Abstract:
Enzymes are immobilized on a solid support material containing essentially cellulose and lignin by a process involving oxidation of the support to provide aldehyde groups, amination of the oxidized support by reacting a diamine with the aldehyde groups, reduction of the aminated support to produce stabilized aminated groups, activation of the aminated groups by reacting the groups with a dialdehyde and immobilization of an enzyme by covalent coupling of the enzyme to the activated groups of the support. The enzyme may be invertase and the immobilized invertase can be used to treat a sugar syrup.

Description:
TECHNICAL FIELD 
     The present invention relates to a process for fixing an enzyme on a water-insoluble solid support. It also comprises as its object an enzyme-support complex and an application procedure for such a complex. 
     PRIOR ART 
     For several years already work has been done to render insoluble enzymes or other agents such as antibodies, antigens or nuclear acids so as to obtain the capability of purifying, modifying, or insulating certain substances with which these &#34;catalysts&#34; form complexes or else favor a specific reaction. 
     The endeavors to achieve insoluble enzyme-support combinations are due to the fact that numerous reactions involve enzymes, and not only is there an economic incentive to recover an expensive enzyme, but moreover the reaction products should not be soiled. 
     Numerous works have been performed on the selection of the supports and the methods of fixing or combining enzymes: the scientific literature is quite abundant and describes several supports such as silica, glass, synthetic polymers, polysaccharides and in particular cellulose. 
     Most of the publications relating to cellulose refer to a very pure cellulose, whereas as a rule cellulose is associated with hemicelluloses and lignin. 
     Thus, N. WELIKY and H. H. WEETHALL in 1965 (Immunochemistry Pergamon Press, Vol. 2, pp. 293-322) published an article entitled &#34;The Chemistry and Use of Cellulose Derivatives for the Study of Biological Systems.&#34; 
     The cellulose reactivity is that of a polyalcohol and can be subjected to oxidation, esterification, etherification, halogenation reactions, etc. 
     In particular, oxycelluloses are coupled to alcohols, amines or proteins, forming thereby esters or amides. 
     In view of the reactions used to modify cellulose, it is clear why as a rule the attempt has been made to eliminate lignin, a crosslinked polymer of which the numerous reactive groups use up the bodies being employed to modify the cellulose or else inhibit the desired reactions. 
     However, the U.S. Pat. No. 3,841,969 (A. N. EMERY et al.) describes a method for preparing immobilized enzymes by chelation with a metal complex and with substances comprising hydroxyl functions: microcrystalline cellulose, diethylaminoethylated cellulose, carboxylmethyl cellulose, but also sawdust and wood chips. 
     In addition to the difficulty of using industrially such metal chlorides as titanium tetrachloride or tin chloride, it is also known that the chelates are not stable over wide pH ranges. 
     On the other hand, the French Pat. No. 2,247,472 (SNPA) describes a method for fixing enzymes on lignin associated cellulose and indicates in surprising manner that the cellulose-enzyme association reaction is enhanced by lignin present by 5 to 25% by weight and preferably 10 to 20% by weight. 
     Cereal stalk granules between 0.3 and 10 mm in size first are surface-delignified and following washing are treated with thionyl chloride in pyridine, and lastly are placed in contact with a buffered enzyme solution. 
     It appears obvious that the lignin exerts no favorable influence at all on the reaction that allows immobilizing the enzyme, because it is mandatory to begin with a stage of surface delignification of the granules. 
    
