 M NaCl); flow rate 1.5 ml/min; chart speed 1.0 cm/min; detection at 280 nm and 0.2 AUFS (AUFS = absorbance ultraviolet full scale) . Figure 4. Time course of production of isoform b on incubation of a mixture of isolated iso¬ forms a and c A mixture of isolated isoforms a and c as described in Figure 3C was incubated at 37*C for various time inter¬ vals from 0 - 240 minutes. The analysis on an analyti¬ cal Mono Q (HR 5/5) column was described in Figure 3. o = incubation at 37"C • = incubation at 50*C Figure 5. Analytical fast performance anion exchange chromatography on Mono Q (HR 5/5) column of isolated Mono Q resolved isoform b of hSOD analog protein Isoform b was preparatively isolated from a mixture of isolated isoforms a and c, following incubation for 24 hours at 37 β C, by preparative fast performance chroma¬ tography on Mono Q (HR 10/10) column. The isoform b peak eluted was collected and immediately frozen at -20°C. The samples were defrosted and diluted 1:3 with H-O (yielding approximately 0.45 mg/ml). 200 μl of the solution was injected onto an analytical fast perfor¬ mance anion exchange Mono Q (HR 5/5) column. The elu¬ tion was performed as described in Figure 3, except the detection at 280 nm was 0.05 AUFS. A: isoform b injected immediately after defrost¬ ing. B: isoform b incubated at 37'C for one hour. C: isoform b incubated at 37*C for 24 hours. Figure 6. Analytical fast performance anion exchange chromatography on Mono Q (HR 5/5) column of a sample from the mixture of the DW4, DEI, C 2, CEl and CE2 fractions (Figure 2) de¬ rived from DEAE-Sepharose I and CM Sepharose I (steps E and F in scheme I) in the process preparation of hSOD analog A: the mixture was injected immediately after the relative fraction mixing. B: the mixture was injected after incubation for two hours at 37'C and 20 hours at room temper¬ ature. In both cases 50 μl (approximately 11 mg/ml) of sample was injected. The elution was performed as described in Figure 3. Figure 7. Preparative ion-exchange chromatography of the hSOD analog preparation produced by the exchange procedure The mixture of the DW4, DEI, CW2, CEl and CE2 hSOD analog fractions derived from Example 3 steps E and F (the grey peaks in Figure 2) were treated as described in Example 4 (exchange procedure including dialysis and incubation steps) . A: after the incubation step of the exchange pro¬ cedure, 4 ml of mixture was applied to the DEAE Sepharose column. Application and elution conditions were the same as Figure 2. The peak shaded black (DMP fraction) was collected and dialyzed against 40 mM sodium acetate pH 4.8. B: the sample after dialysis was applied to a CM Sepharose column. Application and elution conditions are the same as Figure 2. The peak shaded black (CMP) containing hSOD analog is collected, saved and added to the bulk preparation. Figure 8: Analytical fast performance anion exchange chromatography on Mono Q (HR 5/5) column of the fractionated DEAE and CM Sepharose resolved hSOD analog A: a sample from DMP fraction derived from DEAE- Sepharose column (black shaded peak in Figure 7A) was diluted 1:4 in 20 mM bis Tris-HCl, pH 7.0, and 400 μl were injected onto a Mono Q (HR 5/5) column. B: a sample from CMP fraction derived from CM- Sepharose column (black shaded peak in Figure 7B) was adjusted to pH 7.8 with 0.1 M NaOH, and 150 μl were injected onto a Mono Q (HR 5/5) column. The elution conditions of both the columns were as described in Figure 3. Detailed Description of the Invention The plasmids designated pSOD^Tll, pSOD9MAX-12, pSODα2, pMF2005, pMF5534, and pNd-SOD.^-12 were deposited pursuant to, and in satisfaction of, the requirements of the Budapest Treaty on the International Recognition of the Deposit of Microorganisms for the Purposes of Patent Procedure with the American Type Culture Collection (ATCC) , 12301 Parklawn Drive, Rockville, Maryland 20852 under ATCC Accession Nos. 53468, 67177, 39786, 67362, 67703, and 53166, respectively. The Escherichia coli strains designated MC1061, A1645 (containing the plasmid pNd-SOD- jjj -12) , A1637 (contain¬ ing the plasmid pTV104(2)), A2097 (containing the plasmid pSOD 2) , A4200 (containing the plasmid pHG44) , A4255 (containing the plasmid pSOD3MAX-12) , A4048 (con¬ taining the plasmid pHG44) , S^930 (containing the plasmid pMF5534) , 3110 (containing the plasmid pMFS929) , S-^732 (containing the plasmid pMF2005) , S^540 (containing the plasmid pJBF5401) , and S^930 (con¬ taining the plasmid pEFF920) were also deposited pursuant to, and in satisfaction of, the requirements of the Budapest Treaty on the International Recognition of the Deposit of Microorganisms for the Purposes of Patent Procedure with the American Type Culture Collection (ATCC) , 12301 Parklawn Drive, Rockville, Maryland 20852 under ATCC Accession Nos. 67361, 53166, 39384, 39786, 53218, 67177, 53217, 67703, 67705, 67362, 67359, and 67706, respectively. The subject invention provides a method for recovering a solution containing purified, enzymatically active Cu-Zn superoxide dismutase or a polypeptide analog thereof having substantially the same amino acid se- quence as, and the biological activity of, naturally- occurring Cu-Zn superoxide dismutase from a composition which comprises cells containing Cu-Zn superoxide dismutase or a polypeptide analog thereof comprising: (a) treating the composition so as to separate soluble proteins present in the cells from whole cells, cellular debris and insoluble proteins so as to obtain a solu¬ tion containing such soluble proteins; (b) treating the resulting solution containing the soluble proteins with a second solu¬ tion containing a salt at a concentration such that the soluble proteins other than Cu-Zn superoxide dismutase or the polypeptide analog thereof present in the solution containing the soluble proteins are rendered capable of binding to an appropriate hydrophobic substance; (c) contacting the then-resulting solution containing the soluble proteins with an appropriate hydrophobic substance so as to bind the soluble proteins other than Cu-Zn superoxide dismutase or the polypeptide analog thereof present in such solution to the hydrophobic substance and thus sepa¬ rate such other proteins from the Cu-Zn superoxide dismutase or polypeptide analog thereof; and (d) recovering the resulting solution contain¬ ing purified, enzymatically active Cu-Zn superoxide dismutase or polypeptide analog thereof . The Cu-Zn superoxide dismutase or polypeptide analog thereof may be human Cu-Zn superoxide dismutase or polypeptide analog thereof, for example, the non-acety- lated polypeptide analog of human Cu-Zn superoxide dismutase. It should be understood by a person skilled in the art to which the subject invention pertains that any eucaryotic cell which produces naturally-occurring Cu- Zn superoxide dismutase or polypeptide analogs thereof may be used. The eucaryotic cell may be, for example, a yeast cell or a mammalian cell. The method can also use a bacterial cell. The bacterial cell in which the Cu-Zn superoxide dismutase or polypeptide analog thereof is produced may be any bacterium in which a DNA sequence encoding the Cu-Zn superoxide dismutase or polypeptide analog there¬ of has been introduced by recombinant DNA techniques. Of course, the bacteria must be capable of expressing the DNA sequence and producing the desired protein. Presently preferred bacterial cells comprise cells of an Escherichia coli strain. The bacteria may be any strain including auxotrophic, prototrophic and lytic strains; F and F~ strains; 857 strains harboring the cl repressor sequence of the λ prophage; and strains deleted for the deo repressors or the deo gene. Examples of wild type Escherichia coli strains which may be used are the prototrophic strain designated ATCC Accession No. 12435 and auxotrophic strain MC1061 (ATCC Accession No. 67361) . Examples of Escherichia coli strains harboring the λ 8 -_"7 cl represser sequence which may be used are the auxotrophs A1645 containing plasmid pNd-SOD NN ~12 (ATCC Accession No. 53166) , A1637 containing plasmid pTVl04(2) (ATCC Accession No. 39384), and A2097 con¬ taining plasmid pSODα2 (ATCC Accession No. 39786) ; and the prototrophs A4200 containing plasmid pHG44 (ATCC Accession No. 53218) and A4255 containing plasmid pSODøMAX-12 (ATCC Accession No. 67177) . Examples of lytic Escherichia coli strains which may be used include Escherichia coli strain A4048 containing plasmid pGH44 (ATCC Accession No. 53217) . Examples of F " strains which may be used to express DNA encoding superoxide dismutase are Escherichia coli strain S 930 (F-) containing plasmid pMF 5534 deposited under ATCC Accession No. 67703 and Escherichia coli strain W3110 (F~) containing plasmid pMFS 929 deposit¬ ed under ATCC Accession No. 67705; strain 3110 (F~) was obtained from the Escherichia coli Genetic Stock Center (C.G.S.C.), Department of Biology, Yale Univer¬ sity, P.O. Box 6664, New Haven, Connecticut, as C.G.S.C. strain No. 4474. Examples of Escherichia coli strains deleted for the deo gene or deo repressors which may be used include Sφ732 containing plasmid pMF 2005 (ATCC Accession No. 67362) , S ? .540 containing plasmid pJBF 5401 (ATCC Acces¬ sion No. 67359) and S^930 containing plasmid pEFF 920 (ATCC Accession No. 67706) (see European Patent Appli¬ cation Publication No. 0303972, published February 22, 1989) . The bacterial cell may contain the DNA sequence encod¬ ing the superoxide dismutase analog in the body of a vector DNA molecule such as a plasmid. The vector or plasmid is constructed by recombinant DNA techniques, known to those skilled in the art, to have the sequence encoding the superoxide dismutase incorporated at a suitable position in the molecule. The plasmids used for production of superoxide dismutase can harbor a variety of promoters including but not limited to λ promoter or deo promoters. Among the plasmids which may be used for production of superoxide dismutase are the following: Plasmid Deposited In pSOD9 1 T 11 Escherichia coli A4255 (ATCC Accession No. 53468) pSODj9MAX-12 Escherichia coli A4255 (ATCC Accession No. 67177) pSODα2 Escherichia coli A2097 (ATCC Accession No. 39786) PMF2005 Escherichia coli Sd>732 (ATCC Accession No. 67362) PMF5534 Escherichia coli S^930 (ATCC Accession No. 67703) pNd-SOD j ^-12 Escherichia coli A1645 (ATCC Accession No. 53166) The treatment of the composition which comprises cells containing Cu-Zn superoxide dismutase or a polypeptide analog thereof so as to separate soluble proteins present in the cells from whole cells, cellular debris and insoluble proteins may comprise treating the composition so as to disrupt the cells and obtain a cellular extract therefrom, and then subjecting the resulting cellular extract to centrifugation so as to obtain the solution containing the soluble proteins. The cell walls of cultured bacterial cells may be dis¬ rupted by any of a number of methods known to those skilled in the art including, but not limited to, high shear mixing, sonication, mechanical disruption, explo¬ sion by pressure, osmotic shock, etc. It should also be understood that the cells may secrete the Cu-Zn superoxide dismutase or polypeptide analog thereof into the medium, such that no cell disruption is required. The cells may then be removed