Electrochemical assay for nucleic acids and nucleic acid probes
The present invention is concerned with assays for nucleic acids, the said assays extending to both detection of the presence f nucleic acids and to the particular identification of specific nucleic acid molecules. 
     There is a particular need in many areas of biochemical research and commerce to be able to assay for the presence of nucleic acids in a sample, and further, to assay for the existence of particular sequences within the said nucleic acids. 
     The exploitation of the hybridisation between two single-stranded DNA or RNA molecules which have a complementary sequence has been one approach to this problem. Molecules for use in such assays, which are capable of binding to a DNA or RNA sequence which is defined to a particular level of homology, are known as DNA-probes. 
     Known DNA (RNA) probe techniques share a similarity in that the DNA (RNA) polymer is not readily detectable by its inherent biochemical activity. It is therefore necessary to mark the polymer with some signal-producing chemical or biochemical species, such methods as are presently known include the followin prior art: 
     PRIOR ART 
     Avidin-Biotin Reaction; this technology relies on the affinity of the egg-white glycoprotein avidin for biotin. Biotin (Vitamin H) can be covalently linked to the nucleotide residues which comprise the monomeric subunits of the DNA polymer. The modified subunits can still undergo the classical binding reaction between complementary strands of double-stranded DNA, and thus can be incorporated into synthetic DNA probes. At present, to detect the presence of such probes which have formed short double-stranded regions after exposure to complementary sample DNA, the unbound probe must first be separated from the sample DNA/bound probe complex. This is normally done by performing the binding reaction in conditions under which the sample DNA is immobilized on a substrate and washing, although centrifugation may perform the same function. The bound probe is detected by the addition of avidin to which either a fluorescent- marker-labeled antibody or an enzyme has been attached. 
     One problem with the above method is that small oligonucleotide probes (20 nucleotides) contain only a small number of biotinylated sites, limiting the amount of avidin which can be bound. Attempts have been made, with some success, to add long &#34;tails&#34; of up to several thousand bases to the probe DNA, in which case only the tail need be labeled. The method can detect up to a resolution of 10 -13  g of DNA, or about 10 5  copies of a single gene. Although originally the marker on the avidin was horse radish peroxidase the method has now been extended to include alkaline phosphatase. Unfortunately the method is generally difficult to establish for a new diagnostic scheme as either the biotin-linked probe DNA is difficult to prepare or the labeled tail interferes with sensitivity. 
     Use of Mediators 
     Commonly owned European Patent Application No. 84303090.9 published on Nov. 14, 1984 as EP No. 125139A discloses a method in which at least one of a mediator and an enzyme are chemically linked to a nucleic acid probe sequence whereby specific binding of the probe sequence to the target sequence in a single-strand nucleic acid material to be investigated affects the electrochemical availability of the chemically linked species as detected by a sensor electrode in presence of the enzyme substrate, whereby the presence of the target sequence can be detected. 
     As disclosed in the specification, the nucleic acid sequence can be RNA e.g. messenger RNA but is usually DNA. 
     A suitable method for the practice of this technique has been given as follows; 
     (a) providing a single strand nucleic acid material to be investigated for a given target sequence; 
     (b) selecting a probe material with a sequence of nucleic acids complementary to the target sequence; 
     (c) choosing a procedure from among; 
     (i) chemically linking the probe with an enzyme and adding the enzyme-linked probe to a solution containing both a substrate for the enzyme and a mediator, 
     (ii) chemically linking the probe with a mediator and adding the mediator-linked probe to a solution containing both a substrate and an enzyme for the said substrate; 
     (iii) chemically linking the probe with a mediator/enzyme combination and adding the so-modified probe to a solution containing a substrate for the enzyme; 
     (d) contacting the solution containing the chemically linked probe sequence with a sensor electrode whereby charge is transferred by the mediator to the electrode from the enzyme-catalysed substrate reaction; and 
     (e) contacting the solution with the single stranded material, 
     whereby alteration in the amount of charge transferred is an indication of a specific binding reaction between the probe and target affecting the availability of enzyme, mediator or combination. 
