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 into this application in order to more fully describe the state of art as known to those skilled therein as of the date of the invention described and claimed herein.
Superoxide dismutase (SOD) and the phenomenon of oxygen free radicals (O2) was discovered in 1968 by McCord and Fridovich (1). Superoxide radicals and other highly reactive oxygen species are produced in every respiring cell as by-products of oxidative damage to a wide variety of macromolecules and cellular components (for review see 2,3). A group of metalloproteins known as superoxide dismutases catalyze the oxidation-reduction reaction 202+2H+xe2x86x92H2O2+O2 and thus provide a defense mechanism against oxygen toxicity.
There are several known forms of SOD containing different metals and different proteins. Metals present in SOD include iron, manganese, copper and zinc. All of the known forms of SOD catalyze the same reaction. These enzymes are found in several evolutionary groups. Superoxide dismutases containing iron are found primarily in prokaryotic cells. Superoxide dismutases containing copper and zinc has been found in virtually all eukaryotic organisms (4). Superoxide dismutases containing manganese have been found in organisms ranging from microorganisms to man.
Since every biological macromolecule can serve as a target for the damaging action of the abundant superoxide radical, interest has evolved in the therapeutic potential of SOD. The scientific literature suggests that SOD may be useful in a wide range of clinical applications. These include prevention of oncogenesis and of tumor promotion, and reduction of the cytotoxic and cardiotoxic effects of anticancer drugs (10), protection of ischemic tissues (12) and protection of spermatozoa (13). In addition, there is interest in studying the effect of SOD on the aging process (14).
The exploration of the therapeutic potential of human SOD has been limited mainly due to its limited availability.
Superoxide dismutase is also of interest because of its anti-inflammatory properties (11). Bovine-derived superoxide dismutase (orgotein) has been recognized to possess anti-inflammatory properties and is currently marketed in parts of Europe as a human pharmaceutical.
It is also sold in the United States as a veterinary product, particularly for the treatment of inflamed tendons in horses. However, supplies of orgotein are limited. Prior techniques involving recovery from bovine or other animal cells have serious limitations and the orgotein so obtained may produce allergic reactions in humans because of its non-human origin.
Copper zinc superoxide dismutase (CuZn SOD) is the most studied and best characterized of the various forms of superoxide dismutase.
Human CuZn SOD is a dimeric metallic-protein composed of identical non-covalently linked subunits, each having a molecular weight of 16,000 daltons and containing one atom of copper and one of zinc (5). Each subunit is composed of 153 amino acids whose sequence has been established (6,7).
The cDNA encoding human CuZn superoxide dismutase has been cloned (8). The complete sequence of the cloned DNA has also been determined (9). Moreover, expression vectors containing DNA encoding superoxide dismutase for the production and recovery of superoxide dismutase in bacteria have been described (24,25). The expression of a superoxide dismutase DNA and the production of SOD in yeast has also been disclosed (26).
Recently, the CuZn SOD gene locus on human chromosome 21 has been characterized (27) and recent developments relating to CuZn superoxide dismutase have been summarized (28).
Much less is known about manganese superoxide dismutase (MnSOD). The MnSOD of E. coli K-12 has recently been cloned and mapped (22). Barra et al. disclose a 196 amino acid sequence for the MnSOD polypeptide isolated from human liver cells (19). Prior art disclosures differ, however, concerning the structure of the MnSOD molecule, particularly whether it has two or four identical polypeptide subunits (19,23). It is clear, however, that the MnSOD polypeptide and the CuZn SOD polypeptide are not homologous (19). The amino acid sequence homologies of MnSODs and FeSOD from various sources have also been compared (18).
Baret et al. disclose in a rat model that the half life of human MnSOD is substantially longer than the half-life of human copper SOD; they also disclose that in the rat model, human MnSOD and rat copper SOD are not effective as anti-inflammatory agents whereas bovine copper SOD and human copper SOD are fully active (20).
McCord et al. disclose that naturally occurring human manganese superoxide dismutase protects human phagocytosing polymorphonuclear (PMN) leukocytes from superoxide free radicals better than bovine or porcine CuZn superoxide dismutase in xe2x80x9cin vitroxe2x80x9d tests (21).
