Patent Publication Number: US-4925818-A

Title: Method of purifying bioactive substances by biospecific adsorption

Description:
This is a division of application Ser. No. 07/023,861, filed Feb. 5, 1987 issued as U.S. Pat. No. 4,824,578 on Apr. 25, 1989. 
    
    
     The invention relates in general to purification of bioactive substances by biospecific adsorption. More specifically the invention relates to (a) a method of preparing a bioactive substance by adsorption on a mineral substrate in the aqueous phase, the substance being fixable to a given ligand by forming a specific stable reversible complex and the adsorbent carrier medium used being a finely divided mineral material modified by siloxane substituents carrying the ligands. The invention also relates (b) to a method of preparing the adsorbent substrate. 
     As is known, in order to purify biological substances (proteins, enzymes, heparin, hormones, lectins, antigens, antibodies and other polypeptides) by affinity chromatography, a chromatographic carrier material is selected and chemical molecules (ligands) having a strong specific affinity for the substance to be purified are grafted onto it. When the substance, mixed with other substances from which it is to be separated, is placed in the presence of the particulate material, it becomes bonded to the ligand in question by a specific reaction (usually subsequently reversible), e.g. by forming complexes as in antigen-antibody reactions, whereas the other substances remain in solution. Subsequently the substance to be purified is salted out by placing the carrier material under conditions and in a medium such that the complex dissociates and the desired substance can be recovered and also the chromatographic carrier can be recovered and recycled in a new extraction process (see e.g. document US-A 4 066 505). The salting out operation is usually performed in a chromatographic column using an elution solvent or solvent mixture. 
     The following are examples of carrier materials generally used for the aforementioned purposes: organic and mineral gels such as those known commercially as Sephadex, Sepharose, Sephacryl, Spherosil, Ultogel, Affi-Gel, etc. 
     Usually these materials are made up of spherical or differently-shaped porous particles having sizes of the order of a few microns to a few hundred microns and pores measuring from 10 to 300 nm. The bonding agents for bonding a given ligand to the surface of the chromatographic carrier are molecules comprising a group capable of reacting in covalent or other manner with the ligand and also comprising a group for fixing the ligand (chemically or by adsorption) to the surface of the carrier. With regard to mineral carriers such as particles of silica (Spherosil) or porous glass (CPG glass), use is generally made of reactive silanes such as trialkoxy-alkyl silanes where the alkyl group comprises groups such as --OH, --CHO, --NH 2 , --NCO or oxirane which react with the ligands to be fixed. 
     For example K. Roy et al, &#34;Affinity Chromatography and Biological Recognition&#34;, ed. I. M. Chaiken et al, Academic Press 1983, page 257, described the use of a silica gel carrier silanized with γ-glycidyl-oxypropyl-trimethoxy-silane. P. O. Larsson et al in &#34;Some New Techniques Related to Affinity Chromatography&#34;, les Colloques de 1&#39;INSERM: Chromatographie d&#39;affinite et interactions moleculaires, ed. J. M. Egli (1979), page 91, describe silanization of silica gel (10 μm particles, 6 nm pores) by glyceropropyl trialkoxy silane (the alkyl substituent is an HOCH 2  --CH 2  OH--CH 2  O--C 3  H 6  --group), the glycol group being subsequently converted to an aldehyde group by known methods and subsequently used to fix a bioactive ligand (e.g. an antibody) by a SCHIFF reaction with an amino group thereof and reduction of the N═C bond. 
     Document US-A 3 652 761 describes coupling of bioactive molecules (such as antigens or antibodies) to mineral carriers such as glass, silica gel, colloidal silica, bentonite, wollastonite, etc, by means of alkylated alkoxysilanes where the alkyl group comprises groups such as amino, nitroso, carbonyl, carboxy, isocyanato, diazo, isothiocyanato, sulphydryl and halogenocarbonyl. 
     Document WO-A-82/02818 describes a method and device for separating immunoglobulins from milk by conveying the milk over a column filled with a bed of particles, inter alia porous glass silanized by the previously-mentioned conventional method and carrying monoclonal antibodies specific to one or the other of the immunoglobulins to be purified. 
     Other documents such as EP-A 56977 and EP-A 88818 discuss similar subjects. 