by centrifugation or other means known in the art and the medium used directly for purification of the Cu-Zn superoxide dismutase or polypeptide analog thereof. The disrupted cell suspension (if disruption is re¬ quired) may then be centrifuged or separated by other means known to those skilled in the art in order to recover a supernatant containing soluble Cu-Zn superoxide dismutase or polypeptide analog thereof. In order to precipitate cellular components not con¬ taining the superoxide dismutase analog or naturally- occurring superoxide dismutase, the superoxide dismutase solution may subsequently be heated and cooled. The solution containing soluble proteins, including Cu- Zn superoxide dismutase or polypeptide analog thereof, is treated with a second solution containing a salt at a concentration such that the soluble proteins other than Cu-Zn superoxide dismutase or the polypeptide analog thereof present in the solution containing the soluble proteins are rendered capable of binding to an appropriate hydrophobic substance. The salt may be any suitable salt known in the art. Suitable salts which may be used include sodium sulphate and sodium chloride, although ammonium sulphate is preferred. Other salts which may be used include potassium chloride and ammonium acetate. Any combination of the foregoing salts may also be used. The final salt concentration depends on, and is charac¬ teristic of, the salt which is used. For ammonium sulphate, the final salt concentration varies from 1.2- 1.9M and is preferably 1.6M; for sodium sulphate, the concentration range may vary between 0.9-1.25M, prefer¬ ably 1.0M; and for sodium chloride the range is 3.0- 4.5M, preferably 4.0M. Optimal salt concentration ranges for other salts may be readily ascertained by those skilled in the art. The appropriate hydrophobic substance preferably com¬ prises phenyl Sepharose. The appropriate hydrophobic substance may also comprise a suitable resin having a phenyl functional group, preferably a polysaccharide resin having phenyl groups present thereon. The con¬ tact with the hydrophobic substance preferably is ef¬ fected by passing the solution containing the soluble proteins through a column containing the hydrophobic substance. Any hydrophobic column may be used, preferably phenyl- Sepharose. The functional group may be phenyl, benzyl, octyl, butyl and the matrix may be any of those dis¬ cussed below. The soluble proteins other than Cu-Zn superoxide dismutase or the polypeptide analog thereof in the solution are bound to the hydrophobic substance in the column, thus separating the Cu-Zn superoxide dismutase or polypeptide analog thereof from the other soluble proteins. The resulting solution containing purified, enzymatically active Cu-Zn superoxide dismutase or polypeptide analog thereof is then recovered. In one preferred embodiment, the method described above for recovering a solution containing purified, enzymat¬ ically active Cu-Zn superoxide dismutase or a polypeptide analog thereof further comprises the fol¬ lowing steps prior to the treatment with a second solu¬ tion containing a salt: The resulting solution containing the soluble proteins, after its separation from whole cells, cellular debris and insoluble proteins, is treated so as to obtain a more concentrated solution containing purified Cu-Zn superoxide dismutase or polypeptide analog thereof. Preferably, the treatment of the solution to obtain a more concentrated solution of purified Cu-Zn superoxide dismutase or polypeptide ana¬ log thereof comprises ultrafiltration. The ultrafiltration techniques are known to those skilled in the art. The exact filtration conditions, such as permeate flow rate and filter cut-off range are not critical and may be readily ascertained by a person skilled in the art of protein purification. Various sys¬ tems which may be used include, but are not limited to, a PUF-100 Unit and a Pellicon Cas¬ sette System (Millipore) . 10 Ultrafiltration may be further conducted using a smaller molecular weight cut-off. It should be understood that the particular molecular weight cut-off used or the order used for vari¬ ous molecular weight cut-off filters is not 15 critical to an understanding of the invention. The Cu-Zn superoxide dismutase present in the more concentrated solution may comprise a, b and c isoforms of Cu-Zn superoxide dismutase or 20 polypeptide. analog thereof. The resulting more concentrated solution of purified Cu-Zn superoxide dismutase or polypeptide analog thereof is then treated so as to produce three separate solutions, each of which has an in¬ 25 creased concentration of one of either of the a, b or c isoform. Preferably, the treatment of the concentrated solution comprises anion exchange chromatogra¬ 30 phy. 35 The then-resulting solution which has an in¬ creased concentration of the b isoform of puri¬ fied Cu-Zn superoxide dismutase or polypeptide analog thereof is subjected to cation exchange chromatography so as to produce three further separate solutions, each of which has a further increased concentration of one of either of the a, b or c isoform. The then-resulting solution which has a further 10 increased concentration of the b isoform of purified Cu-Zn superoxide dismutase or polypeptide analog thereof is then treated so as to further purify the Cu-Zn superoxide dismutase or polypeptide analog thereof con¬ 15 tained therein prior to treating the resulting solution containing the further purified Cu-Zn superoxide dismutase or polypeptide analog thereof with a second solution containing a salt. 20 Preferably, the treatment of the then-resulting solution comprises strong anion exchange chro¬ matography, preferably by passing the then- resulting solution through a column of a 25 polysaccharide resin having quaternary amino functional groups present thereon. Examples of a strong anion exchange column which may be used include Q-Sepharose and QAE 30 Sephadex A-25. Functional groups may be any quaternary amino group such as quaternary amino ethyl and the matrix may be any of those dis¬ cussed below. 35 Furthermore, the then-resulting solutions which have an increased or further increased concentration of one of either of the a or c isoform may be treated so as to increase the concentration of b isoform and reduce the concentration of a isoform and c isoform of the polypeptide analog contained in each of the solutions prior to treating the then-resulting solution containing the further purified Cu-Zn superoxide dismutase or polypeptide analog 10 thereof with a second solution containing a salt in step (b) . Preferably, the treatment of the then-resulting solutions prior to step (b) comprises: 15 (a) combining the separate solution which has an increased concentration of the a isoform with the separate solution which has an increased concentration of the c 20 isoform; (b) treating the resulting combined solution so as to produce a solution which has an increased concentration of the b isoform; 25 (c) recovering the then-resulting solution which has an increased concentration of the b isoform. 30 Preferably, the treatment of the resulting combined solution so as to produce a solution which has an increased concentration of the b isoform comprises incubation. 35 The solutions with an increased concentration of the b isoform from step (b) can further be treated so as to purify the b isoform present in the solutions. Preferably, the treatment of the solution comprises ion exchange chromatography. The ion exchange chromatography may comprise anion exchange chromatography and further comprise cation exchange chromatography. In one preferred embodiment, after treating the resulting solution with a second solution containing a salt and contacting the then-resulting solution with an appropriate hydrophobic substance, the resulting solution containing purified, enzymatically active Cu- Zn superoxide dismutase or polypeptide analog thereof is recovered. The recovered resulting solution may then be treated so as to obtain a more concentrated solution containing purified Cu-Zn superoxide dismutase or polypeptide analog thereof. Preferably, the treatment of the recovered resulting solution to obtain a more concentrated solution of purified Cu-Zn superoxide dismutase or polypeptide analog thereof comprises ultrafiltration. The resulting more concentrated solution containing purified Cu-Zn superoxide dismutase or polypeptide analog thereof may then be treated so as to obtain a solution containing more purified Cu-Zn superoxide dismutase or polypeptide analog thereof. Preferably, the treatment of the resulting solution comprises strong anion exchange chromatography, prefer- ably by passing the resulting solution through a column of a polysaccharide resin having quaternary amino func¬ tional groups present thereon. In another preferred embodiment, after treating the resulting solution with a second solution containing a salt and contacting the then-resulting solution with an appropriate hydrophobic substance, the resulting solu¬ tion containing purified, enzymatically active Cu-Zn superoxide dismutase or polypeptide analog thereof is recovered and treated so as to obtain a solution con¬ taining more purified Cu-Zn superoxide dismutase or polypeptide analog thereof. Preferably, the treatment of the resulting solution comprises strong anion exchange chromatography, prefer¬ ably by passing the resulting solution through a column of a polysaccharide resin having quaternary amino func¬ tional groups present thereon. In yet another preferred embodiment, the resulting solution containing the soluble proteins, after its separation from whole cells, cellular debris and insol¬ uble proteins, is treated so as to purify the Cu-Zn superoxide dismutase or polypeptide analog thereof contained therein prior to treating the resulting solu¬ tion containing the soluble proteins with a second solution containing a salt. Preferably, the treatment of the resulting solution comprises strong anion exchange chromatography, prefer¬ ably by passing the resulting solution through a column of a polysaccharide resin having quaternary amino func¬ tional groups present thereon. In another preferred embodiment, the method described above for recovering a solution containing purified, enzymatically active Cu-Zn superoxide dismutase or a polypeptide analog thereof further comprises treating the resulting solution containing the soluble proteins, after its separation from whole cells, cellular debris, and insoluble proteins (the soluble proteins including Cu-Zn superoxide dismutase or polypeptide analog there¬ of comprising a, b and c isoforms of Cu-Zn superoxide dismutase or polypeptide analog thereof) so as to in¬ crease the concentration of b isoform and reduce the concentration of a isoform and c isoform of the polypeptide analog contained in the solution prior to treating the resulting solution containing the soluble proteins with a second solution containing a salt. Preferably, the treatment of the resulting solution containing the soluble proteins prior to treatment with a second solution containing a salt comprises: (a) treating the resulting solution containing the soluble proteins so as to produce three separate solutions, each of which has an increased concentration of