     The probe material can be a naturally occuring DNA fragment or a synthetically produced material. 
     Alterations in the sequence of steps can be readily envisaged. Also the sensor electrode itself can include the mediator or the enzyme, although generally it is preferred for the probe sequence and the target sequence both to be present in solution. 
     The mediator can be linked indirectly to the probe sequence by a linker group, and a material reactive to the linker groups can be present on the electrode. In this case the whole complex is present on the electrode; when a target sequence is present, and binds to the probe sequence, an alteration in electrode current is produced. 
     Mediators Disclosed 
     The use of ruthenium complexes on a graphite electrode is disclosed in Koval and Anson [Analytical Chemistry, Vol 50, 223 (1978)] in which the following ruthenium complexes are discussed; 
     Ru(NH 3 ) 6   3+ ,2+ 
     Ru(NH 3 ) 5  py 3+ ,2+ 
     Ru(NH 3 ) 5  L 3+ ,2+ 
     Where L is pyridine, 4-aminomethylpyridine (AMP), or N-(4-picolinic)-benzamide (PBA) 
     Ru(NH 3 ) 5  OH 2   3+ ,2+ 
     Ru(NH 3 ) 5  Cl 3+ ,2+ 
     Errors of Metabolism 
     One aim of the DNA probe technology and the enzyme detection/assay technology so far developed, has been to detect inbuilt errors of metabolism which lead to a variety of &#34;genetic diseases&#34; and inheritable disorders. Among such disorders are: familial Goiter (iodotyrosine dehalogenase defective), Maple syrup urine disease (alpha-keto decarboxylase defective), Xanthinuria (Xanthine oxidase defective) and Methaemoglobinemia (Methaemoglobin reductase defective). A full list of 3500 conditions due to defective genes can be found in McKusick&#39;s &#34;Mendelian Inheritance in Man&#34;. 
     One particular disease of interest is sickle-cell anaemia, in which one of the normal globin gene codons corresponding to the fifth, sixth and seventh amino acids of a particular chain are not, as normal CCT-GAG-GAG, but read CCT-GTG-GAG, thereby replacing valine for glutamic acid at position six. Synthetic oligonucleotides have been made which can act as probes for the sequence variation in prenatal diagnosis of the disease, and successfully distinguish between the wild type and the abberant gene. Assay of the components of the binding mixture was performed by a radioactive tracer method. A similar approach has been taken with the point mutation reported to cause antitrypsin deficiency. 
     As can be seen from the above, however, the methods of detailed metabolic assay presently available have certain disadvantages in that they require some or all of the following; short lifetime reagents (either radioactive, air-sensitive or light-sensitive), highly trained staff or (for autoradiography, ESR measurements or low-level light detection) expensive equipment. 
     Known methods of assay for binding reactions other than antigen/antibody and DNA/complementary DNA are generally similar to the above methods and suffer from similar problems. 
     According to one aspect of the present invention there is provided an assay for nucleic acid which comprises the steps of; 
     (a) providing a probe material comprising; 
     (i) a sequence of nucleic acids complementary to a given target sequence and, 
     (ii) a first ligand chemically linked thereto and capable of a specific binding reaction with an antiligand; 
     (b) contacting the said probe material with an assay system comprising; 
     (i) a suitable mediator, enzyme, substrate system capable of transferring charge to an electrode surface when the enzyme is catalytically active, and; 
     (ii) a second ligand chemically linked to one of said mediator, enzyme or substrate, wherein the second ligand is capable of a competitive binding reaction with the antiligand, and; 
     (iii) the said antiligand, 
      whereby the said first ligand competes with the said second ligand in a specific binding reaction with the antiligand, and; 
     (c) contacting the above system with a solution suspected of containing the said target sequence whereby the binding of any of the said target sequence present to the probe affects the availability of the first ligand and therefore alters the rate of charge transfer to the electrode. 
     By employing a method as described above, it is possible to perform the assay with an amplification step, thereby increasing the resolution of the assay. 