     The methods known hitherto and using the aforementioned techniques have some disadvantages connected with the porous nature of the carriers used, the pore dimensions and the fact that the particles in the carriers must be substantially spherical and have uniform dimensions to avoid channelling when placed in columns. Since most of the active surface of these materials is inside the pores and the pores often have dimensions of the same order of size as the molecules to be fixed, the molecules diffuse slowly inside the pores and, when salted out, have difficulty in diffusing towards the exterior. Also the substances (frequently macromolecules) to be purified only have low intrinsic diffusion speeds. Since the average distance to be travelled by the macromolecules to reach the ligand in question is relatively large (a few tens of microns in the case of adsorbing particles) it is clear that the processes are slow and inefficient, whether for adsorption or desorption of substances to be purified. Small pores are not accessible to large molecules and ligands present in these pores will therefore be incapable of fixing the substance to be purified. 
     For the reasons given hereinbefore, the use of porous biospecific adsorbents has numerous disadvantages, mainly their low fixation capacity and the slowness of the adsorption and desorption steps. With regard to the fixation capacity, it is stated e.g. in document US-A 3 652 761 that if the grafting rate is 14 to 18 mg of antigen (ligand) per gram of porous glass, only about 5 mg of antibody (human gammaglobulin) is fixed per g of carrier. Furthermore, the cost of porous beads of uniform dimensions is very high and incompatible with large-scale industrial use. 
     The method according to the invention as defined in claim 1 obviates the aforementioned disadvantages and can substantially increase the specific extraction yield. Furthermore, the rate at which the products to be extracted are fixed on the substrate is high and therefore the method is more economic to use. Finally the method can without difficulty be used for treating relatively large quantities (several tens to several hundred tonnes per year) of substances to be extracted, in contrast to earlier techniques. Furthermore the cost of non-porous submicronized silica is about a thousand times lower than the cost of carriers having standardized pores as generally used for purification by adsorption. Contrary to expectation, the non-porous submicronic particles used according to the invention are easily separated by filtration after being grafted by a ligand. 
     The following are examples of mineral carriers which are preferably used according to the invention: precipitated or pyrogenic silica having primary (non-agglomerated) particles between 0.01 and 0.1 μm in size depending on categories and having a large specific surface. Al 2  O 3 , TiO 2  and ZrO 2  are used in powder form having particles generally between 0.02 and 1 μm. Hereinafter reference will be made more particularly to silica, though it is understood that the information given applies equally well to alumina, TiO 2  and ZrO 2 . Use may also be made of the silicate carriers described in document US-A 3 652 761. With regard to the aforementioned carriers, the following document should be noted: 
     Document US-A 4 190 425 describes polymer materials based on submicronic silica for filling columns for gas phase chromatography. For example, examples 4 ff disclose a method of grafting organic radicals on to silica particles and subsequent copolymerization thereof with monomers so as to obtain a highly porous medium (see Abstract, line 16). This material, therefore, would be unsuitable for the requirements as per the invention. Also, this document teaches silylation for the purpose of de-activating the chromatographic carrier (using hexamethyldisilazane), but does not teach silanation of silica particles for the purpose of grafting as per the invention. 
     Document FR-A 2 020 527 describes immobilization of bioactive molecules (e.g. enzymes) on mineral carriers using silanes as coupling agents. Porous glass, silica gel, bentonite, alumina, hydroxyapatite and colloidal silica are given as examples of mineral carriers. However, the object of document FR-A 2 020 527, i.e. the preparation of recyclable bioactive mineral carriers, differs from that of the present invention. Immobilized enzymes are catalysts and not biospecific adsorbents, which act in a completely different manner. Also, the silanes described in the citation are grafted in an anhydrous medium (xylene, benzene) instead of an aqueous medium as in the invention. 
     Document FR-A 2 083 067 likewise relates to enzymes which have been made insoluble, inter alia silica-containing substances carrying a re-usable enzyme fixed via a polymer and an organosilane (see page 4, lines 9-17). Owing to the intermediate polymer, the teaching in this document is still remote from that according to the invention. 