one of either of the a, b, or c isoform: (b) recovering the separate solution which has an increased concentration of the b isoform; (c) combining the separate solution which has an increased concentration of the a isoform with the separate solution which has an increased concentration of the c isoform; (d) treating the resulting combined solution so as to produce a solution which has an increased concentration of the b isoform; and (e) recovering the then-resulting solution which has an increased concentration of the b isoform. Preferably, the treatment of the resulting combined solution so as to produce a solution which has an in¬ creased concentration of the b isoform comprises incu¬ bation. Furthermore, the treatment of the resulting solution so as to produce three separate solutions preferably com¬ prises ion exchange chromatography. It will be appreciated by those skilled in the art that any number of ion exchange columns may be used includ¬ ing weak anion exchange, cation exchange or so-called "strong anion" exchange columns. In addition, it will be appreciated by one skilled in the art that the pro¬ tein purification using ion exchange chromatography may involve applying a high concentration of solution through an anion exchange followed by a cation ex¬ change, or a cation exchange followed by an anion ex¬ change or an anion exchange followed by an anion ex¬ change or a cation exchange followed by a cation ex¬ change. Moreover, only one exchange, i.e., either cation or anion, may be required. The anion exchange of this method and the method dis¬ closed above may involve, for example, a DEAE Sepharose fast-flow column and the cation exchange may involve, for example, a CM Sephadex or S-Sepharose column. Weak anion exchange columns used usually have a tertia¬ ry amine, e.g., diaminoethyl, as a functional group, although amino ethyl may also be used. The matrix may be based on inorganic compounds, synthetic resins, polysaccharides or organic polymers. Examples of ma¬ trices which may be used include agarose, cellulose, trisacryl, dextran, glass beads, oxirane acrylic beads, acrylamide, agarose/polyacrylamide copolymer (Ultragel) and hydrophobic vinyl polymer (Fractogel) . Examples of cation columns which may be used instead of carboxymethyl (CM) Sepharose fast flow include CM Sephadex or S Sepharose columns. Functional groups which may be used include carboxymethyl, phospho groups or sulphonic groups such as sulphopropyl. Any of the matrices discussed above for weak anion exchange col¬ umns may be used in cation exchange columns as well. Additionally, the solution with an increased concentra¬ tion of the b isoform which is recovered can further be treated so as to purify the b isoform present in the solution. Preferably, the treatment of the solution comprises ion exchange chromatography. The ion ex¬ change chromatography may comprise anion exchange chro¬ matography, and may further comprise cation exchange chromatography. Using the invention described above, the contaminating Escherichia coli or bacterial proteins and endotoxins are reduced by about one-thousand fold. While not wishing to be bound by theory, it appears that the critical step leading to reduction in contami¬ nating ECP is the hydrophobic separation step following the optional anion and cation exchange chromatography steps. This hydrophobic separation step is novel. In this hydrophobic separation step, it is the desired superoxide dismutase protein which passes through the hydrophobic column while undesirable Escherichia coli proteins are absorbed. The use of such a step in the purification procedure surprisingly and unexpectedly leads to a marked decrease in the level of contamining Escherichia coli proteins. The invention also provides a novel method involving subunit exchange for increasing the yield of recovered solutions having an increased concentration of b isoform of an enzymatically-active polypeptide analog of Cu-Zn superoxide dismutase from a composition which comprises cells containing a, b and c isoforms of the polypeptide analog which comprises: (a) treating the composition so as to separate soluble proteins present in the cells from whole cells, cellular debris and insoluble proteins so as to obtain a solu¬ tion containing such soluble proteins, including the a, b and c isoforms; (b) treating the resulting solution containing the soluble proteins so as to produce three separate solutions, each of which has an increased concentration of one of either of the a, b or c isoform; (c) recovering the separate solution which has an increased concentration of the b isoform; (d) combining the separate solution which has an increased concentration of the a isoform with the separate solution which has an increased concentration of the c isoform; (e) treating the resulting combined solution so as to produce a solution which has an increased concentration of the b isoform; and (f) recovering the then-resulting solution which has an increased concentration of the b isoform. The rationale behind this procedure is that a certain percentage of Cu-Zn superoxide dismutase protein is normally discarded during the purification process and in particular during the cation and anion exchange chromatography steps discussed above. More particular¬ ly, various isoforms of human superoxide dismutase, i.e., the a and c isoforms, contain high amounts of contaminating Escherichia coli proteins (ECP) and therefore peaks containing these isoforms are discard¬ ed, along with the ECP after the chromatography step. Peaks containing the b isoform are kept for further processing. According to the protocol of this inven¬ tion, peaks containing predominantly a and c isoforms are combined and incubated to form b isoforms which may then be used for further purification downstream. Preferably, the treatment of the resulting combined solution so as to produce a solution which has an in¬ creased concentration of the b isoform comprises incu¬ bation. The polypeptide analog of Cu-Zn superoxide dismutase may be a polypeptide analog of human Cu-Zn superoxide dismutase. Preferably, the treatment of the composition so as to separate soluble proteins present in the cells from whole cells, cellular debris and insoluble proteins comprises treating the composition so as to disrupt the cells and obtain a cellular extract therefrom, and then subjecting the resulting cellular extract to cen¬ trifugation so as to obtain the solution containing the soluble proteins. Preferably, the treatment of the resulting solution containing the soluble proteins so as to produce three separate solutions • comprises ion exchange chromatogra¬ phy. In a preferred embodiment, the method further comprises treating the solution with an increased concentration of the b isoform from step (c) or (f) so as to purify the b isoform present in the solution. Preferably, the treatment of the solution comprises ion exchange chro¬ matography. The ion exchange chromatography may com¬ prise anion exchange chromatography and may further comprise cation exchange chromatography. EXAMPLES The examples which follow are set forth to aid in un¬ derstanding the invention but are not intended to and should not be construed as to limit its scope in any way. Example 1 Detailed Description of the hSOD Expression Plasmid (pSOD^Tll) The plasmid used for production of hSOD analog is plasmid pSOD^Tll (Figure 1) . The construction of plasmid pSOD^Tll has been fully described in coassigned, copending European patent application publication no. 0173280, published on May 5, 1986 (corresponding to U.S. patent application Serial No. 644,105, filed August 27, 1984, issued May 2, 1988 as U.S. Patent No. 4,742,004). This plasmid contains the following elements: (a) the origin of replication of plasmid pBR322; p (b) the Tet gene of plasmid pBR322 in counter¬ clockwise orientation; and (c) in clockwise orientation and in 5' to 3' order, the λ P T O T promoter/operator region, the β- lactamase promoter and ribosomal binding site and the coding sequence for hSOD analog. Plasmid pSOD^Tll is a high level expressor of hSOD analog protein under the control of the strong leftward promoter of bacteriophage λ (p ) which is thermoin- ducibly controlled by the CI857 temperature sensitive repressor situated on the host chromosome. Plasmid pSOD^Tll was used to transform prototrophic Escherichia coli strain A4255, and the resulting bacte¬ rium is used for production of hSOD analog. Prototro¬ phic Escherichia coli strain A4255 was prepared from prototrophic Escherichia coli strain ATCC Accession No. 12435 (obtained from the ATCC collection) by trans- fection with TnlO transposon carrying defective λ pro- phage (CI857 ΔHl ΔBam N + ) and subsequently deleted for tetracycline resistance gene (carried by TnlO) . Escherichia coli strain A4255 containing plasmid pSOD^Tll was deposited in the ATCC under Accession No. 53468. Example 2 Growth of Escherichia coli strain No. A4255 containing plasmid pSODfllTll - production of bacterial cake con¬ taining hSOD analog 1. Preparation of Frozen Stock Culture The bacteria producing hSOD analog (Escherichia coli strain No. A4255/pSOD0 1 Tll) are cultivated overnight at 30°C in shake flasks containing CAS medium, which is of the following composition: Casein hydrolysate 20 g Yeast extract 10 g NaCl 5 g Deionized water 1 L 12.5 mg/L sodium tetracycline HCl is also added to the flasks at the time of inoculation. Following incubation, an aliquot of the stationary- phase culture (having an optical density at 660 nm of between 6-12) is thoroughly mixed with an equal volume of sterile freezing medium, containing the following: The cell suspension is dispensed in 2 ml aliquots into sterile, plastic screw-top vials (Nunc Cryo-tubes, cat. No. 3-63401, or equivalent) and frozen at -70 β C until use. 2. Inoculum The inoculum is propagated in 20 g/L casein hydroly- sate, 10 g/L yeast extract, 5 g/L NaCl, and 12.5 mg/L tetracycline HCl. Sterile medium in shaker flasks is inoculated from stock culture and incubated 8-10 hours on a shaker at 30'C at approximately 240 rpm. The flask contents are used to inoculate the seed ferment¬ or. 3. Seed Fermentor The seed fermentor medium is identical to production medium (see below) except that all the glucose (30 g/L) is added at the start of fermentation; the medium lacks added copper and zinc. Sterile medium is inoculated with culture, and incubated 15 hours at 30*C, pH 7 + 0.1 with agitation and aeration to maintain a dissolved oxygen level of about 20% saturation. Contents of the seed fermentor are then transferred to the production fermentor. 4. Production Fermentor The production medium contains: Sterile glucose solution is added initially to supply 10 g/L. During the course of the fermentation, an additional 20 g/L is supplied. The medium is inoculat¬ ed with 5-10% seed culture and incubated at 30*C. Agitation-aeration rates are set to maintain a dis¬ solved oxygen level of about 20% air saturation. The pH is maintained at 7.0 + 0.1 with NH_. Once cell concentration reaches about 6.5 g/L (OD g60 = 16) induc¬ tion is started by raising the temperature to 42*C. The temperature is maintained at 42*C for 2 hours, at which time the cells are harvested from the cell sus¬ pension. 