     Furthermore, the method of the present invention does not require the short-lived radioactive assay components which are employed in other types of assay. 
     Enzyme/substrate pairs whose electrochemical behaviour in association with mediator compounds have been studied by the Applicants include the following: 
     
         ______________________________________                                    
Enzyme                Substrate                                           
______________________________________                                    
Flavo-proteins                                                            
Pyruvate Oxidase      Pyruvates                                           
L-Amino Acid Oxidase  L-Amino Acids                                       
Aldehyde Oxidase      Aldehydes                                           
Xanthine Oxidase      Xanthines                                           
Glucose Oxidase       Glucose                                             
Glycolate Oxidase     Glycolate                                           
Sarcosine Oxidase     Sarcosine                                           
Diaphorase            NADH                                                
Glutathione Reductase NADPH                                               
PQQ Enzymes                                                               
Glucose Dehydrogenase Glucose                                             
Methanol Dehydrogenase                                                    
                      Methanol and                                        
                      other Alkanols                                      
Methylamine Dehydrogenase                                                 
                      Methylamine                                         
Cytochrome b-linked Enzymes                                               
Lactate Oxidase       Lactate                                             
Metalloflavoproteins                                                      
Carbon monoxide Oxidoreductase                                            
                      Carbon Monoxide                                     
______________________________________                                    
 
    
     It is believed that any of these enzyme-substrate pairs could be utilized in association with the mediator in the present invention, given some limitations on the assay conditions which would be obvious to the man skilled in the art. Of these pairs, it is clearly advantageous to utilise those enzyme/substrate pairs whose behaviour is established in most detail and which give good, preferably linear, response over the expected measurement range. 
     Ferrocenes (bis-cyclopentadienyl iron and its derivatives) have advantages over other mediators used with enzyme/substrate reactions for charge-transfer purposes. 
     The unique structure and properties of ferrocene (bis n 5  cyclopentadienyliron: Fecp 2 ) and its derivatives has resulted in a considerable amount of theoretical and experimental studies. First synthesised in 1951, ferrocene was the earliest example of the now well-known metallocene compounds. 
     Whilst ferrocenes, had been found to be of limited value in spectrophotometric assays as a result of their poor solubility in aqueous solution and low extinction coefficients, they have been found to be more suited to a bio-electrochemical system. Ferrocenes have: 
     (a) a wide range of redox potentials accessible through substitution of the cyclopentadienyl rings which can be functionalised; 
     (b) electrochemically reversible one-electron redox properties; 
     (c) a pH-independent redox potential and a slow autoxidation of the reduced form. 
     Within this general class of ferrocenes, i.e. the monomeric or polymeric derivates substituted around one or both rings, we have found certain individual ferrocenes such as are listed below: 
     1,1&#39;-dimethylferrocene; acetic acid ferrocene; hydroxyethylferrocene; ferrocene; 
     1,1&#39; bis(hydroxymethyl)-ferrocene; monocarboxylic acid ferrocene; 1,1&#39;-dicarboxylic acid ferrocene; chloroferrocene; and methyl trimethylaminoferrocene. 
     The E° values of various ferrocenes in phosphate buffer at pH 7.0 given in the above table, span a range of potentials, E°=100 to 400 mV vs SCE. The trend in E° values is in agreement with that expected on the basis of substituent effects. In general electron-donating groups stabilize the positive charge and hence promote oxidation more so than electron withdrawing groups. 
     Ferrocene derivatives which may be utilized in the method of the present invention are not to be considered as limited to the specific examples listed above, for example, the applicants have determined that ferrocene boronic acid, and ferrocene ethylamine exhibit mediator properties. Furthermore, the derivatives of ferrocene listed above may be further modified by suitable techniques known to the man skilled in the art, such as by treatment with nitrene, or diazo precursors. 
     Commonly owned European Patent Application No. 82305597.5, published as EP No. 078636 on May 5, 1983, describes and claims a sensor electrode composed of electrically conductive material and comprising at least at an external surface thereof the combination of an enzyme and a mediator compound which transfers electrons to the electrode when the enzyme is catalytically active. 