     Document FR-A 2 078 427 deals with a similar subject. Example 1 in this document describes fixation of trypsin on to silica type Cab-O-Sil (this is the name used for sub-micronic silica) via a silane and a carbodiimide. This teaching is therefore very similar to that of document FR-A 2 020 527. In order to obtain a reusable catalyst of this kind, trypsin must be strongly bonded to the mineral carrier, which is completely different from the invention. The aim of the invention is to obtain a temporary reversible bond such as that resulting from selected adsorption of trypsin by a ligand such as phenylbutylamine (which is fixed to the non-porous mineral particles), the adsorption being reversible so that the trypsin can be subsequently salted out. 
     In order to carry out step (a) of the present process in the case of silica, an aqueous suspension of the &#34;silica-ligand&#34; carrier is added to an aqueous medium containing the bioactive substance to be isolated, the reactants being contacted under optimum conditions where it is easy to adjust the pH, the ionic strength and the temperature, thus greatly improving the yield on extraction. The conditions must be chosen in each case by the operator in accordance with the rules of the art. The adsorption and fixation yields for the desired substance are excellent. The reaction is also very rapid, since the carrier has no pores into which the molecules (usually bulky molecules) have to penetrate before being purified, as in the case of conventional carriers. All the reactive groupings are available on the outer surface of the particles. Consequently the time for practically complete fixation is normally about 5 to 10 minutes at ambient temperature. It is also possible to work at ambient pressure, which is more economic than in the case of porous supports which need a relatively high presure, depending on circumstances. 
     Generally, when using suspensions of submicronic silica according to the invention, the density of the hydrated adsorbent suspension is about 0.2 (5 ml of adsorbent suspension per g of SiO 2 ) and the suspension has a biospecific adsorption capacity of about 100 to 300 mg of bioactive substance per g of silica. These values correspond to an adsorption capacity per g of silica which is at least one order of magnitude greater than for conventional porous carriers. 
     Step (b) of the present process can be carried out by filtration by conventional means or by centrifuging. Centrifugal gravitation fields of the order of 4,000 to 6,000 g are suitable and can be obtained without difficulty in high-capacity centrifuges, i.e. capable of treating one or more liters of suspension per operation. 
     Separation can also be brought about by the method known as tangential filtration using an ultrafiltration or microfiltration diaphragm of given porosity. In this method, the suspension to be filtered is contacted with the diaphragm surface and conveyed at high speed at a tangent thereto. The permeability of the diaphragm is chosen so that a considerable flow of liquid phase travels into the permeate whereas the actual particles cannot travel through the diaphragm and are therefore progressively separated from the liquid phase containing the non-adsorbed substances. Some commercial tangential filtration devices use hollow filters in which the suspension flows (e.g. Module Microdyn by ENKA GmbH, Wuppertal, FRG; Module CARBOSEP by SFEC, Bollene, France). Other devices use flat diaphragms (Pellikon Cassettes by MILLIPORE). Alternatively the charged particles can be separated from the dissolved substances to be eliminated by the diafiltration method in which a quantity of aqueous phase equal to that eliminated in the permeate is continuously added to the suspension being filtered, and the process is continued until the aqueous phase contains only the particulate substrate carrying the extracted substance and is free from other dissolved products which were originally present in the solution to be purified. 
     In these operations it is not necessary to dry the charged silica after washing or to isolate it from the washing medium, since the medium is usually suitable for the subsequent operations, i.e. step (c). 
     Desorption in step (c) of the substance to be purified from the carrier, i.e. disociation of the ligand complexed with the substance to be purified, is brought about by conventional methods, i.e. by suitably adjusting the characteristics of the aqueous phase (pH, temperature, ionic strength, dilution by a suitable organic solvent, etc.). Operating conditions for desorption are described e.g. in the following reference: E. A. Hill and M. D. Hirtenstein, Adv. in Biotechn. Processs 1, 31 (1983). Some of the most commonly-used methods relate to modifying the ionic strength or the pH, and use of chaotropic agents (e.g. KSCN), water-soluble organic solvents (alcohols, ethylene glycol, isopropanol, etc), &#34;deforming&#34; agents (urea and guanidine) and ligands in the free state. In some cases dissociation is advantageously brought about by using an aqueous 0.5M NaCl phase containing about 20% isopropanol or another comparable water-soluble solvent. 
