Abstract:
The present invention provides a process for the activation of gene-technologically produced, biologically active proteins expressed in prokaryotes after cell digestion by solubilization under denaturing conditions and reducing conditions and subsequent reactivation under oxidizing and renaturing conditions, wherein working is carried out at a protein concentration of 1 to 1000 μg./ml. and, between the solubilization and the reactivation, a dialysis is carried out against a buffer with a pH value of from 1 to 4 containing 4 to 8 mole/liter guanidine hydrochloride or 6 to 10 mole/liter urea.

Description:
The present invention is concerned with a process for the activation of recombinant, biologically-active proteins expressed in prokaryotes. 
     The gene-technological production of proteins, for example antibodies, in heterologous host organisms leads to the formation of inactive, sparingly-soluble protein aggregates, i.e. so-called &#34;inclusion bodies&#34;. It is assumed that the formation of such &#34;inclusion bodies&#34; is, inter alia, a result of the high protein concentrations in the cell arising following expression. In order to obtain biologically-active proteins, the inclusion bodies must be dissolved by renaturation and reduction and then the three-dimensional structure of the protein in its native spatial form again produced by the adjustment of suitable solution conditions (see M. Sela et al., Science, 125, 691/1957). The complete unfolding is achieved by the addition of high concentrations of chaotropic agents, for example urea or guanidine (R. Jaenicke, Prog. Biophys. Molec. Biol., 49, 117-237/1987). For the reduction of disulphide bridges, strong reducing agents, for example β-mercaptoethanol or 1,4-dithioerithritol are used. 
     However, when the renaturing the proteins from the denatured state, two competing reactions occur. Besides the desired folding into the native active state, aggregate formation is observed (see G. Zettlmeissl et al., Biochemistry, 18, 5567). In order to shift the equilibrium to the side of the native molecule, conditions are selected which, both, prevent the establishment of falsely folded and thus more unstable molecules and the non-specific exchange action thereof to aggregates and, does not hinder the refolding into the native state. This is achieved by the addition of chaotropic agents in labilizing concentrations. In addition, it must be observed that the protein concentration has a critical effect on the yield of renatured protein (see European Patent Specification No. 0,241,022). In the case of proteins which have dissulphide bridges in the native state, it is also necessary to provide redox conditions during the renaturing which make it possible to reduce falsely paired cystines producing protein molecules with correctly paired cysteins. So-called &#34;oxido-shuffling solutions&#34; of reduced and oxidised thiol reagent increase the yield of nativestructured and disulphide-bridged proteins. 
     That the renaturing of antibodies is possible after complete denaturing and reduction was shown for the first time by Haber using the example of a Fab fragment (see E. Haber, Biochemistry, 52, 1099-1106/1964). The yields amounted to 12 to 14%. These results were confirmed by the renaturing of a Fab fragment by Whitney and Tanford. They achieved yields of 8% (see P. L. Whitney and C. Tanford, Proc. Natl. Acad. Sci. USA, 53, 524/1965). A complete antibody was successfully renatured for the first time by Freedman and Sela, the yields being from 20 to 25% (see M. H. Freedman and M. Sela, J. Biol. Chem., 241, 2383-2396/1966; J. Biol. Chem., 241, 5225-5232/1966). It is to be noted that the native starting molecule was polyalanylated for the improvement of its solubility. 
     In the cited work, it is to be observed that the concern was with polyclonal antibodies or antibody fragments, i.e. a mixture of antibodies with different paratopes and different affinity constants. Since the heavy and light chains of this heterologous population associate purely statistically during of renaturation, renatured molecules are produced which do not agree with the native ones in terns of binding specificity and affinity. 
     As in the case of other eukaryotic proteins which have been cloned and expressed in Escherichia coli, heavy and light chains of antibodies expressed in Escherichia coli are also obtained in the form of insoluble &#34;inclusion bodies&#34; (see S. Cabilly et al., Proc. Natl. Acad. Sci. USA, 81, 3273-3277/1984; M. Y. Boss et at., Nucleic Acids Research, 12, 3791-3806/1984). For antibodies from transformed micro-organisms, processes were described by Cabilly et al. and Boss et al. which are said to make possible the renaturing of functional antibodies. However, for monoclonal antibodies, these processes only give yields of from about 0.2 to 5%. 
     Therefore, it is an object of the present invention to provide a process for the reactivation of biologically-active proteins expressed in prokaryotes in which the proteins obtained after expression in the form of &#34;inclusion bodies&#34; can be converted into their active renatured form in good yield. 
     Thus, according to the present invention, there is provided a process for the activation of recombinant biologically-active proteins expressed in prokaryotes by solubilization under denaturing and reducing conditions and subsequent reactivation under oxidising and renaturing conditions, wherein working is carried out at a protein concentration of from 1 to 1000 μg./ml. and, between the solubilization and the reactivation, dialysis is carried out against a buffer with a pH value of from 1 to 4 containing 4 to 8 mole/liter guanidine hydrochloride or 6 to 10 mole/liter urea. 
     Surprisingly, we have ascertained that after dialysis in the presence of urea or, especially in the presence of guanidine hydrochloride, extraordinarily high yields of active protein are obtained after the reactivation. 
     The present invention is suitable for use in connection with all recombinant proteins produced in prokaryotes. In particular production of antibodies and fragments thereof and for t-PA and t-PA-like proteins and derivatives thereof can be enhanced. 
     In a preferred embodiment of the present invention, the dialysis buffer contains 5 to 7 mole/liter of guanidine hydrochloride. 
     It is preferred to work at a pH value of from 9 to 12, when reactivating the proteins together with a GSH (reduced glutathione) concentration of 0.1 to 20 mmole/liter, a GSSG (glutathione disulfide) concentration of 0.01 to 3 mmole/liter and with a non-denaturing concentration of the denaturing agent. The reactivation is carried out over a period of from 1 to 300 hours. In an especially preferred embodiment, the GSH concentration is from 0.2 to 10 mmole/liter and/or the GSSG concentration of from 0.05 to 1 mmole/liter. 
     In another preferred embodiment of the present invention, in the reactivation stage, the thiol groups of the antibody are first converted into mixed disulphides of antibody and glutathione, via adding GSSG under denaturing conditions followed by dialysis against the buffer containing guanidine hydrochloride or urea and then reactivated at a pH value of from 6 to 10, using a GSH concentration of 0.1 to 5 mmole/liter and a nondenaturing concentration of a denaturing agent over a period of time of from 1 to 300 hours. 
     As denaturing agent, as a rule, a denaturing agent conventionally employed for the activation of proteins under oxidizing conditions or arginine can be used. Preferably, the agent used is arginine guanidine hydrochloride and/or at least one compound of the general formula: 
     