     The purpose of such an electrode is to detect the presence of, measure the amount of and/or monitor the level of one or more selected components capable of undertaking a reaction catalysed by the said enzyme. 
     Examples of electrode configurations, mediators and uses are given in that patent application. 
     This specification makes use of the chemical properties of the mediators and their derivatives as exemplified in our earlier patent applications. 
     This specification also makes use of the electrode structures and materials disclosed in the commonly owned European Patent Application No. 82305597.5, published as EP No. 078636 on May 5, 1983, included herein by reference, such as electrode materials selected from the group comprising; gold, platinum, silver, carbon or a one-dimensional conductor. 
     While the examples of mediator compounds given above are generally limited to the ferrocene group of mediators, the present invention may also employ the non-ferrocene mediators. Several classes of compounds may perform the functions of a mediator, that is transfer charge from an enzyme to an electrode surface (or in certain cases to a further enzyme). 
     Systems which have been studied have included the following mediator compounds; 
     Carbon-boron compounds (including the carboranes) 
     Viologens (N,N&#39;-dialkyl of or diaryl derivatives of 4,4&#39;-bipyridyl) 
     one-dimensional conductors (including the salts of TCNQ) 
     Phenazine dyes (including phenazine methosulphate and phenazine ethosulphate), and 
     Metalloporphyrins (including cytochrome-C) 
     However the preferred class of mediators are the transition metal complexes, particularly those in which the mediator comprises at least one, and preferably two organic rings, each of which is characterised by at least two double bonds that are conjugated and a metal atom in electron-sharing contact with each of the rings, since these have stability, exhibit rapid electron transfer reactions, have a wide range of redox potentials and are in many cases oxygen insensitive. The preferred mediators are generally those which have iron, nickel or ruthenium as the transition metals. 
     Although the preferable specific binding reaction in the method of the present invention is that between avidin (or streptavidin) and biotin, it should be noted that the invention is not to be taken as limited to this specific binding reaction but also extends to the following pairs of specific binding partners; 
     antigen and antibody, 
     hormone and receptor, 
     lectin and carbohydrate, 
     cofactor and enzyme, and, 
     nucleic acid and complementary nucleic acid. 
     The preferred method of determination of mediator in solution is by the use of cyclic voltametry as described variously in our above-mentioned patent applications, and as incorporated herein by reference. It should be noted that the complete cyclic voltammogram need not be taken in every case, and in many instances it is sufficient (as illustrated by example hereafter) to poise the cell at a particular potential and to make all current and/or voltage and/or time measurements at this potential. As above, the man skilled in the art will be conversant with such techniques and therefore they will not be described in detail herein. 
     According to a second aspect of the present invention there is provided a nucleic acid sequence probe for establishing the presence and/or copy number of a given sequence in a single-stranded nucleic acid molecule wherein a probe nucleic acid sequence is linked to; 
     (a) either a mediator compound or an enzyme electrochemically coupled by the said mediator compound, and, 
     (b) biotin, 
     wherein binding of the probe nucleic acid sequence to the target nucleic acid sequence modifies the electrochemical coupling between the mediator compound and the enzyme. 
     According to a third aspect of the present invention there is provided a method of assay for nucleic acids wherein an electrode is placed in contact with an enzyme and a substrate of the enzyme, and a mediator compound is associated with the enzyme to transfer charge from the enzyme to the electrode as the conversion of substrate into produce proceeds, and, a probe nucleic acid sequence is linked to either the enzyme or the mediator compound, whereby the activity the enzyme and the mediator compound and hence the quantity of charge being transferred to the electrode is modified by the presence or absence of a complementary target nucleic acid sequence bound to the probe nucleic acid sequence. 
     The general comments as to the nature of the mediator, enzyme, methodology and apparatus given in respect of the first aspect of the invention apply equally well to these second and third aspects which should be read accordingly. 
