Magnetic Gel Suitable to Immunoenzymatic Determinations

The present invention relates to the field of the determination of the amounts of proteins, for example antigens and antibodies, which are present in certain biological media. The object of the invention is more particularly a magnetic gel comprising acrylamide, agarose or acrylamide-agarose, a process for obtaining same and its use in immunoenzymatic quantity determinations; the magnetic gel according to the invention makes it possible to effect rapid immunoenzymatic determinations.

The detection of antibodies and antigens by means of enzymes is a known process (see Bull. Soc. Chim. Biol. 1968, 50, no. 5-6). Processes have also already been described for revealing and determining a component of a reaction between a specific binding protein and the corresponding bindable substance by labelling the latter by means of an enzyme; according to such processes determination of the enzymatic activity of the reaction medium, after binding of the bindable substance to the binding protein, necessitates the use of one or more centrifugation steps which considerably increase the time taken for the determination; said processes may not be sufficiently sensitive when there is difficulty in separating insoluble particles from the reaction medium, notably owing to the nature and molecular weight of the binding substance.

Moreover, processes for the immobilization of enzymes on magnetic support are already known [Biotechnology and Bioengineering, Vol. XV (1973) and Biotechnology and Bioengineering Vol. XVI, p. 385-396 (1974)]. In these processes the enzyme is bound directly to the magnetic support; it is thus possible to use enzymes industrially, in fermentors for example; their conservation, recovery and reuse are then easy.

A magnetic gel suitable for immunoenzymatic or even radioimmunological determinations has now been discovered.

The gel according to the present invention is an acrylamide or agarose gel or a mixed acrylamide-agarose gel containing magnetic particles. Acrylamide and agarose gels and the mixed acrylamide-agarose gels are known and are notably useful as migration supports for electrophoretic separation of biological substances or for methods of immunochemical analysis in a gel medium (see French Pat. No. 1,483,742).

The gels of the invention are obtained by using known processes, the magnetic particles being incorporated in said gel while it is being formed, notably in the starting materials useful for obtaining same.

Thus, according to the preferred embodiment of the invention, a magnetic mixed acrylamide-agarose gel can be prepared using the process described by URIEL et al (C.R. Acad. Sc. Paris, v.273, pages 2358-2360, 1971 series D), the magnetic particles being added, according to the invention, to the acrylamide monomer. The magnetic gels of the present invention are advantageously spherical in shape with a mesh size in the range of from 50 to 500 .mu.m.

The magnetic particles introduced into the gels according to the invention are, for example, magnetic, ferrite or iron powder particles; magnetite is particularly preferred. The amount of magnetic particles introduced into the gels of the invention is not critical; it should be sufficient to permit good separation of the gel from a liquid reaction medium by means of a magnet. As an example, it may be pointed out that an amount in the order of 20 g of magnetite for 10 grams of acrylamide monomer and 0.25 g of bisacrylamide is suited to the requirements of the invention when the gel is a polyacrylamide-agarose gel prepared according to the above-mentioned process.

The magnetic gel thus obtained can then be coupled with a protein by a coupling agent; the agents usually employed in this field are used as the coupling agent; amond said agents, glutaraldehyde may be mentioned.

The coupling of proteins and the magnetic gel according to the invention is advantageously effected using the method described by T. TERNYNCK and S. AVRAMEAS (F.E.B.S. letters, 23, 24, 1972).

The magnetic gel of the invention, coupled with a protein, for example an antibody or an antigen, is used for immunoenzymatic determinaton of the corresponding antibody or antigen. Another object of the invention is this magnetic-protein gel complex. For greater clarity in the present description, a magnetic gel coupled with an antigen will be referred to hereafter.