     Step (d) can be brought about by methods similar to those described hereinbefore for step (b). The silica-ligand substrate from which the substance to be purified has been detached can be separated by using the previously-mentioned techniques of centrifuging and filtration, including tangential filtration. In that case, of course, the filtrate will be the phase mainly required, since it contains in solution the substance which is to be purified. The substance is extracted by conventional methods, i.e. the water is evaporated by distillation, e.g. at ordinary pressure or under reduced pressure in the rotovapor, or by lyophilization or spraying, until separation or precipitation occur. Alternatively, precipitation can be brought about by adding a non-solvent or by salification (e.g. by adding (NH 4 ) 2  SO 4 ). The substance is then collected and stored by conventional methods. The resulting silica-ligand system is normally reusable as such in a new extraction operation. 
     The silica carrier (silica-ligand system) used in the method according to the invention can be prepared either by silanization of submicronic silica using an alkoxy silane on to which a suitable ligand has previously been deposited, or by silanization of the silica by an alkoxy silane carrying a group capable of fixing the ligand, which is supplied subsequently. 
     In the first case, submicronic silica, e.g. pyrogenic or precipitated silica having a specific surface of the order of 50 to 600 m 2  /g (AEROSIL types-130, -200, -300 and -380 by DEGUSSA are suitable, as are CAB-O-SIL varieties by CABOT Corp., USA) is dispersed by violent agitation in a liquid, e.g. an organic solvent or water, preferably in the presence of glass balls, until a milky homogeneous liquid is obtained. The balls in the agitated mixture reduce the size of the agglomerates of SiO 2  particles which tend to form under these conditions (by adhering together owing to their high hygroscopicity). Alternatively dispersion can be brought about by means of a homogenizer, e.g. the POLYTRON device by KINEMATICA GmbH, FRG. In the case of alumina, use may be made of the products called &#34;Aluminium oxide-C&#34; by DEGUSSA or ALCAN by CABOT. ZrO 2  and TiO 2  are used in the form of very fine powder having a specific surface of at least 50 m 2  /g (e.g. Titanium dioxide P 25 by DEGUSSA). After a certain dispersion time (half to one hour) at ambient temperature in a suitable device, the size of the agglomerates becomes standardized at about 100 to 1 000 nm, preferably 200-300 nm (note that the primary individual particles of silica initially measure only a few nanometeres). The size of the agglomerates after dispersion can be checked by conventional methods, e.g. using a COULTER Nanosizer. 
     A suitable silane is selected and reacted with the ligand required for subsequent formation of the specific complex with the substance to be purified. Examples of such ligands are antibodies of the substances to be purified, enzyme inhibitors, co-factors, lectins, dyes such as Cobachron blue F3G-A or PROCION (ICI), hydrophobic groupings such as alkyl or phenyl radicals, carbohydrates, heparin, etc. 
     The silanes used in the method are chosen from among those having a reactive group forming part of the alkyl substituent bonded to the silicon atom and suitable for fixing the chosen ligand. We have already seen the various kinds of trialkoxy silanes which may be suitable, depending on the individual case, and are chosen in dependence on the structure of the ligands to be added to them and the nature of the reactive groups thereof. Polypeptide ligands are usually fixed via their reactive groups --OH, --NH 2 , SH or --COOH (modified via intermediate reagents if required) and suitable silanes contain fixing groups such as --NCO, azo, glycidoxy, --CHO, oxirane, amino-, etc. In some cases a bridging agent must be used between the reactive silane group and the ligand to be fixed, e.g. in the case of the bond between an NH 2  grouping and a carboxylic acid (use of a carbodiimide) or in the case of coupling two amino functions where the bridging agent can be a dialdehyde, e.g. glutaraldehyde. According to the invention it is preferred to use trimethoxyaminopropyl-silane and trimethoxyglycidoxypropyl-silane. Of course, other similar alkoxy silanes such as the corresponding ethoxy, propoxy or butoxy silanes are also suitable. The reaction between the silane and the ligand is brought about by conventional means and under the normal conditions for the intended reaction. These conditions are known to the skilled addresee and need not be detailed here. However, it is usual to work in a non-aqueous medium such as THF, acetone, dioxane or other anhydrous (but water-soluble) organic solvents in which siloxane groupings are stable. In some cases it is possible to operate without a solvent. 