     The basis of the present invention is that a current passes in response to the presence of an electrochemically active substance or &#34;mediator&#34; at an electrode surface, as described in our earlier patent applications, especially our European Patent Application 82.305597. The magnitude of the current passed varies either 
     (a) in proportion to the amount of DNA-probe/target-DNA complex formed, or 
     (b) as a perturbation of an existing steady state current which is either enhanced or diminished, in proportion to the quantity of DNA-probe/target-DNA complex formed. 
     In one preferred embodiment of the present invention, the electrode exhibits reversible electron transfer with biotin-labled cytochrome-C. The biotin-labled cytochrome-C acts as a mediator compound, but can also bind to avidin as described in general terms above. Avidin or Streptavidin is added to the reaction mixture and binds to the mediator, reducing the transfer of charge to the electrode. The addition of biotin-labled nucleic acid (DNA or RNA) to the mixture sets up a competitive specific binding reaction between on the one hand a biotin Cytochrome-C (first ligand-mediator) complex with avidin (antiligand) and on the other hand an avidin (antiligand) complex with the biotin-labled nucleic acid (DNA or RNA) (second ligand). As the avidin (or streptavidin) concentration is fixed, an increase in the concentration of the avidin-biotin-nucleic acid complex will cause a corresponding increase in the concentration of the free Biotin Cytochrome-C species, which will cause a related increase in the rate of transfer of charge from the enzyme to the electrode surface. 
     Although the above embodiment of the invention has been described in terms of a first ligand linked to the mediator, it should be understood that the invention is also envisaged as being configured with the mediator linked to either the enzyme or the substrate. 
     In a further preferred embodiment of the present invention the electrode surface detects and measures an enzyme catalysed reaction in the presence of a mediator, which in a more preferable embodiment is cytochrome-C covalently linked to a nucleic acid probe. The electrochemical availability of the mediator is affected by the degree of homologous binding between the nucleic acid probe and the target DNA. 
     In a yet further preferred embodiment of the present invention, the electrode detects and measures, in the presence of a mediator, an enzyme-complex catalysed reaction. In a particular embodiment, the enzyme complex comprises a biotin-peroxidase-avidin complex which is capable of binding further biotin or biotin labled nucleic acid. A steady-state current is obtained using the peroxide substrate. This current decreases when biotin-labled nucleic acid is added to the assay mixture. 
     In each of the three embodiments described above there is a competitive reaction between the probe-containing species and the electrochemical system comprising the mediator, enzyme and substrate. The rate of charge transfer is perturbed by a lowering of the availability of the probe-containing species. Consequently, when a sample containing target nucleic acid is added to the assay system, the degree of homologous binding between the probe nucleic acid and the target is reflected in a lowering of the availability of the free probe containing species, and a corresponding change in the rate of charge transfer. 
     In a still further preferred embodiment of the present invention, the mediator is a ruthenium compound. 
     The rate of reduction of the ruthenium compound pentaammineisonicotinamineruthenium (III) was first noted to be high in 1970 by Taube. Since this time a quantity of work has been done with ruthenium compounds both in liquid systems and on electrodes, however the incorporation of a ruthenium compound into a charge transfer electrode is presented here for the first time. 
     Specific ruthenium compounds which are preferred in embodiments of the present invention include the following; 
     [Ru(NH 3 ) 5  py] 3+ ,2+, 
     [Ru(NH 3 ) 6  ] 3+ ,2+, 
     [Ru(NH 3 ) 5  L] 3+ ,2+, 
     where L is 4-aminomethylpyridine (AMP), or L is N-(4-picolinic)-benzamide (PBA) 
     [Ru(NH 3 ) 5  OH 2  ] 3+ ,2+, and, 
     [Ru(NH 3 ) 5  Cl] 3+ ,2+. 
     A method for attaching the first above listed compound to a graphite electrode is disclosed by Koval and Anson (as referenced above). 
     It is also envisaged that the present invention extends to novel conjugates of DNA or RNA and ruthenium derivatives. It has been known [Clarke and Taube Journal of the American Chemical Society 97, 1397-1403 (1975)] to prepare adducts of ruthenium compounds with xanthines, and it is therefore believed possible that the methods of the present invention may be extended to nucleotide derivatives which are ruthenium adducts. 