         R.sub.2 --CO--NRR.sub.1                                    (I) 
    
     wherein R and R 1  are hydrogen atoms or alkyl radicals containing from 1 to 4 carbon atoms and R 2  is a hydrogen atom or --NRR 1  or an alkyl radical containing from 1 to 3 carbon atoms. These denaturing agents can also be used in the form of mixtures. The concentration of arginine and/or guanidine hydrochloride is preferably from 0.1 to 1.0 mole/liter and an especially preferred concentration is from 0.25 to 0.8 mole/liter. The concentration of the compound of general formula I is preferably from 0.5 to 4 mole/liter and a especially preferred concentration is from 1 to 3.5 mole/liter. 
     In a further preferred embodiment of the present invention, the reactivation step is carried out in the presence of a foreign protein. As such, can be used any foreign protein as long as it is not proteolytically-active. It is preferred to use bovine serum albumin (BSA), for example in an amount of from 1 to 3 mg./ml. The addition of BSA brings about a slight increase of the yield and a stabilization of the protein, probably by protection against surface denaturing and/or proteolytic breakdown. 
     The usual process conditions correspond to the conditions known and usual for reactivation steps. Reactivation is advantageously carried out at a temperature from about 5° C. to 30° C. and preferably at about 10° C. The process steps preceding and following the dialysis and reactivation step (reoxidation/activation), such as cell digestion, solubilization (solubilization, reduction), can be carried out by the methods known from the prior art as taught, for example in European Patent Specifications Nos. A-0,114,506 and A-0,093,619, for the reactivation of heterologously expressed proteins or of t-PA. However, for a result which is optimum with regard to yield and activation, it is advantageous to carry out individual or all process steps using the protocols as follows explained. 
     Cell digestion can be carried out by usual methods usual for this purpose, for example by means of ultrasonics, high pressure dispersion or lysozyme digestion can be used. Preferably, the digestion is carried out in a buffer solution appropriate for the adjustment of a neutral to weakly acidic pH value as suspension medium, for example in 0.1 mole/liter Tris/HCl. After the cell digestion, the insoluble components (inclusion bodies) are separated off in any desired way, preferably by centrifuging or filtration. After washing with agents which do not disturb the proteins but which dissolve foreign proteins as far as possible. The washing agent may be for example, water or a phosphate buffer, either of which may contain mild detergents, such as Triton. Following washing, the precipitate or pellet is subjected to solubilization (solubilization/reduction). 
     Solubilization preferably takes place in the alkaline pH range and especially at pH 8.6±0.4 and in the presence of a reducing agent of the mercaptan group and of a denaturing agent. 
     As denaturing agents, those which are known from the prior art, for example European Patent Specification No. A-0,114,506 can be used, especially preferred are guanidine hydrochloride or compound of general formula I. Especially preferred is a concentration of guanidine hydrochloride of 6 mole/liter or a concentration of the compound of general formula I of 8 mole/liter. 
     As reducing agent from the mercaptan group, there reduced glutathione (GSH) e.g., can be used or 2-mercaptoethanol, for example in a concentration of about 50 to 400 mole/liter and/or especially dithioerythritol (DTE) or dithiothreitol (DTT), for example in a concentration of about 80 to 400 mmole/liter., or cysteine. Solubilization advantageously takes place at ambient temperature for a period of from 0.5 to several hours. Preferably, this incubation is and for 2 hours. For the prevention of oxidation of the reducing agent by atmospheric oxygen, it can be advantageous to added EDTA thereto, preferably in an amount of 1 to 10 mmole/liter. Besides the solubilization reduction, the solubilization step also has a purifying effect since the greater part of the foreign proteins does not go into solution. 