     When the ligand has been fixed to the silane derivative, the reaction mixture is added at ambient temperature and with agitation to the aqueous or organic silica suspension described hereinbefore. It is also possible to operate at a higher temperature, e.g. by heating to between 20 ° and 80° C. for a number of hours (usually from 1 to 8 h). After cooling, the desired suspension of &#34;silica-ligand&#34; substrate is obtained and can be used as such in the method according to the invention (see claim 1). 
     In a second case of preparing the silica-ligand system, trialkoxy silane is first contacted with the silica suspension, preferably an aqueous solution, and the ligand is not added till later. This method is preferable inter alia when the ligand is insoluble in organic solvents or sensitive to the presence thereof (denaturation), as in the case of bacitracin or an antibody or a lectin. In that case the silanization reagent can either be added directly to the aqueous silica suspension or the silane reagent can first be dispersed in water at pH 4-10 in order to bring about preliminary hydrolysis of the alkoxy groups thereof after which the suspension of pre-hydrolyzed alkoxy silane is added to the silica suspension. Note that the time during which the trialkoxy silane is kept in an aqueous medium before adding to the mineral carrier has an influence on the properties of the silanized silica with regard to subsequent fixation of the chosen ligand. It is therefore possible, by adjusting the experimental conditions (the waiting time before adding the carrier to be grafted, the pH during grafting and the temperature) to adapt the properties of the silanized carrier to the nature of the ligands to be used, and this is an additional advantage of the present method, since the pH of the aqueous medium where silanization occurs and the temperature are also very important. The higher the pH (within the range under consideration) the more the aqueous solution tends to thicken and the volume of the carrier (when separated from the liquid by centrifuging) tends to increase. Temperatures of the order of 100° C. are suitable for aminosilanes whereas in the case of the glycidoxy-compound it is preferable not to exceed 50° C. 
     The ligand is fixed to the silanized carrier in aqueous solution under conventional conditions, usually simply by adding the ligand (in aqueous solution) to the silanized silica suspension obtained as described hereinbefore and by leaving the reagents together for sufficient time for the ligand to become bonded to the carrier. In some cases (see hereinbefore) bridging reagents are used at this stage (see Example 5). The resulting &#34;silica-ligand&#34; system is then used in the extraction process as described hereinbefore. In some cases, however, the ligand can be fixed to the silanized carrier in a non-aqueous organic medium. In that case a water-soluble organic solvent is added to the dispersed silanized carrier, the mixture is centrifuged, the residue is dissolved in the solvent and the operation is repeated until the residual water is eliminated, thus yielding a final suspension of silanized particles in the organic solvent. THF, dioxane and ethanol are examples of solvents. The ligand is fixed directly to the silane as in the previously-mentioned case. 
     The following examples illustrate the invention. 
    
    
     EXAMPLE 1 
     23 ml (0.1 mol) of glycidoxypropyl trimethoxysilane in 77 ml of a 10 mM solution of sodium acetate at pH 4 were added dropwise at ambient temperature with agitation. After the addition, the mixture was left at ambient temperature with agitation for 30 minutes. 
     6 g of pyrogenic silica (Aerosil 130, DEGUSSA) were dispersed in 60 ml water by agitation in the presence of glass balls (diameter 3 mm) until a milky suspension was obtained, the average particle size being 200-300 nm. 
     6 ml of the silane solution were then added with agitation to the silica suspension (1 mmol silane/g silica) and the pH was increased to 8.5 with a 0.5M solution of K 2  HPO 4  (approx. 1 ml) and a few drops of a 1N solution of NaOH. After standing overnight, the suspension was centrifuged (10 min at 6000 g) and the residue was redissolved and rinsed twice in 100 ml of pure water. 
     The silanized silica was re-suspended in 50 ml water and a solution of 25 g bacitracin in about 100 ml of 0.2M bicarbonate buffer at pH 8.0 was added with agitation to a quantity of suspension corresponding to 5 g of the initial silica. The mixture was then left to stand at ambient temperature, pH 8, for 24 hours. 