The present invention concerns the preparation of a cDNA molecule encoding the human manganese superoxide dismutase polypeptide or an analog or mutant thereof. It is also directed to inserting this cDNA into efficient bacterial expression vectors, to producing human MnSOD polypeptide, analog, mutant and enzyme in bacteria, to recovering the bacterially produced human MnSOD polypeptide, analog, mutant or enzyme. This invention is also directed to the human MnSOD polypeptides, analogs, or mutants thereof so recovered and their uses.
This invention further provides a method for producing enzymatically active human MnSOD in bacteria, as well as a method for recovering and purifying such enzymatically active human MnSOD.
The present invention also relates to a DNA molecule encoding the human MnSOD gene. It is also directed to inserting the DNA into mammalian cells to produce MnSOD polypeptide, analog, mutant and enzyme.
The present invention also relates to using human manganese superoxide dismutase or analogs or mutants thereof to catalyze the reduction of superoxide radicals to hydrogen peroxide and molecular oxygen. In particular, the present invention concerns using bacterially produced MnSOD or analogs or mutants thereof to reduce reperfusion injury following ischemia and prolong the survival period of excised isolated organs. It also concerns the use of bacterially produced MnSOD or analogs thereof to treat inflammations.
A DNA molecule which includes cDNA encoding the human manganese superoxide dismutase polypeptide or analog or mutant thereof has been isolated from a human T-cell library. The nucleotide sequence of a double-stranded DNA molecule which encodes human manganese superoxide dismutase polypeptide or analog or mutant thereof has been discovered. The sequence of one strand encoding the polypeptide or analog thereof is shown in FIG. 1 from nucleotide 115 downstream to nucleotide 708 inclusive. Other sequences encoding the analog or mutant may be substantially similar to the strand encoding the polypeptide. The nucleotide sequence of one strand of a double stranded DNA molecule which encodes a twenty-four (24) amino acid prepeptide is also shown in FIG. 1, from nucleotides number 43 through 114, inclusive.
The double-stranded cDNA molecule or any other double-stranded DNA molecule which contains a nucleotide strand having the sequence encoding the human manganese superoxide dismutase polypeptide or analog or mutant thereof may be incorporated into a cloning vehicle such as a plasmid or virus. Either DNA molecule may be introduced into a cell, either procaryotic, e.g., bacterial, or eukaryotic, e.g., yeast or mammalian, using known methods, including but not limited to methods involving cloning vehicles containing either molecule.
Preferably the cDNA or DNA encoding the human manganese superoxide dismutase polypeptide or analog or mutant thereof is incorporated into a plasmid, e.g., pMSE-4 or pMSxcex94RB4, and then introduced into a suitable host cell where the DNA can be expressed and the human manganese superoxide dismutase (hMnSOD) polypeptide or analog or mutant thereof produced. Preferred host cells include Escherichia coli, in particular E. coli A4255 and E. coli A1645. The plasmid pMSE-4 in E. coli strain A4255 has been deposited with the American Type Culture Collection under ATCC Accession No. 53250. The plasmid pMS RB4 may be obtained as shown in FIG. 4 and described in the Description of the Figures.
Cells into which such DNA molecules have been introduced may be cultured or grown in accordance with methods known to those skilled in the art under suitable conditions permitting transcription of the DNA into mRNA and expression of the mRNA as protein. The resulting manganese superoxide dismutase protein may then be recovered.
Veterinary and pharmaceutical compositions containing human MnSOD or analogs or mutants thereof and suitable carriers may also be prepared. This human manganese superoxide dismutase or analogs or mutants may be used to catalyze the following reaction:
202+2H+xe2x86x92H2O2+O2
and thereby reduce cell injury caused by superoxide radicals.
More particularly, these enzymes or analogs or mutants thereof may be used to reduce injury caused by reperfusion following ischemia, increase the survival time of excised isolated organs, or treat inflammations.
This invention is directed to a method of producing enzymatically active human manganese superoxide dismutase or an analog or mutant thereof in a bacterial cell. The bacterial cell contains and is capable of expressing a DNA sequence encoding the manganese superoxide dismutase or analog or mutant thereof. The method comprises maintaining the bacterial cell under suitable conditions and in a suitable production medium. The production medium is supplemented with an amount of Mn++ so that the concentration of Mn++ available to the cell in the medium is greater than about 2 ppm.