5. Harvest of Bacterial Cake Five hundred liters of cell suspension (containing 12 DCW/L and 200 g hSOD/L) obtained as described in Sec¬ tion 4 is cooled to room temperature and transferred to a holding tank. The cell suspension is centrifuged at 14,000 rpm (16,000 x g) in a CEPA 101 tubular bowl -centrifuge at a feed rate of 250 liters/hr. The CEPA 101 centrifuge has a tube length of 73.7 cm, a radius to the discharge dam of 3.25 cm and a radius to the bowl wall of 7.35 cm. It is equipped with acceleration fins and has cooling on the exterior jacket. The clear supernatant has a density of 1.0 g/ml, con¬ tains no detectable hSOD analog and is discarded. The cake, weighing 16 kg (75% wet) , contains hSOD analog and is washed. 6. Washing of Bacterial Cake The cake obtained as described above is resuspended in 25 mM potassium phosphate buffer, pH 7.8, containing 155 mM NaCl. The composition of the buffer is: 7 L buffer is used per kg net weight of cake. The cake is suspended in the buffer, mixed thoroughly, and then centrifuged in the CEPA 101 centrifuge at a feed rate of 150 L/hr. The supernatant contains no detectable hSOD analog and is discarded. The cake, weighing 16 kg and containing the hSOD analog, is saved. Example 3 Improved Method for Purification of Recombinant Cu-Zn hSOD Analog The following method gives greatly improved yield and purity of the hSOD analog. The general scheme of the downstream process (Scheme I) consists of steps A through I, as follows: Scheme I - Downstream Processing of the Recombinant Human Cu-Zn SOD Analog (A) CELL DISRUPTION (B) HEAT TREATMENT (C) 100K ULTRAFILTRATION (D) 10K ULTRAFILTRATION (E) DEAE CHROMATOGRAPHY (F) CM CHROMATOGRAPHY (G) PHENYL-SEPHAROSE (PS) CHROMATOGRAPHY (HYDROPHOBIC CHROMATOGRAPHY) (H) Q SEPHAROSE (QS) CHROMATOGRAPHY (I) FREEZE DRYING and PACKAGING The following detailed example of the steps in purifi¬ cation of the Cu-Zn hSOD analog is a scaled-up example involving hundreds of grams of product. It should be understood that the following is a preferred method only and that the invention is not limited to specific embodiments described or particular order of steps recited. A. CELL DISRUPTION Harvested cell cakes are weighed and stored frozen at -20"C. Cakes weighing 50-60 kg from one or more fer¬ mentations are processed downstream. The wet cell cake (51.6 kg containing 260 gr hSOD ana¬ log) is thawed at 4-10*C and suspended in 4 liters of 25 mM Tris-75 mM NaCl buffer pH 7.8 for each kilogram of wet cells (total volume of suspension is 258 li¬ ters) . A Polytron high-shear mixer (Kinematica, Luzern) aids in dispersing the cake. The pH of the suspension is adjusted to 7.3 - 7.5 if necessary. The suspended cells are then disrupted by any suitable method, e.g., by sonication or mechanical disruption. The following method is preferred. The cells are fed into a Dynomill KD-5 bead mill disrupter (Willy A. Bachofen, Basel) at 80 L/hr. The bead mill is circu¬ lated with a cooling liquid at -20*C to maintain the temperature of the bead mill discharge at 8-25*C. The bead mill discharge is fed again into the Dynomill and grinding of the suspension is repeated according to the above conditions. The pH of the disrupted cell suspen¬ sion is adjusted to 7.3 - 7.5 if necessary. The sus¬ pension is centrifuged on the Cepa 101 tubular bowl centrifuge (Carl Padberg, Lahr/Schwarzwald) at 14,000 rpm (16,000 G's) . The feed rate to the centrifuge is 45-55 L/hr. The cake (weighing 23.4 kg) is discarded. The supernatant (243 L) contains the soluble hSOD ana¬ log and is saved. Disruption Conditions: Dynomill Model KD-5 Speed 1810 rpm Bead diameter 0.25 to 0.50 mm or 0.50 to 0.75 mm Shell volume 4.2 L Centrifuge Conditions: Cepa 101 Centrifuge Tube length 73.70 cm Radius to discharge dam 3.25 cm Radius to bowl wall 7.35 cm Typical cake thickness 2.0 cm Speed 14,000 rpm - 16,000 G's Inlet temperature 30 * C Outlet temperature 35*C Cooling on exterior jacket Acceleration fins - yes NOTE: Temperatures quoted above result from use of frozen or thawed cells at 4-10*C. It may be assumed that temperatures up to 37*C are not harmful to the hSOD analog. B. HEAT TREATMENT The supernatant from the previous step (243 L contain¬ ing 250 gr hSOD) is supplemented with 0.25 N NaCl by adding 5 N NaCl providing a final concentration of 0.325 N NaCl and the pH is adjusted to 7.3-7.5 if nec¬ essary. The solution is transferred into a jacketed bioreactor (Bioengineering AG. , Wald, Switzerland) . An ICI silco- lapse 5000 antifoam is added to a final concentration of 10-15 ppm active material. The temperature is raised at a slow rate to 65*C while stirring gently to avoid foaming. The heat treatment at 65"C is carried out for 2 hours and terminated by cooling the solution in the bioreactor to 40*C and then transferred to a cold room for incubation at 4*C overnight (10-16 hours) without stirring. The cooled solution is centrifuged on the Cepa-101 at 40-60 L/hr. All other centrifugation conditions are identical to those of cell disruption. The cake (weighing 11.3 kg) is discarded. The clarified super¬ natant (233 L) containing the soluble hSOD analog is saved and the pH is adjusted to 7.3-7.5. C. 100K ULTRAFILTRATION (PUF-100 UNIT) Any ultrafiltrate method with a cut-off point at 100K may be used in this step. Examples of such methods are the Pellicon cassette system or Amicon's hollow fiber system. The preferred ultrafiltration method is the Millipore PUF-100 Unit, Process Ultrafiltration System 2 equipped with two 50 ft spiral-wound membrane car¬ tridges with 100,000 molecular weight cut-off ratings (Millipore Intertech, Inc., Bedford, Massachusetts). This is used to concentrate the protein solution of the previous step (230 L containing 232 gr hSOD) to about 10% of the initial volume (20 L) and the filtrate (210 L containing some of the hSOD) is saved. The initial permeate rate is 6 L/min. The retentate containing the rest of the hSOD is diafiltered by continuous addition of fresh 20 mM Tris buffer at pH 7.8 to the well-mixed retentate at a rate just equal to the permeate rate. In this way the volume of retentate remains constant while the hSOD passes through the membrane. Diafil¬ tration is continued until 2-3 times the volume of the clarified protein solution used in the beginning of this step is collected (460 liters of filtrate) . At this point the permeate OD is checked and found to be in the range 0.1-0.2. [During this period, the mem¬ branes are depolarized several times whenever a de¬ crease in permeate flow rate is observed (below 4 L/min) . This is achieved by diluting the retentate to 40 liters and recirculating for a few minutes with the permeate valves shut off. The retentate is concentrat¬ ed again to 20 liters and diafiltration resumes.] The filtrate is collected, combined with the permeate (210 L) of the concentration step (total of 670 L) and the retentate is discarded. Ultrafiltration conditions: Membrane - PTHK cartridges Initial Final Inlet Pressure - psig Outlet Pressure - psig Recirculation rate - L/min - L/hr-ft' Permeate rate - L/min - L/hr-ft : The feed, retentate and permeate are all examined by Superose 6B gel chromatography in order to determine the extent and quality of the separation. The condi¬ tions of the gel column are: 1.6-cm dia. x 55-cm long (Pharmacia, Uppsala) Total volume: 110 ml Void volume: 30 ml Flow rate: 60 ml/hr Loading: Feed 20-25 OD's in 0.25 ml Retentate 3-5 OD's in 0.25 ml Permeate 4-6 OD's in 0.25 ml Eluent : 20 mM Tris - 150 mM NaCl, pH 7.8 Normal running time: 2.3 hours Detection: 280 nm UV The 100 K molecular weight cut off fractionation of hSOD analog has been practiced by ultrafiltration with both the PUF-100 Unit and the Pellicon Cassette System. The Pellicon System is suitable for smaller batches of hSOD, as described below. The cell cake weight of this batch was 9.25 Kg containing 45 gr hSOD analog. The clarified protein solution obtained at the end of the heat treatment step (44 liters containing 41.6 gr hSOD) is ultrafiltered through two Pellicon Cassette Systems each of them equipped with five 100,000 molecu- lar weight cut-off cassettes (type PTHK) of 5 ft 2 area each (Millipore Intertech, Inc. , Bedford, Massachu¬ setts) . The protein solution is concentrated to about 10% of the initial clarified protein solution volume (5.5 L of retentate) and the permeate is saved. The retentate is diafiltered by continuous addition of fresh 20 mM Tris solution at pH 7.8 to the well-mixed retentate at a rate just equal to the permeate rate. In this way the volume of retentate remains constant while the hSOD passes through the membrane. Diafil¬ tration is continued until the permeate has an absor- bance at 280 nm (1-cm cell) of less than 0.2. This normally produces 2-3 times the volume of the clarified protein solution (150 L permeate) . The permeates of the concentration and diafiltraton steps are combined (190 L containing 40.5 gr hSOD analog) while the reten¬ tate is discarded. Ultrafiltration Conditions: Millipore Pellicon System Membrane: PTHK cassette (100 K cut-off) Inlet Pressure: 50 psig Backpressure: 5-8 psig UF rate with 10 cassettes (50 ft 2 ) Ultrafiltration rate when retentate is most concentrated 3.6 L/hr ft' 180 L/hr Ultrafiltration rate when retentate is most dilute 4.2 L/hr ft' 210 L/hr Recirculation rate when retentate is most concentrated 12 L/hr ft' 10 L/min Recirculation rate when retentate is most dilute 14.4 L/hr ft' 12 L/min D. 10K ULTRAFILTRATION (PUF-100 UNIT) Any ultrafiltration method with a cut-off point of 10 K may be used in this step. The preferred method is described here, where a Millipore PUF-100 unit and a Pellicon Cassette System (10 K cut-off) are used se¬ quentially. The combined permeate from the previous step (670 L containing 229 gr hSOD) is inline fed into a Millipore PUF-100 Unit, Process Ultrafiltration System equipped with two 50 ft 2 spiral-wound membrane cartridges with 10,000 molecular weight cut-off ratings (Millipore Intertech, Inc. , Bedford, Massachusetts) , and the per¬ meate rate is checked. The initial permeate rate is 9 L/min. The solution is concentrated to 2-5% (20 L) of the permeate combined volume. The retentate is then diafiltered by continuously adding fresh 20 mM Tris buffer at pH 7.8 to the well-mixed retentate at a rate just equal to the permeate rate. In this way, the volume of retentate remains constant and the hSOD ana¬ log is retained while lower molecular weight proteins pass through the membrane. [During this period, the membranes are depolarized several times whenever a decrease in permeate flow rate is observed (below 4 L/min) . This is achieved by diluting the retentate to 40 liters and recirculating for a few minutes with the permeate valves shut off. The retentate is concentrat¬ ed again to 20 liters and diafiltration resumes.] The diafiltration is completed when the permeate conductiv¬ ity is equal to that of the diafiltration buffer (at least 20 volumes per volume of retentate are required) . The retentate is drained out of the PUF-100 Unit con¬ tainer and transferred for a final concentration through a Pellicon Cassette System equipped with three 10K molecular weight cut off cassettes (type PTGC) of 5 ft 2 area each (Millipore Intertech, Inc., Bedford, Mas¬ sachusetts) . The hSOD analog is concentrated to