     It was then centrifuged to separate the mineral carrier from the excess bacitracin solution and the residue was re-suspended in an excess of a 1M solution of ethanolamine at pH 8 (50 ml) in order to deactivate any epoxy groups still present. The suspension was centrifuged (10 min, 6000 g) and the residue was then washed 4 times (each time with intermediate separation by centrifuging), i.e. twice in succession with water, once in 1M NaCl and finally in Tris buffer (10 mM) and 0.2 mM of CaCl 2 , pH 8. 
     The &#34;silica-bacitracin&#34; carrier was then used for extraction and purification by biospecific adsorption of the enzyme subtilisin BPN&#39;, which forms a specific complex with bacitracin. 
     To this end, the aforementioned carrier was introduced, in the proportion of 100 mg solid substrate per ml of solution, into a previously-clarified fermentation medium at pH 8 containing 47 mg/ml proteins and having an enzymatic activity of 2000 U/ml. Hereinbefore, the unit of subtilisin BPN&#39; is defined as the quantity of enzyme capable of hydrolyzing 1 μmol of the ethyl ester of N-acetyl-L-tyrosine per minute at ambient temperature at pH 8. After incubation for 30 min the mixture was centrifuged (6000 g, 10 min) and the supernatant liquid was analyzed, showing that its enzymatic citivity corresponded to 3.7% of the initial activity and it contained 74% of the initially-present proteins in solution. 
     The residue was suspended in a 1M solution of NaCl containing 25% by volume of isopropanol, using 30 ml of the solution per g of carrier. After 1 hour a liquid fraction E1 was obtained by further centrifuging. The residue was dissolved and re-extracted in identical manner, yielding a second fraction E2. Fractions E1 and E2 were combined and analyzed, showing the presence of 96.4% of the original enzymatic acrivity combined with 23% by weight of the quantity of proteins present in the starting solution. The subtilisin purification factor was therefore 4.2. 
     EXAMPLE 2 
     Silanized silica was used as in Example 1 and coupled with concanavalin A lectin (Con A), operating in a 0.2M, pH 9 carbonate buffer. The solution of Con A dissolved in the same buffer (21 mg/ml) was added at the rate of 247 mg of Con A per g of silanized silica (calculated as dry product) and was left under agitation at ambient temperature overnight. The free epoxy groups were then blocked at ethanolamine in 1M solution, pH 9, and left to stand overnight. 
     The solid was then separated by centrifuging and the residue was washed (by re-suspension in an aqueous medium followed by centrifuging) using the following solutions: 
     Water twice; 1M NaCl in Tris 10 mM buffer, pH 9; 1M NaCl in 10 mM acetate buffer, pH 5; finally 0.05M NaCl in Tris 0.05M at pH 8. The solution still contained 2 mmols per liter of each of the following ions: Ca ++ , Mg ++ , Mn ++ . 
     The silica-Con A substrate was then used to extract immunoglobulin IgG conjugated with horseradish peroxidase. To this end, a preliminary check was made of absense of affinity of the silica-Con A system of IgG alone. To this end, 10 mg of human IgG (Miles) dissolved in a Tris 50 mM, NaCl 50 mM buffer at pH 8, were treated with 100 mg of silica-Con A. After contact for 30 minutes, the mixture was centrifuged as previously described (6,000 g; 10 min), the residue was re-dispersed in the same medium and re-centrifuged. The supernatant liquids from the two operations were combined and analyzed, showing the presence of 98% of the initial protein content. The human IgG in solution was then tagged with peroxydase as per S. AVRAMEAS, Immunochemistry (1969) 6, 43. The tagged IgG fraction was then extracted, using the silica-Con A carrier. 
     The solution simultaneously containing free IgG and IgG tagged with peroxidase was then treated with 100 mg of silica-Con A carrier per 6 mg of proteins in solution. The medium consisted of the following buffer: 50 mM NaCl, 50 mM Tris, pH 8, Ca ++ , Mg ++ , Mn ++  2 mM. After 30 minutes at ambient temperature, the particles of solid were separated by centrifuging. An analysis shows that the supernatant liquid contained only 8.2% of the initial peroxidase activity but about 92% of the non-conjugated portion of IgG. 