According to a preferred embodiment of the invention determination of the antibodies contained in a biological liquid is effected as follows:

A predetermined amount of magnetic gel coupled with the antigen is contacted with the biological liquid containing the said antibodies. After incubation for about 0.5 to 1 hour, the gel is washed several times with a phosphate buffer, the gel being maintained on the test tube walls by means of a magnetic field, this being possible owing to the presence of magnetic particles in the gel, and thus avoiding the centrifugation step necessary after each wash as in a conventional process of determination of this type. After said washes, the gel is incubated for about 30 minutes in the presence of antibodies labelled with an enzyme. Labelling of the antibodies with an enzyme is effected by a process well known to one man skilled in the art; the enzymes conventionally used in immunoenzymatic determination, such as glucose-oxidase and peroxidase, are used as enzymes. After incubation of the gel in the presence of antibodies labelled with an enzyme, the enzymatic activity of the gel is measured after removal of the supernatent by retaining the gel on the walls of the tube by means of a magnetic field; the enzymatic activity is determined by reading the optical density. Determinations take approximately 2 hours; the gel of the invention therefore enables the duration of conventional immunoenzymatic determinations to be reduced about four-fold.

The magnetic gels of the present invention can notably be used for determining anti-medicament, antiallergene and anti-parasitic IgE, specific anti-German measles IgMs making it possible to detect a recent attack of German measles during pregnancy; they also permit rapid determination of antibodies, tetanic anti-toxin, for example in persons who have been involved in acidents, or anti-ADN antibodies for example in the case of Lupus erythematosus.

It is also possible to seek the specificity of the anti-pollen allergy, for example of graminaceous plants (Dactylis glomerata, Lolium perenne, Phleum pratense, Secale Cereale) and to measure the intensity of an humoral reaction (strong or weak amount of specific IgE).

The magnetic gel of the invention can form part of a kit of reactants for immunoenzymatic determination; this kit of reactants which constitutes another object of the invention is composed essentially:

of magnetic gel coupled with an antigen or an antibody,

a buffer,

an antibody or an antigen specific to the antigen or antibody to be tested, coupled with an enzyme,

a magnet,

an enzyme substrate to reveal its enzymatic activity,

normal control serum.

The kit can further contain accessories making it possible to carry out a series of dilutions of the sample to be determined. The invention will be illustrated in greater detail by the following non-limitative examples.

The process described by URIEL et al (C.R. Acd. Sci. Paris, v.273, 2358-2360, 1971, series D) was used.

50 g of Fe.sub.3 O.sub.4 was used for 10 g of acrylamide monomer, 0.25 g of bis-acrylamide and 5 g of agarose. The magnetite was added to the acrylamide and bis-acrylamide solution before the latter was mixed with the solution containing the melted agarose.

The gel thus obtained was coupled with proteins by means of glutaraldehyde according to the method devised by T. TERNYNCK and S. AVRAMEAS (F.E.B.S. Letters 23, 24, 1972). The following proteins were attached:

sheep Ig

sheep antibody, rabbit anti-Ig

tetanic anatoxin

bovine albumine.

The amounts of these bound substances are shown hereinafter as a function of the size of the magnetic gel particles.

The method of determination is as follows:

Each tube containing 50 .mu.l of antigen bound to the magnetic gel receives 1 ml of a dilution of immune serum or normal serum (control).

After incubation for one hour at 20.degree. C., the gel is washed three times with phosphate buffer (PBS). The gel is maintained on the walls of the tube by means of magnet during the washes.

The gel is then incubated for 30 minutes in the presence of anti-immunoglobulin antibodies labelled with glucose-oxidase.

The enzymatic reaction is effected on the gel after three further washes (PBS).

Reading of optical density is effected at 400 nm after 10 minutes enzymatic reaction at laboratory temperature. The enzymatic activity measured is compared with that of a control (normal serum).

The gel used was magnetic gel coupled with sheep Ig prepared according to example 1; said gel was incubated in accordance with the procedure described above with the rabbit immune serum sheep anti Ig and the optical density of the gel for different dilutions of this immune serum was noted. The results obtained are given in the single figure appended, which shows the optical density measured at 400 nm in ordinates and the dilutions in abscissa; curve (A) refers to the immune serum and curve (B) relates to the normal serum used as control.

Similar results were obtained using the gel coupled with the bovine albumine prepared according to Example 1 and immune serum of rabbit bovine albumin antiserum.

The sensitivity of the determination realized according to the invention is approximately 50 ng of antibodies per ml.