     Another embodiment of the present invention depends upon the formation between the mixed disulphides of biologically-active proteins expressed in prokaryotes and glutathione before the reactivation step. For the formation of these mixed disulphides of protein and glutathione, the dialysed and reduced proteins which have been separated from reducing agents are incubated with a denaturing agent-containing dilute solution of GSSG, at for example a concentration of 0.2 mole/liter. Activation takes place after separating the oxidation agent by renewed dialysis against the guanidine hydrochloride- or urea-containing buffer at a pH value of from 6 to 10, with a GSH concentration of 0.1 to 5 mmole/liter and with a non-denaturing concentration of a denaturing agent over a period of time of from 1 to 300 hours. 
     In all other reaction steps, the activation of the protein via the formation of the mixed disulphides with GSSG corresponds to the embodiments for the activation of proteins of the previously described part of the present invention. In the case of this embodiment, the pH optimum is from 6 to 8 and the activated proteins are stable for a comparatively long period of time in the renaturing buffer. 
     According to the present invention, it is possible, for example, to reactivate as antibodies as which have been expressed in prokaryotes with a yield of up to 30% with reference to their immune reactivity. This corresponds to an increase of about 10 fold compared to processes known from the prior art. &#34;Antibodies&#34;, as used herein includes whole antibodies, as well as all known fragments thereof. 
     The following Examples are given for the purpose of illustrating the present invention, with reference to the accompanying drawings. If not stated otherwise, the statements of percentage are percentages by weight. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 shows the sequence of plasmid pBT111 for the expression of the kappa chain of MAB 33 (nucleotide positions 7 to 663); and 
     FIG. 2 shows the DNA sequence of plasmid p10169 for the expression of the Fd chain of MAB 33 (nucleotide positions 240 to 917). 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Example 1 
     Expression of antibody fragments in Escherichia coli.1.1. Construction of a plasmid for the expression of the MAB 33 kappa chain in Escherichia coli. 
     The cloning of kappa chain cDNA for MAB 33 as Pst I fragment in pBR 322 has been described (see P. Buckel et al., Gene, 51, 13-19/1987). With the restriction endonuclease Mnl I, the cDNA was cleaved nucleotide-neighbouring the first amino acid codon of the mature kappa chain. This cDNA was cloned with the help of an adapter (5&#39;CATG3&#39; hybridized with 5&#39;CATGAATT3&#39;) as an Eco RI-Pst I fragment into the vector pKK 223-3, DSM 3694P, which had also been cleaved with Eco RI and Pst I (see Brosius et al., Plasmid, 6, 112-118/1981). For shortening the 3&#39;-untranslated region of the cDNA, the resulting plasmid was cleaved with Pst I, shortened nucleolytically with Bal 31 and subsequently the Eco RI-Bal 31 fragment corresponding to the kappa chain cDNA was cloned back into the vector with the help of a Hind III linker. The resulting plasmid is designated as pBT 111 (FIG. 1 of the accompanying drawings shows the sequence of the expression plasmid; kappa chain runs from nucleotide position 7 to 663). 
     1.2. Construction of a plasmid for the expression of the Fc Fd fragment of the gamma chain of MAB 33 in E. coli 
     The cloning of the MAB 33 gamma chain cDNA as a Pst I fragment into pBR 322 has also been described (see Buckel et al., v. supra, 1987). For the expression, immediately before the first amino acid of the mature gamma chain an Xma I cleavage position was introduced by means of oligonucleotide-directed mutagenesis. A gamma Fc fragment was produced by the introduction of a stop codon after amino acid position 225 by means of the same technique, in which case Bcl I and a Sal I cleavage position were also introduced. The resulting Xma I-Sal I fragment, which codes for the gamma Fc fragment, was cloned into pUC8 (see Vieira and Messing, Gene, 19, 259-268/1982). FIG. 2 of the accompanying drawings shows the sequence of the resulting expression plasmid p10169 (gamma Fc from nucleotide positions 240 to 917). 