a maximum of 100 gr/L (10 L containing 229 gr) . The retentate is drained out of the Pellicon and the ultra- filter is washed with a small amount of 20 mM Tris pH 7.8 which is combined with the retentate. The combined retentate volume after addition of the wash is divided into portions containing an amount equivalent to about 75 gr hSOD analog each (3 por¬ tions) which are processed directly or stored at -20*C. Prior to proceeding to the next purification step, frozen hSOD analog solutions are thawed at 37'C. (a) PUF-100 UNIT Membrane PTCG Cartridges Inlet Pressure psig Outlet Pressure psig Recirculation Rate L/min. L/hr-ft 2 Permeate Rate L/min L/hr-ft 2 (b) PELLICON CASSETTE SYSTEM Membrane PTCG Cartridges Inlet Pressure 20-30 psig •Backpressure 5-10 psig UF Concentration Conditions: UF rate with 3 2 cassettes (15 ft ) Ultrafiltration rate when retentate is most concentrated 6 L/hr ft' 90 L/hr Ultrafiltration rate when retentate is most dilute 8 L/hr ft' 120 L/hr Recirculation rate when retentate is most concentrated 54 L/hr ft' 13.5 L/min Recirculation rate when retentate is most dilute 36 L/hr ft' 9.0 L/min The retentate and permeate are examined by Superose 6B gel chromatography to determine the extent and quality of the separation. The conditions of the gel column are: Loading : Retentate: 30-40 OD's in 0.25 ml, Permeate : 3-4 OD's in 0.25 ml. All other gel column conditions are identical to those described in the 100 K ultrafiltration step (PUF-100 Unit) . E. DEAE CHROMATOGRAPHY Any weak anion exchange (e.g. tertiary amine) method can be used in this step but DEAE Sepharose fast-flow chromatography is preferred. From this step on, protein solutions obtained in the preceding manner from several runs can be combined and processed downstream. The amount of hSOD analog which is processed at each step is a function of the equip¬ ment capacity. A retentate solution from the previous step (3300 ml of 23 mg hSOD analog/ml) is diluted to 10 mg hSOD/ml with 20 mM Tris, pH 7.8. The solution is loaded onto a KS- 370 stack column [37 cm x 15 cm (Pharmacia, Uppsala)], packed with 16 L of DEAE Sepharose C1-6B fast flow anion exchanger at a maximum linear flow velocity of 164 cm/hr (180 L/hr) . An equivalent of 75 gr hSOD analog maximum is loaded onto 16 liters of resin with¬ out peak separation interference. From this step on, pyrogen-free water purified by reverse osmosis system (Hydro-Hammer, Tel Aviv) is used. The progress of the run is monitored by continuously following the absor- bance of the eluate at 280 nm and the elution profile of this column is shown in Figure 2A. The column is washed at the above flow rate with three to four bed volumes (48 - 64 L) of 20 mM Tris-25 mM NaCl at pH 7.8 until the blue peak is fully eluted. The eluate from the loading and washing steps is discarded (however, see Example 4). A step change in eluent to 20 mM Tris- 80 mM NaCl at pH 7.8 displaces the hSOD analog off the column; this is the peak shaded in black in Figure 2A. The elution flow velocity is up to 180 L/hr and the hSOD analog peak is collected in 0.8 to 1.5 bed volumes (21 liters containing 50 gr hSOD) and is saved. The average concentration of the hSOD analog in the pool is approximately 2.5 gr hSOD/L. Concentration after DEAE: The eluted peaks from one or more loadings are concentrated by ultrafiltration through a Millipore Pellicon Cassette System equipped with three 10,000 molecular weight cut-off cassettes (type PTGC) of 5 ft 2 each until the calculated activity in the retentate is up to 100 mg hSOD/ml. The ultra¬ filtrate should contain no hSOD analog and is discard¬ ed. The retentate is usually concentrated by a factor of roughly 20 (2 liters of retentate) . After the hSOD analog has been concentrated, the reten¬ tate containing the hSOD analog is drained from the Pellicon, and the ultrafilter is washed with a small amount of 20 mM Tris pH 7.8 which is combined with the retentate. The combined retentate volume after addi¬ tion of the wash is divided into portions each contain¬ ing not more than an equivalent of 75 gr hSOD analog and that is processed directly or stored at -20*C. Prior to proceeding to the next purification step, frozen hSOD solutions are always thawed at 37 * C. Ultrafiltration Conditions: Millipore Pellicon System Membrane: PTGC (10 K cut-off) Inlet Pressure: 20-30 psig Backpressure: 10 psig UF rate with 3 cassettes (15 ft 2 ) Ultrafiltration rate when retentate is most concentrated 4.8 L/hr ft 72 L/hr Ultrafiltration rate when retentate is most dilute 6.4 L/hr ft * 96 L/hr Recirculation rate when retentate is most concentrated 54 L/hr ft' 13.5 L/min Recirculation rate when retentate is most dilute 36 L/hr ft' 9.0 L/min F . CM CHROMATOGRAPHY In this step any cation exchange method can be used, e.g. CM Sephadex or S-Sepharose. The preferred method is carboxymethyl (CM) Sepharose Fast Flow (FF) . A retentate solution from the previous step (4000 ml containing 21.15 mg hSOD analog/ml) is dialyzed on the day of loading onto the CM column. The dialysis is carried out by continuously adding 40 mM sodium acetate at pH 7.8 to the retentate using the Millipore Pellicon Cassette System. All other concen¬ tration and dialysis conditions are identical to those described in the DEAE Chromatography step. The dialy¬ sis is completed when the filtrate conductivity is equal to that of the dialysis buffer (20-25 volumes per volume of retentate) . The permeate is discarded. The retentate is drained out of the Pellicon and the ultra- filter is washed with a small amount of 40 mM sodium acetate pH 7.8 which is combined with the retentate. An equivalent of about 75 gr hSOD analog of retentate solution (2000 ml containing 31.25 g hSOD/ml) is ad¬ justed to pH 4.8 by adding 1 M glacial acetic acid. Then the solution is diluted to 10 mg hSOD/ml with 40 mM sodium acetate pH 4.8 and immediately loaded onto a KS-370 stack column [37 cm x 15 cm (Pharmacia, Uppsala) ], packed with 16 L of CM Sepharose C1-6B fast flow cation exchanger at a linear flow velocity of up to 164 cm/hr (180 L/hr) . Up to 75 gr hSOD analog can be loaded onto 16 L of resin. The progress of the run is monitored by continuously following the absorbance of the eluate at 280 nm, and the elution profile of this column is shown in Figure 2B. The column is washed at the above flow rate with three to four bed volumes (48-64 L) of 40 mM sodium acetate adjusted to pH 4.8 with glacial acetic acid. The elu¬ ate from the loading and washing steps is discarded (however, see Example 4) . A step change in eluent to 85 mM sodium acetate adjusted to pH 4.8 with glacial acetic acid displaces the hSOD analog off the column; this is the peak shaded in black in Figure 2B. The maximum elution flow velocity is 180 L/hr and the hSOD analog peak is collected in 0.75 to 1.25 bed vol¬ ume (12 L) and is saved. The protein solution is ti¬ trated immediately to pH 7.8 with 1 N NaOH. At this point in the process most of the impurities have been eliminated and U.V. absorbance fairly moni¬ tors hSOD analog concentration. Concentration and dialysis after CM: The eluted peaks from one or more loadings carried out on the same day are concentrated up to 100 mg hSOD analog/ml. The concentration conditions are identical to those de¬ scribed in the DEAE Chromatography step. The retentate is then dialyzed against 50 mM Tris pH 7.8 using the Millipore Pellicon Cassette System. The dialfiltration is performed until the permeate conductivity is equal to that of the dialysis buffer (at least 20 volumes of dialysis buffer per one volume of retentate are re¬ quired) . The retentate containing the dialyzed hSOD analog is drained from the Pellicon and the ultrafilter is washed with a small amount of 50 mM Tris pH 7.8 which is com¬ bined with the retentate. The combined retentate is divided into portions of 150-160 gr hSOD analog, each of which are processed directly or stored at -20*C. Ultrafiltration Conditions: Millipore Pellicon System Membrane: PTGC (10 K cut-off) Inlet Pressure: 20-30 psig Backpressure: 10 psig UF Diafiltration Conditions UF rate with 3 cassettes (15 ft 2 ) Ultrafiltration rate 4.8 L/hr ft' 72 L/hr Recirculation rate 54 L/hr ft 2 13.5 L/min G. PHENYL - SEPHAROSE (PS) CHROMATOGRAPHY In this step phenyl-Sepharose chromatography is used to achieve hydrophobic separation. The hSOD analog, which is applied in overcapacity and in high salt concentra¬ tion, does not attach to the column but goes straight through; this achieves purification of hSOD analog from residual Escherichia coli protein (ECP) and endotoxin which are absorbed. This novel step achieves a sur¬ prising degree of reduction of ECP and endotoxin con¬ tamination. The ECP levels are reduced by about 3 orders of magnitude; the measurement of ECP by a solid- phase immunoradiometric assay is described in Example 5 where the reduction of ECP by means of the phenyl-Seph¬ arose column is detailed (Table 2) . The endotoxin level is reduced by a similar extent, from about 10,000 ppm to less than 10 ppm, as measured by the Limulus Amebocyte Lysate (LAL) assay described in U.S. Pharmacopeia (U.S.P.) XXI 1165-1166 (1985). A retentate solution from the previous step (5000 ml containing 31.6 mg hSOD analog/ml), is brought by 50 mM Tris pH 7.8 to a protein concentration of 25-30 mg hSOD/ml. A buffer containing 50 mM Tris - 3.2 M ammo¬ nium sulfate at pH 7.8 is used to dilute volume per volume the proteinaceous solution to a final concentra¬ tion of 12.5 to 15 mg hSOD analog/ml (15 mg hSOD/ml is preferable) in 1.6 M ammonium sulfate and 50 mM Tris at pH 7.8. The solution is stirred gently for 30-60 minutes at room temperature and then loaded on a 11.3 x 30 cm column (113 Bioprocess, Pharmacia, Uppsala) packed with 3 L of phenyl-Sepharose fast flow at a linear flow velocity of 3-6 L/hr. A maximum of 150 gr hSOD analog can be loaded on the 11.3 x 30 cm column. The column is eluted by 50 mM Tris-1.6 M ammonium sul- fate, pH 7.8 which displaces the hSOD analog off the column. The elution flow velocity is 3-6 L/hr. The progress of the run is monitored continuously by fol¬ lowing the absorbance of the eluate at 280 nm. The eluted hSOD analog peak has maximal absorbance after about one and one-half bed volume of eluant. Material eluted at the beginning of the peak (up to 2% of the maximum absorbance) is discarded. The major portion of the peak is saved (from where the absorbance is 2% of the maximum absorbance at the beginning of the peak to 5% of the maximum at the end (9-12L) ) . Concentration and Dialysis After Phenyl-Sepharose The eluted peak is concentrated by ultrafiltration through a Millipore Pellicon Cassette System equipped with three 3000 molecular weight cut-off cassettes of 5 2 ft each until the calculated activity in the retentate is up to 25 mg hSOD analog/ml. The ultrafiltrate should contain no hSOD and is discarded. The retentate is then dialyzed against 20 mM Tris pH 8.8. The dialy¬ sis is completed when the filtrate pH and conductivity are equal to that of the dialysis buffer (at least 20 volumes per volume of retentate are needed) . The re¬ tentate containing the dialyzed hSOD analog is drained from the Pellicon and the ultrafilter is washed with a small amount of 10 mM ammonium bicarbonate, pH 8.0 which is combined with the