    
     DESCRIPTION OF THE INVENTION 
     The present invention concerns a method for obtaining enzyme derivatives wherein the enzyme is fixed by a covalent bond on a water-insoluble support, characterized by the following steps: 
     (a) a substrate essentially containing cellulose and lignin is washed with water, 
     (b) controlled oxidation of the oside fraction of the substrate, 
     (c) condensation of a diamine on the aldehyde groups of the oxidized support, 
     (d) stabilization by reduction of the amine-substrate bond, 
     (e) activation by means of a dialdehyde of the aminated groups, 
     (f) coupling the enzyme on the activated substrate. 
     Preferably the oxidizer in stage (b) is sodium periodate, the diamine is ethylene diamine, the reducer is sodium borohydride or cyanoborohydride, dialdehyde, glutaraldehyde. The enzyme in particular may be invertase, saccharase levan, lactase or dextran sucrase. 
     Even though the whole of the support containing cellulose and lignin can be used, especially significant results are obtained using the most lignified fraction of cereal stalks, in particular maize (i.e., &#34;corn&#34;) stalks. 
     The maize stalks consist of: 
     (1) soft elements called &#34;feeds&#34; containing for the most part cellulose and proteins, meant for cattle feed, 
     (2) hard elements called &#34;grits&#34; containing a high proportion of lignin (exceeding 25%), cellulose and xylanes. 
     As a rule, these elements are ground and due to their hardness are used as fillers or abrasives. The quasi-spherical particles that are obtained vary in sizes between a few dozen microns and several millimeters. 
     Another object of the invention is an enzyme derivative wherein the enzyme is covalently bonded to the support and characterized in that the support consists of that part of the cereal stalks of which the lignin content exceeds 25%, the enzyme being invertase, saccharase levan, lactase or dextran sucrase. 
     Another object of the invention concerns a treatment process of juices sweetened by means of enzymes fixated on that part of the cornstalks of which the lignin content exceeds 25%. 
     In particular, concentrated saccharose juice is being treated by being made to pass through a bed of cornstalk particles acting as the invertase support for the purpose of obtaining an invert syrup. 
     In another implementation of the invention, saccharose juice is being treated by being made to pass over a bed of cornstalk particles acting as the support for the saccharase levan: a juice containing polyfructose and glucose is obtained. 
     The examples provided below better explain the invention: 
     Invertase is an enzyme catalyzing the hydrolysis of saccharose into glucose and fructose, and for which the invention provides the fixation on solid water-insoluble substrates containing both cellulose and lignin. 
     More particularly, the fixation of the &#34;hard&#34; part of the cornstalk with a lignin content in excess of 25% has been researched. This &#34;hard&#34; part was ground in a manner to obtain particles with diameters between 170 and 280 microns. Their apparent density is 0.4 kg/liter in the dry state and 1.8 kg/liter in the wet state. Their specific surface is 1 m 2  /g. 
     The activity of the enzyme preparation is ascertained by establishing contact for 4 minutes at 40° C. between 10 ml of 0.4 M saccharose solution in 0.1 M acetate buffer, and 0.1 ml of invertase solution (or immobilized invertase solution) in the same buffer. 
     The reaction is stopped by adding 5 ml of the medium to 0.5 ml of 2 N soda solution. Then the content in released reducing sugars is determined by the dinitrosalicylate method (mn absorbance reading). Calibration is obtained using equimolecular solutions of glucose and fructose in the 0.1 M acetate buffer (pH=4.5). 
     One enzyme activity unit (U) is defined as that amount of enzyme causing the release of 1 g of reducing sugars a minute under the test conditions. 
     The specific activity of the invertase preparation (SIGMA Chem. Co.) is 93.3 U per mg of enzyme. 
     The treatment procedure of the cornstalk particles comprises the following stages: 
     (a) preliminary distilled-water washing for 24 h at 25° C. to eliminate the soluble substances, followed by drying at 60° C., 
     (b) controlled oxidation by placing a sample of 100 mg in contact with 20 ml of sodium metoperiodate solution in distilled water at 25° C., in darkness, 
     (c) amination by reacting oxidized stalks with 20 ml of diamine in methanol solution at 25° C. 
     Concerning this stage, the following diamines were tested: 
     ethylenediamine 
     hexamethylenediamine 
     octamethylenediamine 
     diamino dicyclohexyl methane 
     diamino diphenyl methane, 
     (d) reduction at 25° C. by placing in contact either 25 ml of sodium borohydride solution in 0.05 M carbonate buffer and at a pH of 10.5 or in methanol, or 20 ml of sodium cyanoborohydride solution in 0.05 M phosphate buffer at a pH of 6.5, 
     (e) activation by a glutaraldehyde solution in a 0.05 M pyrophosphate buffer at a pH of 8.6 and at 25° C., 
     (f) fixation of the invertase in solution in 0.1 M acetate buffer at a pH of 4.5 and at 4° C. 
     Each stage is followed by two washings of the stalks by return to suspension for 15 min in 20 ml of the medium used during the previous stage--containing no reactive sample--and then by three washings by return to suspension during the following stage. 
     The various stages can be schematically shown in principle as follows: ##STR1## 
     Following grafting of the enzyme, the support is washed with 20 ml of 1 M NaCl solution in distilled water in order to eliminate the simply adsorbed invertase. Then the activity retained on the sample of the stalks is ascertained. 
     Obviously the attempt was made to optimize the various stages: 
     (1) Oxidation 
     (a) Effect Of The Sodium Periodate Concentration 
     The experimental conditions selected were as follows: 
     