     The peroxydase IgG complex fixed to the silica-Con A carrier was dissociated as follows. The charged carrier was suspended in a 0.1M solution of α-methylglucoside in the following buffer solution: 50 mM NaCl, 50 mM Tris; pH 8; Ca ++ , Mg ++ , Mn ++  2 mM. Incubation for 30 minutes was followed by centrifuging (6,000 g, 10 min). The supernatant liquid contained 86.2% of the original peroxidase activity. 
     EXAMPLE 3 
     6 mmols of 4-phenylbutylamine (ligand) was added dropwise at 4° C. to 5 mmols of glycidoxypropyl trimethoxysilane. The mixture was agitated for a further hour at 4° C. after which the temperature was allowed to rise and agitation was stopped after a total of 5 hours. 
     Next, 10 ml of an aqueous slution of CH 3  COONa (10 mM, pH 4) was dded to the solution and prehydrolysis of the alkoxy groups was allowed to occur at ambient temperature for about 30 minutes. 
     Meanwhile an aqueous dispersion of pyrogenic silica (AEROSIL-130) was prepared as described in Example 1 and the aqueous solution of silane obtained as described hereinbefore was added to the dispersion at the rate of 50 μmols of silane per g of silica. The pH was adjusted to 8.5 and the grafting reaction was allowed to occur at ambient temperature for 72 hours. 
     The silanized silica carrier was isolated and coupled to the ligand by the methods described in Example 1 and resuspended in a 50 mM CaCl 2 , 50 mM Tris (pH 8) buffer in the proportion of 40 mg of solid per ml of aqueous phase. The suspension was then added to a crude pancreatic extract in the same buffer as before (pH 8). In addition to contaminants of pancreatic origin, the extract contained trypsin and chymotrypsin. The proportion of carrier added to the solution of extract as 10 g per g of dry extract material. After contact for 30 minutes, the solid was separated as described in the preceding examples and the supernatant liquid (S1) was placed on one side. The operation was repeated after suspending the residue in the same buffer in the proportion of 25 ml/g silica, thus obtaining a second portion of liquid (S2). Finally the solid was re-suspended a third time but in a 1M solution of tetraethylammonium bromide (25 ml/g of solid) and centrifuged, obtaining a third portion of liquid (E1). 
     The various harvested fractions were then analyzed for enzymes. The analyses were based on the use of N-benzoylarginine-p-nitroanilide (BAPNA) as substrate for tripsin and glutaryl-phenylalanine-p-nitroanilide (GLUPHEPA) as substrate for chymotrypsin. The techniques were as follows: 
     Trypsin: 10 μl of the solution for analysis was added to an excess of a 10 -3  M solution of BAPNA in a 10 mM CaCl 2  and 50 mM Tris-HCl buffer, pH 8, and the quantity of p-nitroaniline formed at 410 nm was measured by spectrophotometry. A BAPNA unit was defined as the quantity of trypsin capable of hydrolyzing 1 μmol of BAPNA per minute at ambient temperature under the aforementioned conditions. By way of example, a sample of very pure trypsin (origin SIGMA, type 1X) having a given BAEE activity of 15600 units/mg has a strength of 2.35 BAPNA units/mg. 
     Chymotrypsin: 10-20 μl of the solution for analysis was added to an excess of a 10 -3  M solution of GLUPHEPA in a 10 mM CaCl 2 , 50 mM Tris-HCl buffer, pH 7.6, and the rate of liberation of p-nitroanaline by hydrolysis at 410 nm was measured. A GLUPHEPA unit was defined as the amount of tripsin capable of liberating 1 μmol of p-nitroaniline/min at ambient temperature under the given conditions. For example a highly purified preparation of chymotrypsin (SIGMA type II) given as having an acitivity of 65 BTEE units/mg has a strength of 68×10 -3  GLUPHEPA units (mg). 
     These methods were used for analyzing the various fractions in the present example, giving the following results: 
     
         ______________________________________                                    
        BAPNA activity                                                    
                     GLUPHEPA activity                                    
Sample    % units        % units                                          
______________________________________                                    
Starting  5,450    100       816    100                                   
sample:                                                                   
S1        3,690     77        62    8.5                                   
S2          490               7                                           
E1          420    7.7       626     77                                   
______________________________________                                    
 
    
     As these results clearly show, the present system of silica-4-phenyl-butylamine selectively fixes chymotrypsin in the presence of tripsin. The fixed chymotrypsin is also salted out by the tetraethyl ammonium solution and contains less than 10% of the starting tripsin. The purification yield for chymotrypsin was 77%. 