retentate. All mentioned ultrafiltration activities are performed in a cold room (4'-10*C). The combined retentate (5460 ml containing ■ 23.9 hSOD/ml) can be transferred to the next step or stored frozen at -20 * C. UF Diafiltration Conditions Millipore Pellicon System Membrane : 3 K cut-off (Filtron, Massachusetts, USA) Inlet Pressure: 20-30 psig Backpressure : 5-10 psig UF rate with 3 cassettes (15 ft 2 ) Ultrafiltration rate when retentate is most concentrated 4.8 L/hr ft' 60 L/hr Ultrafiltration rate when retentate is most dilute 6.4 L/hr ft' 30 L/hr Recirculation rate when retentate is most concentrated 25.2 L/hr ft' 6.3 L/min Recirculation rate when retentate is most dilute 16.8 L/hr ft' 4.2 L/min H. Q-SEPHAROSE (QS) CHROMATOGRAPHY In this step a strong anion exchanger is used. This can be, for example, QAE Sephadex A-25, but the pre¬ ferred method is to use Q-Sepharose (QS) . Note also that the order of Steps G and H can be interchanged, but the order given here is preferred. It is also possible for the superoxide dismutase analog solution to be of sufficient purity after the hydrophobic column exchange step that this step may be unnecessary. The retentate solution from the previous step (3.88 L of 28.6 mg hSOD analog/ml) is diluted to 5 mg hSOD analog/ml with 20 mM Tris, pH 8.8 and supplemented with 20 mM NaCl. The solution is stirred gently at 20-30*C for 30-45 minutes and loaded on a KS 370 stack column (37 x 15 cm, Pharmacia, Uppsala) packed with 16 L of Q- Sepharose fast flow anion exchanger, at a linear flow velocity of 90-180 L/hr. A maximum of 200 gr hSOD analog can be loaded on the KS 370 column. The column is washed with one to two bed volumes of 20 mM Tris-45 mM NaCl, pH 8.8 and the eluate is discarded. A step change in eluent to 20 mM Tris-130 mM NaCl, pH 8.8 displaces the hSOD analog off the column. The elution flow velocity is 90-180 L/hr. The progress of the run is monitored continuously by following the absorbance of the eluate at 280 nm The eluted hSOD analog peak has maximal absorbance after about one bed volume of eluent. The major portion of the peak (50-60 liters) is saved. Material eluted at the end of the peak (where the absorbance is lower than 5% of the maximal absorbance) is discarded. Concentration and dialysis after QS: The eluted peak is concentrated to an OD of up to 45 then dialyzed against 10 mM ammonium bicarbonate pH 8.0 using the Millipore Pellicon Cassette System in a cold room (5'C) . The dialysis is completed when the filtrate pH and conductivity are equal to that of the dialysis buffer (at least 20 volumes per volume of retentate are needed) . All other concentration and dialysis condi¬ tions are identical to those described in the DEAE and the CM Chromatography steps. The retentate containing the dialyzed hSOD analog is drained from the Pellicon and the ultrafilter is washed with a small amount of 10 mM ammonium bicarbonate, pH 8.0 which is combined with the retentate. The combined retentate wash (2.34 L of 44.45 mg hSOD analog/ml) is transferred to the next step. I. FREEZE DRYING AND PACKAGING Freeze drying of the bulk hSOD analog is carried out as follows: The hSOD analog solution is diluted using 10 mM ammonium bicarbonate, pH 8.0 to a final concentra¬ tion of 7 to 10 mg hSOD/ml (10.4 L of 10 mg hSOD/ml) and then divided into approximately 600 ml fractions under aseptic conditions in a laminar flow hood (Con¬ tamination Control Inc., Lansdale, Pennsylvania). Each portion is added to a 2 liter glass jar preheated for 4 hours in a 180*C oven. The jars are frozen quickly using an acetone-dry ice mixture and put on a freeze dryer (Freezemobile-24, Virtis Company, New York). The outer surface of the jars is exposed to ambient temper¬ ature for 60-120 hours throughout the drying cycle until the hSOD analog is dry, and it is then packaged. The overall yield of hSOD analog is about 25% and it is extremely pure, containing less than 10 ng ECP per mg hSOD analog. Increase in yield with no loss of purity may be achieved by following the procedure described in Exam¬ ple 4. Example 4 A Novel Procedure for Improving the Yield of Cu-Zn hSOD Analog Production by Means of Subunit Exchange The following novel procedure may be used as an addi¬ tion to the purification protocol described in Example 3 (Scheme I) . It significantly increases the yield of hSOD analog without affecting the required high degree of purification achieved and with relatively low addi¬ tional cost. A. RATIONALE FOR THE EXCHANGE PROCEDURE Cu-Zn SOD enzymes from different species consist of dimers built from two chemically identical subunits and yet are known to appear in several discrete forms on isoelectric focusing gel (IEF) . The existence of electroforms of various Cu-Zn SOD enzymes have been reported (1-8) and recently reviewed by I. Fridovich (9). As demonstrated by IEF analysis and FPLC/Mono-Q anion- exchange chromatography, the dimeric human Cu-Zn SOD can be separated into three major electroforms. The electroforms (or isoforms) may represent post-transla¬ tion modifications. The IEF and Mono-Q patterns of natural Cu-Zn hSOD can be explained in terms of there being two types of sub- units, which can result in one or two extra negative charges per dimer. These differences in charge provide the different pi values. The subunits designated x and y could associate to yield xx, xy and yy dimers which would correspond to the observed three major dimeric isoforms, designated a, b, and c, respectively. We have shown by IEF analysis and also by analytical Mono-Q anion exchange chromatography that the recombi¬ nant hSOD analog also consists of three discrete iso¬ forms with pis of 5.16, 4.95, and 4.85 that are the a, b and c isoforms, respectively. The Cu-Zn hSOD that has been extracted from red blood cells also has three isoforms with pis of 4.85, 4.75, and 4.65 i.e., natural hSOD is more acidic than the recombinant Cu-Zn hSOD analog. The acidic shift appearing in the IEF pattern of the authentic hSOD versus the recombinant analog can be explained by the net decrease of two positive charges at the N-terminal residues of the authentic Cu- Zn hSOD which are blocked by one N-acetyl group per polypeptide chain (1, 6, 10). The question remains as to the chemical nature of the different isoforms. We postulate, although not wishing to be bound by theory, that the cause for charge heter¬ ogeneity might be due to residue modification at posi¬ tion Cys-lll of one or both subunits. It is known that Cu-Zn SOD enzymes from different sources have different pi values (6, 9) . Moreover, studies have shown that intraspecies hybridization of Cu-Zn SOD subunits can be performed and the reformed hybrid SOD dimer has an intermediate electrophoretic mobility on isoelectro- focusing gels compared to the "parental" dimers (2-5) . We have shown that the recombinant Cu-Zn hSOD analog -can undergo subunit exchange of the different dimeric isoforms and this is described below; we have also shown that the recombinant hSOD analog can hybridize to authentic Cu-Zn SOD from human and bovine sources. We isolated the three isoforms denoted a, b, and c of the recombinant hSOD analog (Figures 3A, 5A and 3B, respectively) . Incubation of a mixture of equal amounts of isolated isoforms a and c produced a mixture which contained about 50% of the hybridized b isoform and about 25% each of the a and c isoforms (Figure 3F) . The new b isoform is produced by random hybridization between one subunit derived from the a dimer and one subunit derived from the c dimer. Thermal incubation proved to be the best method for the hybridization (Figure 3, D-F and Figure 4). The reversibility of the subunit exchange phenomenon can be demonstrated by heat incubation of isolated isoform b produced by hybridization of isolated a and c isoforms; the re-formation of a and c isoforms to a level of about 25% each is shown (Figure 5) . The column procedures for hSOD analog purification (Scheme I) are described in Example 3, steps E and F (DEAE Sepharose and CM Sepharose respectively) . During the hSOD analog purification, the b isoform becomes predominant and the a-rich and c-rich fractions, which contain relatively high amounts of Escherichia coli proteins (ECP) and which run on either side of the main hSOD analog peak, are preferentially discarded. In Figures 2A and 2B, these a-rich and c-rich peaks are shaded grey. We have invented a novel protocol in which these previ¬ ously discarded fractions from DEAE and CM-Sepharose are collected, mixed together in the right proportions and incubated under conditions which effect the maximum amounts of conversion of the a and c isoforms into the b isoform through the subunit exchange mechanism. The reconstituted b dimer can then be recovered and repuri- fied on DEAE and CM columns before it is added to the main batch (see Figures 6, 7, 8). This novel proce¬ dure significantly raises the recovery of hSOD analog without affecting the purity of the product. B. DETAILED PROCEDURE i. DEAE Sepharose Column (see Example 3E) . The elution profile of this column is shown in Figure 2A; the main b-rich hSOD analog peak is shaded black. The peak (shaded grey) appearing immediately prior to the main hSOD peak is a- rich. This material mostly elutes in the wash step with 20 mM Tris, 25 mM NaCl, pH 7.8. The peak which follows the main hSOD peak is shown to be c-rich (also shaded grey) . In our new purification protocol these two samples (which are normally discarded) are collected and treated as described below. ii. CM Sepharose Column (see Example 3F) The elution profile of this column is shown in Figure 2B; the main b-rich hSOD analog peak is shaded black. The two peaks appearing prior to the main hSOD peak are c-rich, and these peaks are shaded grey. This material elutes in the wash step with 40 mM sodium acetate pH 4.8. The peak eluting later, on the addition of 0.5M NaCl, is a-rich; this peak is also shaded grey. In our new purification protocol these two c- rich and a-rich samples (which are normally discarded) are now collected and treated as described below. iii. The bulk hSOD analog preparation is kept at the end of step F in Example 3 for the a-rich and c-rich samples to be treated. iv. All four additional samples, two from the DEAE- Sepharose column (i) and two from the CM-Sepha¬ rose column (ii) , are treated as follows: each sample is first concentrated by ultrafiltra¬ tion, as described in Example 3 step E, and then dialyzed against 20 mM tris pH 7.8. The four samples are then combined. v. Incubation of the combined samples is per¬ formed under conditions that can be varied e.g., 4 hours at 37'C, or 2 hours at 37*C fol¬ lowed by overnight at room temperature, or 30 minutes at 50*C. All three sets of incubation conditions have a fairly similar effect on the formation of the intermediate isoform b by means of subunit exchange from isoforms a and c. The kinetics of formation of the new b isoform is temperature dependent. The amount of the new b dimer formed is, of course, dependent on the relative amounts of a and c dimers added to the incubation mixture. Thus, the proportion in which the four samples are mixed is critical for. the overall success of the exchange process. vi. Following the incubation the mixture becomes b- rich (Figure 