                                           TABLE I__________________________________________________________________________Stage   Oxidation        Amination              Reduction                    Activation                            Fixation__________________________________________________________________________Product NaIO.sub.4        C.sub.2 H.sub.8 N.sub.2              NaBH.sub.2                    glutaraldehyde                            invertaseMedium  H.sub.2 O        CH.sub.3 OH              CH.sub.3 OH                    pyrophosphate                            acetate                    buffer  bufferpH      --   --    --    8.6     4.5Concentration   variable        3M    10 g/l                    1.25%   0.5 g/lTime (h)   30   72    4     5       10__________________________________________________________________________ 
    
     By varying the NaIO 4  concentration, the results in Table II were obtained: 
     
                       TABLE II______________________________________ConcentrationNaIO.sub.4 (M)         0     0.05       0.2  0.4Activity(U/g of stalks)         0     0.28       1.28 1.33______________________________________ 
    
     By carrying out the reduction with sodium cyanoborohydride (10 g/l concentration, phosphate buffer pH 6.5, time=2 h) in lieu of sodium borohydride, all other conditions being the same, the results listed in Table III are obtained: 
     
                       TABLE III______________________________________ConcentrationNaIO.sub.4 (M)         0     0.05       0.2  0.4Activity(U/g of stalks)         0     0.53       2.20 2.18______________________________________ 
    
     Accordingly, whatever the reducing method used, the optimal enzyme grafting is obtained for a sodium metaperiodate concentration in excess of 0.2 M. 
     (b) Effect Of Duration 
     The metaperiodate concentration being set at 0.2 M and all other test conditions being identical with those selected in (a) (except that the methanol reducing medium is replaced by a phosphate buffer), the results listed in Table IV are obtained: 
     
                       TABLE IV______________________________________Time of Oxidation (h)        0     2      5    8.75 13   24   48Activity(U/g of stalks)        0     0.42   0.55 0.77 0.97 1.49 1.53______________________________________ 
    
     The optimal oxidation time is set at 30 h. Be it noted that the oxidation appears to proceed by two phases 
     a first rapid phase (0-2 h) 
     a second slower phase (2-24 h). 
     (2) Oxidation 
     (a) Effect Of The Nature Of The Diamine 
     The oxidation time by 0.2 M sodium periodate being set at 30 h, the other test conditions being those selected in 1(a), the results listed in Table V are obtained: 
     
                       TABLE V______________________________________Diamine   C.sub.2 H.sub.8 N.sub.2               C.sub.6 H.sub.16 N.sub.2                        C.sub.8 H.sub.20 N.sub.2                                C.sub.13 H.sub.26 N.sub.2Activity(U/g of stalks)     1.28      0.61     0.43    0.40______________________________________ 
    
     Accordingly the highest enzyme activities are obtained when using ethylenediamine (C 2  H 8  N 2 ). 
     (b) Effect Of The Ethylenediamine Concentration 
     The test conditions being those of example 1(a) (except that the time of oxidation is set at 13 h and that the methanol reducing medium is replaced by a carbonate buffer), the results listed in Table VI are obtained: 
     