     1.3 Expression of antibody chains in E. coli 
     The expression plasmids pBT111 and p10169 were each transformed individually into Escherichia coli (DSM 3689) which also contained a plasmid for the expression of the lac repressor (lac I q ) in trans. The Escherichia coli cells were cultured in LB medium up to an optical density of OD 550  nm =0.5, then induced with 1 g./liter isopropyl-β-D-thiogalactoside (IPTG) and incubated for a further 4 hours at 37° C. Subsequently, the cells were centrifuged off. 
     1.4 Preparation of the &#34;inclusion bodies&#34; 
     For this purpose, there were taken, per immunoglobulin chain, about 25 g. Escherichia coli (cf. Example 1.3) Moist cell mass in 580 ml. 0.1 mole/liter Tris/HCl (pH 6.5) and 20 mmole/liter EDTA and the cells homogenized with a shearing rod (Ultraturax). Lysozyme at 0.25 mg./ml. was added, followed by incubation for 30 minutes at ambient temperature. This was subsequently suspended in 0.5 mole/liter aqueous sodium chloride solution containing 5% v/v Triton X-100 and homogenized with a shearing rod (Ultraturax) and further stirred for 30 minutes at ambient temperature. Thereafter, centrifugation was carried out in a Sorvall GSA rotor for 50 minutes at 4° C. and 13000 r.p.m. The pellets were taken up in 300 ml. 0.1 mole/liter Tris/HCl (pH 6.5), 20 mmole/liter EDTA and 2.5% v/v Triton X-100 and homogenized. Further centrifugation was then carried out for 30 minutes at 4° C. and 13000 r.p.m., again in a Sorvall GSA rotor. The pellets were taken up in 300 ml. 0.1 mole/liter Tris/HCl (pH 6.5), 20 mmole/liter EDTA and 0.5% v/v Triton X-100 and homogenized. Thereafter, there were two further centrifugations were carried out, each for 30 minutes at 4° C. at 13000 r.p.m., in a Sorvall GSA rotor and in each case thereafter the pellets were taken up in 300 ml. and 250 ml., respectively, of 0.1 mole/liter Tris/HCl (pH 6.5) and 20 mmole/liter EDTA and homogenised. 
     Example 2 
     Denaturing of the antibody 
     Lyophilisates of antibody MAB 33 obtained from hybridoma cells lines (ECACC 88091404) or Fab fragments thereof (for the production of Fab fragments, cf. A. Johnstone and R. Thorpe in Immunochemistry in Practice, pub. Blackwell Scientific Publications, 1982, pp. 52-53), as well as the pellets of the &#34;inclusion bodies&#34; prepared according to Example 1, were incubated in 0.1 mole/liter Tris. HCl (pH 8.5), 6 mole/liter guanidine hydrochloride, 2 mmole/liter EDTA and 0.3 mole/liter DTE for 3 hours at ambient temperature. The protein concentration was from 4 to 6 mg./ml. Chain separation was examined with SDS-PAGE under non-reducing conditions. The complete reduction of the disulphide bridges was confirmed by the determination of the free SH groups according to the method of G. L. Ellmann (Arch. Biochem. Biophys., 82, 70/1959). Subsequently, the solutions were adjusted to pH 3 with concentrated hydrochloric acid. 
     The Examples given in the following for renaturation were carried out, as described in Example 2, with completely denatured, reduced MAB 33 or MAB 33 Fab or the antibody chains from the &#34;inclusion bodies&#34; in 1:100 dilution in reoxidation buffer after dialysis against 6 mole/liter guanidine hydrochloride (pH 2). The renaturing batches were thermostated at 20° C. 
     Example 3 
     Renaturing of MAB 33 Fab fragments. 
     The reoxidation buffer contained 0.1 mole/liter Tris. HCl (pH 8.5), 0.5 mole/liter L-arginine and 2 mmole/liter EDTA. 
     The protein concentration ranged from 30 to 60 μg./ml. The period for renaturation was up to 200 hours. The reoxidation was examined with the conformation-specific ELISA test system (see Example 8) for passive immunoreactivity. 
     The following Tables 1A to 1C show the yield of active MAB 33 Fab fragment depending upon the variation of: 
     1A the DTE concentration in the case of constant GSSG concentration (5 mmole/liter GSSG); 
     1B the GSSG concentration in the case of constant DTE concentration (3 mmole/liter DTE); 
     1C the GSSG concentration in the case of constant GSH concentration (1 mmole/liter). 
     