In a preferred embodiment of the invention the bacterial cell is an Escherichia coli cell containing a plasmid which contains a DNA sequence encoding for the human manganese superoxide dismutase polypeptide e.g. pMSE-4 or pMSxcex94RB4 in E. coli strain A4255. The concentration of Mn++ in the production medium ranges from about 50 to about 1500 ppm, with concentrations of 150 and 750 ppm being preferred.
This invention also concerns a method of recovering manganese superoxide dismutase or analog thereof from bacterial cells which contain the same. The cells are first treated to recover a protein fraction containing proteins present in the cells including human manganese superoxide dismutase or analog or mutant thereof and then the protein fraction is treated to recover human manganese superoxide dismutase or analog or mutant thereof. In a preferred embodiment of the invention, the cells are first treated to separate soluble proteins from insoluble proteins and cell wall debris and the soluble proteins are recovered. The soluble proteins are then treated to separate, e.g. precipitate, a fraction of the soluble proteins containing the hMnSOD or analog or mutant thereof and the fraction containing the hMnSOD or analog or mutant is recovered. The recovered fraction of soluble proteins is then treated to separately recover the human manganese superoxide dismutase or analog thereof.
A more preferred embodiment of the invention concerns a method of recovering human manganese superoxide dismutase or analog or mutant thereof from bacterial cells which contain human manganese superoxide dismutase or analog or mutant thereof. The method involves first isolating the bacterial cells from the production medium and suspending them in suitable solution having a pH of about 7.0 to 8.0. The cells are then disrupted and centrifuged and the resulting supernatant is heated for about 30 to 120 minutes at a temperature between 55 and 65xc2x0 C., preferably for 45-75 minutes at 58-62xc2x0 C. and more preferably for 1 hour at 60xc2x0 C. and then cooled to below 10xc2x0 C., preferably to 4xc2x0 C. Any precipitate which forms is to be removed e.g. by centrifugation, and the cooled supernatant is dialyzed against an appropriate buffer e.g. 2 mM potassium phosphate buffer having a pH of about 7.8. Preferably, the dialysis is by ultrafiltration using a filtration membrane smaller than 30K. Simultaneously with or after dialysis the cooled supernatant optionally may be concentrated to an appropriate convenient volume e.g. 0.03 of its original volume. The retentate is then eluted on an anion exchange chromatography column with an appropriate buffered solution e.g. a solution of at least 20 mM potassium phosphate buffer having a pH of about 7.8. The fractions of eluent containing superoxide dismutase are collected, pooled and dialyzed against about 40 mM potassium acetate, pH 5.5. The dialyzed pooled fractions are then eluted through a cation exchange chromatography column having a linear gradient of about 40 to about 200 mM potassium acetate and a pH of 5.5. The peak fractions containing the superoxide dismutase are collected and pooled. Optionally the pooled peak fractions may then be dialyzed against an appropriate solution e.g. water or a buffer solution of about 10 mM potassium phosphate buffer having a pH of about 7.8.
The invention also concerns purified enzymatically active human manganese superoxide dismutase or analogs thereof e.g. met-hMnSOD, or mutants produced by the methods of this invention.
The present invention also relates to a DNA molecule encoding the human MnSOD gene. The nucleotide sequence of the exon coding regions of one strand of the MnSOD gene is shown in 6. The DNA encoding the MnSOD gene may be incorporated into a cloning vehicle such as a plasmid or a virus. The DNA or the cloning vehicle may be introduced into eucaryotic cells using known methods. Preferably, the DNA encoding the human manganese superoxide dismutase gene is encoded in the plasmids pMSG11-1, pMSG4 and pMSG-1b. The eucaryotic cells which are transformed are preferably mammalian cell lines such as the human HeLa cell line or the mouse L cell line. Another aspect of this invention is the production of the human manganese superoxide dismutase polypeptide, analog, mutant or enzyme by growing the cells of this invention in suitable medium under suitable conditions.