6) . At the end of the exchange step, the material is chromatographed on DEAE- Sepharose and on CM-Sepharose (i.e. steps E and F of Example 3 are repeated) ; this re-chroma- tography is shown in Figures 7A and B. This purified preparation is then added to the bulk preparation held at step F(iii) . steps G, H and I are then performed on the combined mate¬ rial as one batch. The novel procedure described herein is shown in Scheme II, shown on the next page. The 10 recombinant Cu-Zn hSOD analog produced is as pure as that achieved by Scheme I (described in Example 3) and the yield is increased from the level of 25% achieved by Scheme I to 33% achieved by Scheme II. 15 20 25 30 35 SCHEKB II DOWNSTREAM PROCESSING OF THE RECOMBINANT HUMAN CU-ZN SOD ANALOG UTILIZING THE SUBUNIT EXCHANGE PROCEDURE Downstream Processing Main Downstream Processing Subunit Exchange Steps A through D as Scheme I (Example 3) [ I •r DEAE-Chromatography (Step E in Scheme I) a and c-Rich Isoforms —ι b-Rich Isoform —*Incubation 4 b-Rich Isoform CM-Chromatography (Step F in Scheme I) ^ a and c-Rich Iso orms. DEAE-Ch iromatography I b-Rich IIs-oform CM-Chromatography Phenyl-Chromatography (Step G) Q-Chromatography (Step H) Freeze Drying and Packaging (Step I) Example 5 Detection of Escherichia coli Protein (ECP) Contamin¬ ants in Purified SOD Analog By a Solid-Phase Immuno- radiometric Assay The purpose of this assay is to quantify residual ECP presence in the purified recombinant hSOD analog prepa¬ ration. In order to evaluate ECP appropriately, we developed an immunoradiometric assay (IRMA) that shows a high degree of specificity and sensitivity. The ECP standard was prepared from a mock purification process (Example 3) of Escherichia coli reference (Escherichia coli containing the plasmid without the gene coding for hSOD analog) ; the fraction used was that which eluted from the CM-Sepharose column. Anti¬ body to this ECP was raised in rabbits and purified IgG's to the ECP were obtained. The solid-phase IRMA is accomplished by first coating the wells of a plastic 96-well plate with purified IgG's against the mock-purified, CM-eluted ECP frac¬ tion. CM-purified ECP, at serial dilutions, is allowed to bind to the IgG in the wells. Subsequently, 125I- labeled anti-ECP antibody is added, thereby providing a "sandwich" complex which allows accurate and sensitive assessment of binding and the construction of an ECP standard curve. TABLE I STANDARD CURVE: TITRATION OF ECP EE _ cpm (triplicates) 20,000 10,000 5,000 2,500 1,250 625 312.5 156.25 78 39 19.5 9.76 4.88 2.44 Buffer Based on this curve, the IRMA proved to be sensitive to 1 ng/ml of ECP, which is significantly different from the zero point, as defined, by the minimum detectable dose method of Rodbard et al. , Radioimmunoassay And Related Procedures In Medicine I (International Atomic Energy Agency, Vienna) p. 469, (1977). The above assay was used to monitor the ECP in various steps of hSOD analog purification. The antigen used was produced at Step F after the CM column. (See Exam¬ ple 3, Scheme I.) As can be seen from the results shown below, (Table 2) the two subsequent steps G and H (phenyl-Sepharose chromatography and Q-Sepharose chromatography) remove efficiently virtually all ECP contaminants from the SOD analog preparation. Note that here steps G and H are reversed. This is a possi¬ ble alternative order, as described in Example 3. TABLE II Measurement of ECP levels after various stages in puri¬ fication of recombinant hSOD analog. Stage in Purification DEAE Sepharose Chromatography CM Sepharose Chromatography Q-Sepharose Chromatography Phenyl-Sepharose Chromatograp Example 6 Preparation Of Naturally-Occurring Cu-Zn Superoxide Dismutase From Liver A homogenized human liver preparation was heat-treated at 65'C and then centrifuged. Ammonium sulphate was added to the supernatant solution and the 65%-80% ammo¬ nium sulphate precipitate was obtained. A solution of this precipitate was applied to a DEAE ion-exchange column to separate the manganese superoxide dismutase (MnSOD) from the Cu-Zn superoxide dismutase (Cu-Zn SOD) . The MnSOD eluted with 5 mM potassium phosphate pH = 7.8 and the Cu-Zn SOD eluted with 50 mM potassium phosphate pH = 7.8. The clear solution of Cu-Zn SOD was dialyzed to 50 mM tris pH 7.8, and ammonium sulphate was added to a concentration of 1.6 M. This high salt solution of Cu-Zn SOD was then applied to a phenyl-Sepharose column as described in Example 3, stop G. The resulting fractions which eluted in high salt con¬ centration contained highly purified naturally-occur¬ ring Cu-Zn superoxide dismutase. The contaminant eucaryotic proteins remained bound to the column until eluted with water. These results show that the hydrophobic column in the presence of high salt solution binds the majority of liver proteins, and allows the naturally-occurring Cu- Zn superoxide dismutase to pass through, highly puri¬ fied. Naturally-occurring Cu-Zn superoxide dismutase or analogs thereof may also be purified from other eucaryotic sources such as blood and yeast by this means. REFERENCES 1. I. Fridovich, Adv. Enzymology 4_1: 35-97 (1974). 2. J.V. Weisiger, et al., Comp. Biochem. Physiol. 568: 235-238 (1977) . 3. J.H. Bloor, et al., Biochem. Genet. 2 : 349-364 (1983) . 4. H. Tegelstrom, Hereditas 8_1: 185-198 (1975). 5. L. Civalleri, et al., Mol. Cell. Biochem. 4_7: 3- 9 (1982). 6. S. Marklund, et al., Eur. J. Biochem. 65_: 415- 422 (1976) . 7. C. Beauchamp, et al.. Anal. Biochem. 4_4_: 276-278 (1971) . 8. B. Lonnerdal, et al., FEBS Lett. 108: 51-58 (1979) . 9. I. Fridovich, Adv. Enzymology ]58_: 61-97 (1986) . 10. J.S. Valentine, et al.. Metal Ions Biol. 3_ : 291- 358 (1981). 11. Koyama, et al.. Transplantation 4_0(6) : 590-595 (1985) . 12. Hoo Lim, et al., American College of Surgeons, Surgical Forum Volume XXXVII, 443-445 (1986). 13. Ambrosio, et al., Circulation 75_(1) : 282-291 (1987) . 14. Ambrosio, et al., Circulation 74(6): 1424-1433 (1986) . 15. Lieman-Hurwitz, et al., Proc. Natl. Acad. Sci. USA 79: 2808-2811 (1982) . 16. Hallewell, et al., Nucleic Acids Res. ^3_: 2017- 2034 (1983). 17. Hallewell, et al., Biotechnology 5_: 363-366 (1987) . 18. Takahara, et al., Biotechnology 6_: 195-198 (1988)

Claims: What is claimed is: 1. A method for recovering a solution containing purified, enzymatically active Cu-Zn superoxide dismutase or a polypeptide analog thereof having substantially the same amino acid sequence as, and the biological activity of, naturally-occurring Cu-Zn superoxide dismutase from a composition which comprises cells containing Cu-Zn superoxide dismutase or a polypeptide analog thereof compris¬ ing: (a) treating the composition so as to separate soluble proteins present in the cells from whole cells, cellular debris and insoluble proteins so as to obtain a solu¬ tion containing such soluble proteins; (b) treating the resulting solution containing the soluble proteins with a second solu¬ tion containing a salt at a concentration such that the soluble proteins other than Cu-Zn superoxide dismutase or the polypeptide analog thereof present in the solution containing the soluble proteins are rendered capable of binding to an appropriate hydrophobic substance; (c) contacting the then-resulting solution containing the soluble proteins with an appropriate hydrophobic substance so as to bind the soluble proteins other than Cu-Zn superoxide dismutase or the polypeptide analog thereof present in such solution to the hydrophobic substance and thus sepa¬ rate such other proteins from the Cu-Zn superoxide dismutase or polypeptide analog thereof; and (d) recovering the resulting solution contain¬ ing purified, enzymatically active Cu-Zn superoxide dismutase or polypeptide analog thereof. 2. A method of claim 1, wherein the Cu-Zn superoxide dismutase or polypeptide analog thereof is human Cu-Zn superoxide dismutase or a polypeptide analog thereof. 3. A method of claim 2, wherein the polypeptide ana¬ log comprises a non-acetylated polypeptide analog of human Cu-Zn superoxide dismutase. 4. A method of claim 1, wherein the cells comprise bacterial cells. 5. A method of claim 4, wherein the bacterial cells comprise cells of an Escherichia coli strain. 6. A method of claim 5, wherein the Escherichia coli strain comprises Escherichia coli A4255 containing the plasmid designated pSOD^T,, and deposited under ATCC Accession No. 53468. 7. A method of claim 5, wherein the Escherichia coli strain comprises Escherichia coli A4255 containing the plasmid pSOD0MAX-12 and deposited under ATCC Accession No. 67177. 8. A method of claim 5, wherein the Escherichia coli strain comprises Escherichia coli A2097 containing the plasmid designated pSOD 2 and deposited under ATCC Accession No. 39786. 9. A method of claim 5, wherein the Escherichia coli strain comprises Escherichia coli S^732 containing the plasmid designated pMF2005 and deposited under ATCC Accession No. 67362. 10. A method of claim 5, wherein the Escherichia coli strain comprises Escherichia coli S 930 containing the plasmid designated pMF5534 and deposited under ATCC Accession No. 67703. 11. A method of claim 5, wherein the Escherichia coli strain comprises Escherichia coli A1645 containing the plasmid designated pNd-S0D NN -12 and deposited under ATCC Accession No. 53166. 12. A method of claim 1, wherein the cells comprise eucaryotic cells. 13. A method of claim 12, wherein the eucaryotic cells comprise yeast cells. 14. A method of claim 12, wherein the eucaryotic cells comprise mammalian cells. 15. A method of claim 1, wherein the treatment of the composition in step (a) comprises treating the composition so as to disrupt the cells and obtain a cellular extract therefrom, and then subjecting the resulting cellular extract to centrifugation so as to obtain the solution containing the solu¬ ble proteins. 16. A method of claim 1, wherein the salt in the solu¬ tion of step (b) is selected from the group con¬ sisting of sodium sulfate, sodium chloride, ammo¬ nium sulfate, potassium chloride, ammonium acetate or a combination of any of the foregoing. 17. A method of claim 1, wherein the appropriate hy¬ drophobic substance comprises phenyl-Sepharose. 18. A method of claim 1, wherein the appropriate hy¬ drophobic substance comprises a suitable resin having a phenyl functional group. 19. A method of claim 18, wherein the suitable resin is a polysaccharide resin having phenyl groups present thereon. 20. A method of claim 1, wherein the contacting of step (c) is effected by passing the solution con¬ taining the soluble proteins through a column containing the hydrophobic substance. 21. A method of claim 1, which further comprises treating the resulting solution containing the soluble proteins from step (a) so as to obtain a more concentrated solution containing purified Cu- Zn superoxide dismutase or polypeptide analog thereof. 22. A method of claim 21, wherein the treatment of the solution to obtain a more concentrated solution of purified Cu-Zn superoxide dismutase or polypeptide analog thereof comprises ultrafiltration. 23. A method of claim 22, wherein the resulting more concentrated solution of purified Cu-Zn superoxide dismutase or polypeptide analog thereof comprises a, b and c isoforms of Cu-Zn superoxide dismutase or polypeptide analog thereof. 