                       TABLE 1A______________________________________DTE (mmole/liter)            yield (%)______________________________________0                531                533                415                 810                3______________________________________ 
    
     
                       TABLE 1B______________________________________GSSG (mmole/liter)             yield (%)______________________________________0                 181                 153                 235                 3210                37______________________________________ 
    
     
                       TABLE 1C______________________________________GSSG (mmole/liter)             yield (%)______________________________________0                 220.1               531                 385                 28______________________________________ 
    
     Example 4 
     Completely denatured, reduced MAB 33 Fab fragments were dialysed against 6 mole/liter guanidine hydrochloride (pH 2) and subsequently reoxidised in 0.1 mole/liter Tris. HCl (pH 8.5), 0.3 mole/liter guanidine hydrochloride, 0.2 mole/liter GSSG, 2 mmole/liter GSH and 2 mmole/liter EDTA at a temperature of 20° C. and a period of renaturing of about 200 hours. The following Table 2 shows the yield of active MAB 33 Fab depending upon the variation of the protein concentration in the case of the renaturing. The reoxidation was examined with the Elisa test system for active immunoreactivity (see Example 8). 
     
                       TABLE 2______________________________________Fab concentration            yield(μg./ml.)     (%)______________________________________ 5               1310               920               430               370               1130              0.5660              0______________________________________ 
    
     Example 5 
     Completely denatured, reduced MAB 33 antibodies were dialysed against 6 mole/liter guanidine hydrochloride (pH 2) and were subsequently reoxidized in 0.1 mole/liter Tris.HCl (pH 8.5), 0.5 mole/liter L-arginine, 2 mmole/liter EDTA and 1 mmole/liter GSH at a temperature of 20° C. and a period of renaturing of about 200 hours. Reoxidation was examined with the Elisa test system (see Example 8) for active immunoreactivity. The following Table 3 shows the yield of active antibody depending upon the variation of the GSSG concentration at constant GSH concentration (1 mmole/liter). 
     
                       TABLE 3______________________________________GSSG (mmole/liter)             yield (%)______________________________________0                 2.00.1               3.50.5               4.71                 4.82                 4.24                 3.96                 3.410                2.7______________________________________ 
    
     Example 6 
     Renaturing after derivatisation to the mixed disulphide. 
     The denaturing of MAB 33 or MAB 33 Fab was carried out as described in Example 2. Dialysis against 6 mole/liter guanidine hydrochloride followed (pH 2) and subsequently derivatisation with 6 mole/liter guanidine hydrochloride, 0.2 mole/liter GSSG and 0.1 mole/liter Tris.HCl (pH 8.5) at ambient temperature over a period of time of about 5 hours took place. After renewed dialysis against 6 mole/liter guanidine hydrochloride (pH 2), the renaturing was carried out in different variants. 
     Table 4A shows the renaturing of MAB 33 Fab in 0.1 mole/liter Tris.HCl (pH 7), 0.5 mole/liter L-arginine and 2 mmole/liter EDTA at 20° C. over a period of about 200 hours. The renaturing was tested with the conformation-specific Elisa test system for passive (see Example 8) immunoreactivity depending upon the GSH concentration. 
     Table 4B shows the yield of active MAB 33 Fab in the above buffer with 2 mmole/liter GSH depending upon the pH of the buffer. 
     Table 5A shows the renaturing of MAB 33 IgG in 0.1 mole/liter Tris.HCl (pH 7.0), 0.5 mole/liter L-arginine and 2 mmole/liter EDTA at a temperature of 20° C. over a period of renaturing of about 200 hours depending upon the GSH concentration. The renaturing was examined with the conformation-specific Elisa Test system for passive immunoreactivity (see Example 8). 
     Table 5B shows the yield of active antibody in the above buffer with 2 mmole/liter GSH depending upon the pH value. 
     
                       TABLE 4A______________________________________GSH (mmole/liter)            yield (%)______________________________________0                 50.1              480.5              481                412                363                314                277                2410               23______________________________________ 
    
     
                       TABLE 4B______________________________________   pH   yield (%)______________________________________   7    40   8    39   9    30   10   19   11    1______________________________________ 
    
     
                       TABLE 5A______________________________________GSH (mmole/liter)            yield (%)______________________________________0                 0.30.1              3.00.2              4.00.5              3.81                3.12                2.53                2.06                1.410               1.4______________________________________ 
    
     
                       TABLE 4B______________________________________   pH   yield (%)______________________________________   6    3.9   7    3.8   8    2.9   9    1.5   10   0.8   11   0.6______________________________________ 
    