                       TABLE VI______________________________________Diamine Concentration (M)           0     0.05   0.5  1    5    7.5Activity (U/g of stalks)           0     0.50   0.72 0.97 1.24 1.23______________________________________ 
    
     It will be noted that a substantial catalytic activity is obtained for a relatively low diamine concentration (0.05 M). However, the optimal concentration is taken to be 3 M. 
     (c) Effect Of Duration 
     The test conditions are the following: 
     
                                           TABLE VII__________________________________________________________________________Stage   Oxidation        Amination              Reduction                    Activation                            Fixation__________________________________________________________________________Product NaIO.sub.4        C.sub.2 H.sub.8 N.sub.2              NaBH.sub.3 CN                    glutaraldehyde                            invertaseMedium  H.sub.2 O        CH.sub.3 OH              phosphate                    pyrophosphate                            acetate              buffer                    buffer  bufferpH      --   --    6.5   8.6     4.5Concentration   0.2 M        3M    10 g/l                    1.25%   0.5 g/lDuration (h)   30   variable              2     5       10__________________________________________________________________________ 
    
     The results are listed in Table VIII: 
     
                       TABLE VIII______________________________________Amination Time (h)         0      17     24   48    72   79Activity (U/g of stalks)         0      0.98   1.21 1.84  2.20 2.12______________________________________ 
    
     Accordingly a time of 72 h was set for the amination stage. 
     (3) Reduction 
     (a) Effects Due To The Kind And Concentration Of The Reducer 
     The test conditions are as follows: 
     
                                           TABLE IX__________________________________________________________________________Stage   Oxidation        Amination              Reduction                    Activation                            Fixation__________________________________________________________________________Product NaIO.sub.4        C.sub.2 H.sub.8 N.sub.2              variable                    glutaraldehyde                            invertaseMedium  H.sub.2 O        CH.sub.3 OH              variable                    pyrophosphate                            acetate                    buffer  bufferpH      --   --    --    8.6     4.5Concentration   0.2 M        3M    variable                    1.25%   0.5 g/lDuration (h)   30   72    2     5       10__________________________________________________________________________ 
    
     The results are listed in Table X: 
     
                       TABLE X______________________________________                Reducer                Concentration (g/l)Reducer                0      10      20______________________________________NaBH.sub.4 (0.05 M carbonate buffer) pH 10.5                  1.11   1.29    1.28NaBH.sub.3 CN (0.05 M phosphate buffer) pH 6.5                  1.11   2.20    2.25______________________________________ 
    
     Accordingly cyanoborohydride is the more effective reducer. 
     (b) Effect Of The Time Of Reaction 
     The test conditions are the following: 
     
                                           TABLE XI__________________________________________________________________________Stage   Oxidation        Amination              Reduction                    Activation                            Fixation__________________________________________________________________________Product NaIO.sub.4        C.sub.2 H.sub.8 N.sub.2              NaBH.sub.3 CN                    glutaraldehyde                            invertaseMedium  H.sub.2 O        CH.sub.3 OH              phosphate                    pyrophosphate                            acetate              buffer                    buffer  bufferpH      --   --    6.5   8.6     4.5Concentration   0.2 M        3M    10 g/l                    1.25%   0.5 g/lDuration (h)   30   72    variable                    5       10__________________________________________________________________________ 
    
     The test results are listed in Table XII: 
     
                       TABLE XII______________________________________Reducing Time (h)           0          2      4Activity (U/g of stalks)           1.11       2.10   2.15______________________________________ 
    
     (4) Activation 
     (a) Effect Of The Concentration Of Glutaraldehyde And Of Duration 
     The test conditions are the same as for test 3(b). 
     The results, expressed in U/g of stalks are provided in Table XIII: 
     
                       TABLE XIII______________________________________      Glutaraldehyde Concentration (%, v/v)Activation Time (h)        0       0.5     1.25  5     10______________________________________0            --      --      1.34  --    --0.5          --      --      1.42  --    --3            --      --      1.70  --    --5            1.34    2.26    2.33  2.30  2.3211           --      --      2.34  --    --______________________________________ 
    
     Accordingly, the optimal concentration is 1.25% and the duration is 5 h. 
     (5) Invertase Fixation 
     (a) Effect Of The Invertase Concentration And Of The Fixation Time 
     The test conditions are the same as for test 3(b). 
     Table XIV lists the results. 
     