24. A method of claim 23, which further comprises treating the resulting more concentrated solution of purified Cu-Zn superoxide dismutase or polypeptide analog thereof so as to produce three separate solutions, each of which has an increased concentration of one of either of the a, b or c isoform. 25. A method of claim 24, wherein the treatment of the concentrated solution comprises anion exchange chromatography. 26. A method of claim 25, which further comprises subjecting the then-resulting solution which has an increased concentration of the b isoform of purified Cu-Zn superoxide dismutase or polypeptide analog thereof to cation exchange chromatography so as to produce three further separate solutions. each of which has a further increased concentra¬ tion of one of either of the a, b or c isoform. 27. A method of claim 26, which further comprises treating the then-resulting solution which has a further increased concentration of the b isoform of purified Cu-Zn superoxide dismutase or polypeptide analog thereof so as to further purify the Cu-Zn superoxide dismutase or polypeptide analog thereof contained therein prior to treating the resulting solution containing the further purified Cu-Zn superoxide dismutase or polypeptide analog thereof with a second solution containing a salt in step (b) . 28. A method of claim 27, wherein the treatment of the then-resulting solution comprises strong anion exchange chromatography. 29. A method of claim 28, wherein the strong anion exchange chromatography comprises passing the then-resulting solution through a column of a polysaccharide resin having quaternary amino func¬ tion groups present thereon. 30. A method of claim 25, which further comprises treating the then-resulting solutions which have an increased concentration of one of either of the a or c isoform so as to increase the concentration of b isoform and reduce the concentration of a isoform and c isoform of the polypeptide analog contained in each of the solutions prior to treat¬ ing the then-resulting solution containing the further purified Cu-Zn superoxide dismutase or polypeptide analog thereof with a second solution containing a salt in step (b) . 31. A method of claim 26, which further comprises treating the then-resulting solutions which have a further increased concentration of one of either of the a or c isoform so as to increase the con¬ centration of b isoform and reduce the concentra¬ tion of a isoform and c isoform of the polypeptide analog contained in each of the solutions prior to treating the then-resulting solution containing the further purified Cu-Zn superoxide dismutase or polypeptide analog thereof with a second solution containing a salt in step (b) . 32. A method of claim 30 or 31, wherein the treatment of the then-resulting solutions prior to step (b) comprises: (a) combining the separate solution which has an increased concentration of the a isoform with the separate solution which has an increased concentration of the c isoform; (b) treating the resulting combined solution so as to produce a solution which has an increased concentration of the b isoform; (c) recovering the then-resulting solution which has an increased concentration of the b isoform. 33. A method of claim 32, wherein the treatment of the resulting combined solution in step (b) comprises incubation. 34. A method of claim 32, which further comprises treating the solution with an increased concentra¬ tion of the b isoform from step (b) so as to puri¬ fy the b isoform present in the solution. 35. A method of claim 34, wherein the treatment of the solution comprises ion exchange chromatography. 36. A method of claim 35, wherein the ion exchange chromatography comprises anion exchange chromatog¬ raphy. 37. A method of claim 36, wherein the ion exchange chromatography further comprises cation exchange chromatography. 38. A method of claim 1, which further comprises treating the resulting solution containing puri¬ fied, enzymatically active Cu-Zn superoxide dismutase or polypeptide analog thereof recovered in step (d) so as to obtain a more concentrated solution containing purified Cu-Zn superoxide dismutase or polypeptide analog thereof. 39. A method of claim 38, wherein the treatment of the resulting solution to obtain a more concentrated solution of purified Cu-Zn superoxide dismutase or polypeptide analog thereof comprises ultra-filtra¬ tion. 40. A method of claim 38, which further comprises treating the resulting more concentrated solution containing purified Cu-Zn superoxide dismutase or polypeptide analog thereof so as to obtain a solu¬ tion containing more purified Cu-Zn superoxide dismutase or polypeptide analog thereof. 41. A method of claim 40, wherein the treatment of the resulting solution comprises strong anion exchange chromatography. 42. A method of claim 41, wherein the strong anion exchange chromatography comprises passing the resulting solution through a column of a polysaccharide resin having quaternary amino func¬ tional groups present thereon. 43. A method of claim 1, which further comprises treating the resulting solution containing puri¬ fied, enzymatically active Cu-Zn superoxide dismutase or polypeptide analog thereof recovered in step (d) so as to obtain a solution containing more purified Cu-Zn superoxide dismutase or polypeptide analog thereof. 44. A method of claim 43, wherein the treatment of the resulting solution comprises strong anion exchange chromatography. 45. A method of claim 44, wherein the strong anion exchange chromatography comprises passing the resulting solution through a column of a polysaccharide resin having quaternary amino func¬ tional groups present thereon. 46. A method of claim 1, which further comprises treating the resulting solution containing the soluble proteins from step (a) so as to purify the Cu-Zn superoxide dismutase or polypeptide analog thereof contained therein prior to treating the resulting solution containing the soluble proteins with a second solution containing a salt in step (b). 47. A method of claim 46, wherein the treatment of the resulting solution comprises strong anion exchange chromatography. 48. A method of claim 47, wherein the strong anion exchange chromatography comprises passing the resulting solution through a column of a polysaccharide resin having quaternary amino func¬ tional groups present thereon. 49. A method of claim 1, which further comprises treating the resulting solution containing the soluble proteins from step (a) , the soluble pro¬ teins including Cu-Zn superoxide dismutase or polypeptide analog thereof comprising a, b and c isoforms of Cu-Zn superoxide dismutase or polypeptide analog thereof, so as to increase the concentration of b isoform and reduce the concen- tration of a isoform and c isoform of the polypeptide analog contained in the solution prior to treating the resulting solution containing the soluble proteins with a second solution containing a salt in step (b) . 50. A method of claim 49, wherein the treatment of the resulting solution prior to step (b) comprises: (a) treating the resulting solution containing the soluble proteins so as to produce three separate solutions, each of which has an increased concentration of one of either of the a, b, or c isoform; (b) recovering the separate solution which has an increased concentration of the b isoform; (c) combining the separate solution which has an increased concentration of the a isoform with the separate solution which has an increased concentration of the c isoform; (d) treating the resulting combined solution so as to produce a solution which has an increased concentration of the b isoform; and (e) recovering the then-resulting solution which has an increased concentration of the b isoform. 51. A method of claim 50, wherein the treatment of the resulting combined solution in step (d) comprises incubation. 52. A method of claim 50, wherein the treatment of the resulting solution in step (a) comprises ion ex¬ change chromatography. 53. A method of claim 50, which further comprises treating the solution with an increased concentra¬ tion of the b isoform from step (b) or (e) so as to purify the b isoform present in the solution. 54. A method of claim 53, wherein the treatment of the solution comprises ion exchange chromatography. 55. A method of claim 54, wherein the ion exchange chromatography comprises anion exchange chroma¬ tography. 56. A method of claim 55, wherein the ion exchange chromatography further comprises cation exchange chromatography. 57. A method of increasing the yield of recovered solutions having an increased concentration of b isoform of an enzymatically-active polypeptide analog of Cu-Zn superoxide dismutase from a compo¬ sition which comprises cells containing a, b and c isoforms of the polypeptide analog which compris¬ es: (a) treating the composition so as to separate soluble proteins present in the cells from whole cells, cellular debris and insoluble proteins so as to obtain a solu- tion containing such soluble proteins, including the a, b and c isoforms; (b) treating the resulting solution contain¬ ing the soluble proteins so as to produce three separate solutions, each of which has an increased concentration of one of either of the a, b or c isoform; (c) recovering the separate solution which has an increased concentration of the b isoform; (d) combining the separate solution which has an increased concentration of the a isoform with the separate solution which has an increased concentration of the c isoform; (e) treating the resulting combined solution so as to produce a solution which has an increased concentration of the b isoform; and (f) recovering the then-resulting solution which has an increased concentration of the b isoform. 58. A method of claim 57, wherein the treatment of the resulting combined solution in step (e) comprises incubation. 59. A method of claim 57, wherein the polypeptide analog of Cu-Zn superoxide dismutase is a polypeptide analog of human Cu-Zn superoxide dismutase. 60. A method of claim 57, wherein the treatment of the composition in step (a) comprises treating the composition so as to disrupt the cells and obtain a cellular extract therefrom, and then subjecting the resulting cellular extract to centrifugation so as to obtain the solution containing the solu¬ ble proteins. 61. A method of claim 57, wherein the treatment of the resulting solution containing the soluble proteins so as to produce three separate solutions compris¬ es ion exchange chromatography. 62. A method of claim 57, which further comprises treating the solution with an increased concentra¬ tion of the b isoform from step (c) or (f) so as to purify the b isoform present in the solution. 63. A method of claim 62, wherein the treatment of the solution comprises ion exchange chromatography. 64. A method of claim 63, wherein the ion exchange chromatography comprises anion exchange chromato¬ graphy. 65. A method of claim 64, wherein the ion exchange chromatography further comprises cation exchange chromatography.

Inventors: BARTFELD DANIEL, HADARY DANY, LIFSHITZ RUTH, BARTFELD, DANIEL, HADARY, DANY, LIFSHITZ, RUTH
Applicants: BIO TECHNOLOGY GENERAL CORP, BIO-TECHNOLOGY GENERAL CORP.
Application year: 1990
Main classification: C12N9
IPC codes: C12N9