     Example 7 
     Renaturing of antibody chains expressed in Escherichia coli. 
     Inclusion bodies of the complementary single chain (kappa and Fd, Example 1) were solubilized as described in Example 2. Subsequently, dialysis was carried out against 6 mole/liter guanidine hydrochloride (pH 2). Solubilization and dialysis took place separately for both chains. 
     For the reoxidation, equimolar amounts of the reduced single chains were simultaneously diluted in a 100 fold volume of 0.1 mole/liter Tris-HCl (pH 8.5), 0.6 mole/liter L-arginine, 0.1 mmole/liter GSSG, 1 mmole/liter GSH and 2 mmole/liter EDTA. Renaturing took place by incubation over a period of 200 hours at 20° C. 
     The yield of native, biologically-active protein amounted to 18%. The proportion of native antibodies was determined by measurement of the passive immunoreactivity (see Example 8). 
     Example 8 
     Detection of passive and active immunoreactivity, as well as inhibiting activity 
     8.1. Detection of the passive immunoreactivity of the monoclonal antibody against human CK-MM (skeletal muscle isoenzyme of human creatine kinase) 
     By passive immunoreactivity is here to be understood the formation of native-structured epitopes on the native or renatured antibodies. The detection of these epitopes takes place by conformation-specific anti-mouse antibodies, which have been obtained from another type of animal (here sheep), in an Elisa test system. 
     Preparation of conformation-specific, polyclonal sheep anti-mouse Fab antibodies. 
     Antiserum against polyclonal mouse immunoglobulin was prepared in sheep according to the procedure of A. Johnstone and R. Thorpe, Immunochemistry in Practice, pub. Blackwell Scientific Publications, Oxford, 1982, pp. 27-31. 
     The IgG fraction of the antiserum was isolated according to Johnstone and Thorpe (loc. cit., pp. 44-46) by ammonium sulphate fractionation and DEAE-ion exchange chromatography. 
     500 mg. of this IgG fraction were adsorbed on 20 ml. of a mouse kappa chain Spherosil immunoadsorber (1.5 mg. mouse kappa chain/ml. Spherosil) until anti-mouse kappa activity was no longer detectable by an enzyme immune test. The IgG fraction was then passed over a column with 20 ml. of a mouse Fab Spherosil filling (5 mg. mouse Fab/ml. Spherosil) and the specifically adsorbed portion of the IgG fraction eluted with 0.2 mole/liter glycine (pH 2.8). After dialysis against 1 mmole/liter acetic acid, this fraction was lyophilized. 
     By means of immune tests, it was ascertained that the so produced IgG did not react with free M-kappa, M-gamma and M-Fd chains or with incompletely folded M-kappa/M-Fd complexes. Binding takes place with M-Fab, M-IgG and completely folded complexes of M-kappa/M-Fd and M-kappa/M-gamma. 
     Renatured antibodies and native standard samples were pre-incubated with conformation-specific antimouse Fab antibodies. Depending upon the number of native formed conformation epitopes, differing numbers of binding positions of the conformation-specific antimouse Fab antibody were saturated. The pre-incubation solution was introduced into test tubes, together with murine antibody-enzyme conjugate (antibody-peroxidase conjugate), the walls of the test tubes being coated with mouse antibodies. 
     The unsaturated conformation-specific anti-mouse antibodies from sheep then bridged the wall-bound mouse antibodies with the mouse antibody-enzyme conjugates, the amount of bound conjugate there being indirectly proportional to the amount of native or renatured antibodies in the pre-incubation solution. The spectroscopic detection took place, after reaction of the chromogenic substrate ABTS® (2,2&#39;-azino-bis-(3-ethylbenzothiazoline-6-sulphonate) by the antibody-conjugated enzyme, by absorption measurement at 405 nm (see H. U. Bergmeyer, Methods in Enzymology, 3rd edition, Volume 9, pp. 15-37). 
     8.2. Detection of the active immunoreactivity of the monoclonal antibody 
     MAB 33 IgG specifically recognises the skeletal muscle isoenzyme of human creatine kinase (CK-MM; cf. P. Buckel et al., Gene, 51, 13-19/1987). 
     By active immunoreactivity is here to be understood the reaction of the native or regenerated antibody with the specific antigen (in the present case, human CK-MM). 
     In an Elisa test system, this reaction was colorimetrically quantitatively determined quantitatively with the help of enzyme-conjugated, polyclonal anti-mouse antibodies via the reaction of the chromogenic substrate ABTS. 
     8.3. Detection of the inhibition activity of the monoclonal antibodies 
     MAB 33 IgG recognises an epitope which is only formed on the M subunits of the skeletal muscle-specific isoenzyme CK-MM. By binding to the epitope, the enzymatic activity is inhibited by 80% (see P. Buckel et al., 1987). The CK-MM inhibition test is thus a very conclusive test for the detection of the reconstitution of MAB 33 after complete denaturing and reduction since only a completely renatured antigen binding position can inhibit the enzyme. 
     The activity of the creatine kinase was determined with the help of a coupled enzymatic test of Boehringer Mannheim GmbH (see H. U. Bergmeyer, Methods of Enzymatic Analysis, 3rd edition Volume III, pp 510-540). Creatine kinase thereby reacts with creatine phosphate and ADP to give creatine and ATP. In order to obtain a spectroscopically detectable reaction, the resultant ATP is utilized by hexokinase for the phosphorylation of glucose to give glucose-6-phosphate. Glucose-6-phosphate is oxidized by glucose-6-phosphate dehydrogenase with the formation of NADPH+H +  from NADP +  to give gluconate-6-phosphate. The activity of the creatine kinase can be calculated from the change of extinction per minute. 
     From a calibration curve with native antibody against CK-MM, the amount of native material which corresponds to the inhibition action in the renatured batch can be determined. The proportion of inhibition-active protein, referred to the total amount of protein in the renaturing batch, gives the percentage yield of inhibition-active antibodies. 
     8.4. Comparative determination of the renaturing yield by means of passive/active immunoreactivity and inhibition activity 
     Completely denatured, reduced MAB 33 Fab fragments were dialysed against 6 mole/liter guanidine hydrochloride (pH 2) and subsequently reoxidised by 1:100 dilution in 0.1 mole/liter Tris-HCl (pH 8.5), 0.5 mole/liter L-arginine, 0.2 mmole/liter GSSG, 2 mmole/liter GSH and 2 mmole/liter EDTA at a temperature of 20° C. and a period of renaturing of about 200 hours. 
     On the basis of the relatively high antibody concentration which must be used in the case of the measurement of the inhibition activity, the renaturing batch was concentrated. For this purpose, the renatured solution was dialysed against 1 mmole/liter acetic acid and subsequently lyophilised. The lyophilisate was taken up in water (water:renaturing volume=1:200) and dialysed against 50 mmole/liter potassium phosphate, 0.15 mole/liter sodium chloride (pH 7.5). 
     The following Table 6 shows the yield of active Fab fragment in the dialysate, determined by passive-active immunoreactivity and inhibition activity. 
     