                       TABLE XIV______________________________________     Invertase Concentration (g/l)Fixation Time (h)       0.05   0.1     0.5  1     2    5______________________________________10          0.39   1.01    2.36 3.30  3.52 3.5624          --     --      --   3.82  --   --35          --     --      --   4.08  --   --______________________________________ 
    
     Accordingly, the optimal invertase concentration is 2 g/l and the optimal duration for fixation is 30 h. 
     Table XV summarizes the optimal conditions for fixating invertase on cornstalks: 
     
                                           TABLE XV__________________________________________________________________________Stage   Oxidation        Amination              Reduction                    Activation                            Fixation__________________________________________________________________________Product NaIO.sub.4        C.sub.2 H.sub.8 N.sub.2              NaBH.sub.3 CN                    glutaraldehyde                            invertaseMedium  H.sub.2 O        CH.sub.3 OH              phosphate                    pyrophosphate                            acetate              buffer                    buffer  bufferpH      --   --    6.5   8.6     4.5Concentration   0.2 M        3M    10 g/l                    1.25%   2 g/lDuration (h)   30   72    2     5       30__________________________________________________________________________ 
    
     Other immobilization methods for invertase on cornstalks were tested: 
     absorption 
     absorption followed by crosslinking due to glutaraldehyde 
     direct fixation on oxidized stalks 
     grafting aromatic diamines activated by diazotation 
     oxidizing aldehydes into acids, followed by activation by the nitridation process. 
     None of these methods offered results comparable to those obtained by the procedure of the invention. 
     In particular, the method described in the French Pat. No. 2,247,472 was carried out; two supports were used: 
     (1) a sample of whole stalks mechanically ground and sifted so as to only keep the fraction with a diameter between 100 and 200 microns, 
     (2) a sample of &#34;grits&#34; with a diameter between 170 and 280 microns. 
     The invertase was immobilized by placing in contact 100 g of activated supports with 20 ml of an invertase solution (SIGMA Chem. Co. I 5875) at 2 g/l in 0.1 M acetate buffer at a pH of 4.5, for 30 hours with rotary agitation and at 4° C. 
     Following exhaustive elimination of the unfixed enzyme by washing the supports with 0.1 M acetate buffer at a pH of 4.5, the retained activation was determined by measuring the amount of reducing sugars (glucose and fructose) which were released by the action of the enzyme on a 0.4 M saccharose solution at 40° C. 
     Table XVI lists the results. 
     
                       TABLE XVI______________________________________Immobilization Method         Support       Enzyme Activity______________________________________French Patent whole stalks  0.049No. 2,247,472               0.0505         grits         0.036                       0.015Present       whole stalks  2.12Invention                   2.10         grits         3.10                       2.94______________________________________ 
    
     The enzyme activities obtained with supports treated by the invention are 40 to 80 times higher than those obtained with the same supports treated according to the French Pat. No. 2,247,472. 
     Again the possibilities of continuously using invertase immobilized on cornstalks in a reactor with a 10 ml fixed volume, supplied with 1.9 g of stalks bearing the enzyme were tested. This reactor was placed in a thermostatically controlled enclosure and kept at 40° C. and was fed continuously by means of variable concentrations of saccharose solutions in 0.1 M acetate buffer at a pH of 4.5. 
     Table XVII lists the results: 
     
                                           TABLE XVII__________________________________________________________________________Initial Saccharose    Feed Rate (l/h)Concentration    0.15        0.40 0.68                 0.82 0.97                          1.05 1.2__________________________________________________________________________0.1 M (34.2 g/l)    100%        100% 100%                 100% 100%                          100% 100%1 M (342 g/l)     97%         96%  80%                  73%  62%                           57%  53%__________________________________________________________________________ 
    