     EXAMPLE 4 
     In this Example, aluminium oxide (type C, DEGUSSA) was used as the mineral carrier. The alumina (5 g) was dispersed in 50 ml water, using a Polytron homogenizer. The average size of the particles after dispersion, as measured with the Coulter Nanosizer, was 220 nm. Also, an aqueous solution of glycidoxypropyl trimethoxysilane (1 mmol/ml) was prepared as described in Example 1. 
     An aliquot part of the aqueous silane solution was then added with agitation to the alumina suspension (1 mmol of silane per g of alumina) and the pH was adjusted to 7.5 by adding a few drops of aqueous 1N NaOH solution. After standing overnight, the suspension was centrifuged (6000 g, 10 mn) and the residue was dissolved in pure water and re-centrifuged under the same conditions. The silanized alumina was then coupled with m-aminophenyl boronic acid (0.5M) in 0.2M bicarbonate buffer at pH 9.0 and the mixture was left to stand at pH 8.0 and ambient temperature for 72 hours. The particles of the alumina and m aminophenyl boronic acid system were then washed four times by centrifuging and resuspended in water. After the last centrifuging operation, the carrier was suspended in a 10 mM Tris, 0.2 mM CaCl 2  buffer at pH 8.0. 
     The carrier was then used to extract and purify subtilisin BPN&#39;. The same clarified fermentation medium was used as in Example 1, containing 47 mg/ml of proteins and having an enzymatic activity of 2000 U/ml, and 100 mg of carrier were added per ml of solution. After 20 minutes at ambient temperature, the mixture was centrifuged (6000 g, 10 min). The supernatant liquid was analyzed, showing that the enzymatic activity of the liquid corresponded to 32% of the initial enzymatic activity. The residue was suspended in a 0.5M solution of pentaerythritol and was re-centrifuged after an hour, yielding a liquid fraction E. The fraction contained 59% of the enzymatic activity initially present and 24% of the quantity of proteins present in the starting solution. By means of this operation, therefore, the initial subtilisin was purified with a purification factor of 2.25. 
     EXAMPLE 5 
     10 mmols of gamma-aminopropyl-trimethoxysilane was added dropwise to 5 ml of a solution of 0.02M sodium acetate, pH 4.0, care being taken to keep the pH between 4 and 5 by progressively adding 4N hydrochloric acid. The solution was then kept for 30 minutes under agitation at ambient temperature. Its volume was then made up to 10 ml with distilled water. 
     Pyrogenic silica (Aerosil 300, 5 g) was dispersed in 50 ml water, using a POLYTRON homogenizer. The aqueous solution of silane was then added to the dispersed silica suspension in the proportion of 1 mmol silane per g silica and the pH of the solution was adjusted to 7.4 by adding 1M K 2  HPO 4 . The suspension was left under agitation overnight at ambient temperature. 
     The particles were then centrifuged (10 min, 6,000 g) and the residue was suspended in distilled water. This operation was repeated four times, the particles being successively suspended in 1M NaCl, distilled water, a 50:50 water/alcohol mixture, and finally in pure ethanol. The final suspension in ethanol contained about 50 mg silica per ml. Next, a 0.5M solution of N-carbobenzoxy-L-phenylalanine (Z-L-Phe) was prepared in ethanol and a 0.5M solution of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). 10 ml of the solution of Z-L-Phe was added to 100 ml of the silanized silica suspension, followed by 10 ml of the solution of EEDQ; the mixture was left under agitation at ambient temperature for 24 hours. 
     The particles were then washed by repeated centrifuging followed by re-suspension in the initial ethanol (three times) then in the ethanol/water mixture (once) and finally in distilled water. 
     The particles were used to purify a solution containing subtilisin BPN&#39; as described in Example 1. 100 mg of silica-Z-L-Phe per 200 U of enzyme were used. Under these conditions, the supernatant liquid after centrifuging (S1) contained less than 1% of the initial enzymatic activity. The particles were then suspended in a 1M NaCl solution containing 25% isopropanol and the suspension was centrifuged one hour later. This time the supernatant E1 contained 87% of the original enzymatic activity.