                       TABLE 6______________________________________test method         yield (%)______________________________________passive immunoreactivity               28active immunoreactivity               26inhibition activity 25______________________________________ 
    
     Example 9 
     Activation of t-PA from Escherichia coli 
     As described in Example 1.4, &#34;inclusion bodies&#34; are prepared as pellets from moist cell mass of Escherichia coli, DSM 3689, transformed with pePa 133 (see European Patent Specification No. 0,242,835). The pellets of the &#34;inclusion body&#34; preparation were incubated in 0.1 mole/liter Tris-HCl (pH 8.5); 6 mole/liter guanidine hydrochloride; 2 mmole/liter EDTA and 0.3 mole/liter DTE for 3 hours at ambient temperature at a protein concentration of from 4 to 6 mg./ml. Subsequent to the solubilisation, the pH of the solution was adjusted to pH 3 with concentrated hydrochloric acid. The reducing agent and buffer components were separated off by dialysis against 6 mole/liter guanidine hydrochloride (pH 2) at 4° C. 
     Renaturing of t-PA 
     Renaturing of the so obtained denatured, reduced protein took place by dilution with 0.1 mole/liter Tris-HCl (pH 10.5), 0.5 mole/liter L-arginine, 1 mmole/liter EDTA and 1 mg./ml. bovine serum albumin. The protein concentration was from 10 to 30 μg./ml., the temperature 20° C. and the period of renaturing 24 hours. 
     The reactivation was determined according to the test procedure for t-PA standard of Boehringer Mannheim GmbH, Order No. 1080954. The following Tables 7A and 7B show the yield of active t-PA depending upon the variations of 
     7A the GSSG concentration at constant DTE concentration (3 mmole/liter); and 
     7B the GSSG concentration at constant GSH concentration (1 mmole/litre). 
     
                       TABLE 7A______________________________________GSSG (mmole/liter)             yield (%)______________________________________0                 01                 13                 75                 1210                6______________________________________ 
    
     
                       TABLE 7B______________________________________GSSG (mmole/liter)             yield (%)______________________________________0                  60.1               141                 105                  1______________________________________ 
    
     Example 10 
     Renaturing of tPA derivative &#34;K2P&#34; 
     Cells of E. coli DSM 2093 were transformed with plasmids pA27fd and pUBS500, which are now described, as is the transformation protocol and the resulting product. 
     First, the plasmid pA27fd was made. This involves several steps, which are now described. 
     a. Preparation of Plasmid pA27.3 
     This plasmid was made from starting plasmid pREM7685, described in EPA 0 242 836, which contains a tac-promotor, a lacoperator region with ATG start codon, the region coding for t-PA derivative FK2p (i.e., nucleotides 190-336, (F domain) 715-1809 (K2 domain, P domain, small portion of 3&#39;UT), the transcription terminator of plasmid pKK223-3, a β-lactamase gene, a kanamycin resistance gene, and the ori of plasmid pACY177. The above nucleotide sequences are according to Pennica et al., Nature 301: 214-221 (1983). 
     The F-domain of FK2P in pREM7685 was deleted essentially following Morinaga et al., Biotechnology 21:634 (1984). Two fragments were isolated from pREM7685 for heteroduplex formation. Fragment A was cleaved with EcoRI, and the cleavage products were separated by gel electrophoresis and the largest EcoRI fragment was eluted from the gel. Fragment B: plasmid pREM7685 was linearized with the restriction enzyme KhoI. The linearized plasmid was also obtained preparatively by gel electrophoresis. The following oligonucleotide was prepared synthetically for the mutagenesis. 
     