     Accordingly, maximal hourly productivities are obtained, which are: 
     22 g of saccharose hydrolyzed per hour and per g of stalks for an initial concentration of 0.1 M 
     117 g of saccharose hydrolyzed per hour and per g of stalks for an initial concentration of 1 M saccharose. 
     The following experiments were run in order to establish the industrial application of the continuous hydrolysis of saccharose juice: 
     Two reactors are used: 
     Reactor A: total volume is 100 ml, stalk quantity is 44.1 g; 
     Reactor B: total volume is 1 liter, stalk quantity is about 400 g. 
     A commercial invertase preparation (SIGMA Chem. Co.) was immobilized on cornstalks with an average diameter of 0.81 mm. The results obtained from the two successively prepared reactors are listed in Table XVIII: 
     
                       TABLE XVIII______________________________________                 % Immobilization       Activity (U).sup.(a)                 Efficiency.sup.(b)______________________________________Reactor A     0.32         3Reactor B     0.84        16Optimal Conditions.sup.(c)         1.10        19______________________________________ .sup.(a) The activity is expressed in g of reducing sugars released per m and per g of support for the test conditions. .sup.(b) The immobilization efficiency is expressed as the ratio of the activity per mg of immobilized enzyme to the activity per mg of free enzyme. .sup.(c) The optimal immobilization conditions were previously determined for samples of 100 mg of stalks. 
    
     It will be noted that the problem of extrapolating the immobilization of the invertase for a scale of supports 100 mg to the scale of stalks up to 1 kg was resolved because the results for Reactor B are close to those obtained under optimal conditions. 
     Table XIX lists the results concerning the production of glucose and fructose (expressed as g of reducing sugars produced per h and per g of stalks). 
     The maximum productivity was determined, respectively, for an initial saccharose conversion rate of 90% to 100%. 
     
                       TABLE XIX______________________________________              Maximum Productivity              (g/h × g of support)Reactor  Saccharose  Temper-   For 90% For 100%Volume Concentration              ature     Conversion                                Conversion______________________________________0.1 liter  1 M, (342 g/l)              40°                        3.55    2.32  2 M         40°                        2.90    2.10  (684 g/l)   50°                        4.10    2.40              55°                        5.30    3.30              60°                        5.00    2.501 liter  1 M         40°                        9.65    5.25  2 M         40°                        6.50    4.90              50°                        11.00   6.50              55°                        7.90    6.50  Solution 70 50°                        5.40    1.90  BRIX (Akg/l)______________________________________ 
    
     The extrapolation from the 100 ml reactor results to those of the 1 liter reactor is essentially linear, the maximum productivities being in the same ratio as the enzyme activities per g of support (0.32 U and 0.84 U, respectively). 
     The immobilized invertase hence reveals itself being exceedingly effective as for the results obtained from the 1 liter reactor for a solution of 68.4% saccharose, a productivity (at 50° C.) of 11  g of reducing sugars a day is obtained for the 1 liter volume reactor (the conversion rate being 90%). 
     The catalyst stability is very good as no significant drop in activity at all was noted after 20 days of continuous operation (at 40° C., 50° C., 55° C., and 60° C.) for the 1 liter reactor. 
     A positive treatment was obtained for a remolten pure sugar syrup or an industrial syrup from beets or sugar cane at a temperature between 40° and 60° C. and about 70 Brix. 
     However, the maximum productivity is about twice less than for an approximately 70% saccharose solution. 
     Nevertheless, solutions equally highly concentrated in saccharose may be continuously hydrolyzed in principle, provided the syrup be preheated to lessen its viscosity. 
     It must be stressed that the enzyme derivatives obtained in conformity with the invention allow the continuous treatment of very high saccharose concentration juices without thereby decreasing their activities. 
     The appreciable advantage thus obtained industrially is easily seen; there is less bulk of equipment, saving in energy, and long life of the enzyme support.