         5&#39;TG TCT TAC CAA GGA AAC AGT GA 3&#39; 
    
     In order to form the heteroduplex, fragment A, fragment B (450 fmol of each) and the oligonucleotide (75 pmol) were mixed and incubated initially for 3 minutes at 100° C. in the presence of 50 mmol/l NaCl, 10 mmol/l Tris-HCl, pH 7.5 and 10 mmol/l MgSO 4  and then transferred immediately onto ice. The renaturation of the DNA was carried out for 30 minutes at 60° C. The following were added to the heteroduplex for repair synthesis. 
     Deoxynucleotide triphosphate (0.25 mmol/l), ATP (1 mmol/l), NaCl (100 mmol/l), Tris-HCl, pH 7.5 (6.5 mmol/l), MgCl 2  (8 mmol/l), β-mercaptoethanol (1 mmol/l), Klenow-fragment of the DNA-polymerase from E. coli (0.125 U/μl reaction mixture) and T4-ligase (0.1 U/μl reaction mixture). The repair synthesis was carried out for 4 hours at 16° C. Subsequently, this preparation was transformed into E. coli cells (RM82, DSM 3689) with a lac I q  -plasmid and the transformed cells were selected by the addition of 25 μg/ml kanamycin to the culture medium. 
     Those clones which contain the plasmid pA27.3 which encode the t-PA derivative K2P according to the present invention were selected by the colony hybridization technique using the mutagenesis oligonucleotide described above as a probe. This plasmid differs from the starting plasmid pREM7685 inter alia by the absence of a PstI or a SspI cleavage site. Both these cleavage sites are contained in the region of the starting plasmid which codes for the F-domain. 
     b. Production of plasmid pA27fd 
     Plasmid pA27.3 described supra was used in connection with other plasmids to make pA27fd. This was done to increase the yield of expression product. 
     The plasmid pKK223-3 (DSM 3694P) was split and DNA coding for tPA was inserted therein. This generated plasmid pePA 126.1. 
     Following generation of pePA 126.1, an fd-terminator sequence was first integrated into this plasmid. For this, the plasmid pePA 126.1 was linearized with the restriction enzyme Hind III. The plasmid cleaved in this manner was separated by gel electrophoresis and isolated preparatively. The plasmid pLBU1 (Beck et al., (1978), Nucl. Acids. Res., 5, 4495-4503; Gentz et al., (1981) PNAS 78 (8): 4963) was cleaved with Hind III and a Hind III fragment of about 360 bp which contained the fd-terminator was isolated preparatively by gel electrophoresis and gel elution. The linearized plasmid pePA 126.1 and the 360 bp Hind III fragment from pLBU1 were ligated. The ligation preparation was cotransformed with the plasmid pUBS 500, in E. coli, DMS 2102. From the clones, those were selected that contained the desired plasmid pePA 126 fd which differs from the starting plasmid pePA 126.1 in that it contains a second Hind III cleavage site. 
     Two fragments were isolated from the plasmid pePA 126 fd: a BamHI/PvuI-fragment of 3.4 kb size and a PvuI/XmaI fragment of 1.3 kb size. Both these fragments were ligated with a BamHI/XmaI fragment of about 1.3 kb from plasmid pA27.3 and transformed with the plasmid pUBS 500 into E. coli. The resultant plasmid was named pA27 fd and can be distinguished from pePA 126 fd in that in a restriction digest with EcoRI the second smallest EcoRI fragment from pePA 126 fd of about 610 bp length is about 515 bp shorter in pA27 fd. 
     c. Production of the K2P derivative 
     Cells of E. coli DSM 2093 were then transformed with plasmids pA27fd, which codes for K2P, and pUBS 500, cultured, and the cell mass recovered. Denatured, reduced K2P was obtained from this cell mass, following Example 9. 
     The solution of the denatured, reduced protein in 6 mole/liter guanidine hydrochloride (pH 2) was adjusted with GSSG to 0.1 mole/liter and with Tris to 0.1 mole/liter. Subsequently, the pH value was adjusted to 8.5 with sodium hydroxide. After incubating for 2 hours at ambient temperature, the oxidation agent and the buffer components were separated off by renewed dialysis against 6 mole/liter guanidine hydrochloride (pH 2) at 4° C. 
     The renaturing of the denatured, oxidised protein (&#34;mixed disulphide&#34;) took place after dilution in renaturing buffer. The protein concentration ranged from 30 to 60 μg./ml, the temperature 20° C. and the period of renaturing 12 hours. 
     The following Table 8A shows the reactivation of K2P in 0.1 mole/liter Tris-HCl (pH 8.5), 0.8 mole/liter L-arginine and 2 mmole/liter EDTA depending upon the GSH concentration and the following Table 8B shows the yield of active K2P in the above buffer with 0.5 mmole/liter GSH depending upon the pH value of the buffer. 
     
                       TABLE 8A______________________________________GSH (mmole/liter)            yield (%)______________________________________0                 00.1              140.5              261                202                10______________________________________ 
    
     
                       TABLE 8B______________________________________   pH   yield (%)______________________________________   7     6   8    15   9    23   10    7______________________________________