     According to a fourth aspect of the present invention there are provided kits of reagents and/or apparatus for carrying out the methods of assay for nucleic acids described, supra. 
    
    
     In order that the present invention may be further understood, it will be further illustrated by way of example with reference to the accompanying figures wherein; 
     FIG. 1; shows an assay protocol in which the nucleic acid is bound to the enzyme, 
     FIG. 2; shows an assay in which the nucleic acid is bound to the mediator, 
     FIG. 3; shows an assay in which the nucleic acid is bound to a linker which has an affinity for the electrode surface, 
     FIG. 4a; shows the cyclic voltammogram of 2.8 mg biotin cytochrome-C in the presence of horseradish peroxidase. 
     FIG. 4b; shows the same as 4a on addition of 300 mM hydrogen peroxide, 
     FIG. 5; shows a y-t recorder trace of the current produced in the presence of 1.4 mg/ml of biotin-cytochrome C, 105 m/ml horseradish peroxidase, at a BPS 4,4-dipyridyldisulphide surface modified gold working electrode, 
     FIG. 6; shows the cyclic voltammogram of [Ru(NH 3 ) 5  ]py 2+ , with gold as a working electrode in the presence of 6 mM biotin-peroxidase avidin enzyme complex, 
     FIG. 7; shows further details of FIG. 6, and, 
     FIG. 8; shows a y-t plot of the catalytic current against time with the addition of biotinylated DNA to the assay mixture. 
    
    
     EXAMPLE 1 
     Enzyme Attached to DNA Probe 
     In FIG. 1, E is an enzyme, for example, glucose oxidase, which catalyses the conversion of a substrate S (for example glucose) to a product, P, (for example gluconolactone which then gives gluconic acid) and liberates electrons, e, which reduce the oxidised form of a mediator M ox  (for example the ferricinium ion) to the reduced form, M red  (for example ferrocene). M red  is oxidised at the electrode G, the current passed is measured and is proportional to the amount of substrate S present. 
     Bound to the enzyme E by any suitable method is a DNA fragment, D, which may be either derived from a naturally occurring DNA sequence or may be synthetic. In the presence of an excess of the substrate S, a steady state current is obtained. A sample of DNA which is to be assayed for a particular target sequence complementary to the sequence of the DNA fragment D is converted to single strands by any suitable method, and then added to the reaction mixture. 
     If a target sequence complementary to the DNA fragment D is present in the mixture, it will bind to the fragment D and inhibit the enzymatic reaction of the enzyme E with the substrate S. Consequently the throughput of substrate S to product P will be reduced and the coupled reduction of M ox  will be diminished. The change in the rate of reduction of M ox  is reflected in a reduction of the current at the electrode G. 
     The change in current is proportional to the amount of the fragmented DNA D which is now bound to target DNA and hence to the amount of target DNA present. 
     EXAMPLE 2 
     Mediator Attached to DNA Probe 
     In example 2, the redox-active mediator M (in this example a substituted ferrocene) is attached directly (as shown in FIG. 2) to the fragmented DNA which is to be used as a probe. The formation of mediator-linked DNA-probe does not impede either the amperometric response of the mediator M nor the binding interaction of the mediator-DNA probe with the complementary target sequence contained in the DNA which is being assayed. 
     The amperometric response caused by the addition of the mediator-DNA probe to the assay mixture is measured. If genetic material in a single-stranded form to which the probe is complementary is present, the probe binds to the complementary sequence in the sample DNA. This greatly reduces or completely inhibits the amperometric response, that is, the complex of the mediator-DNA probe and the target DNA is not amperometrically active. The reduction in the initial amperometric response is in direct proportion to the amount of mediator-DNA probe/target DNA complex formed and hence to the amount of genetic material containing a sequence complementary to the known sequence of the mediator-DNA probe. 
     EXAMPLE 3 
     Mediator- and Linker-Attached to DNA Probe 
     In this example (as shown in FIG. 3) the mediator-DNA probe also contains one or more linker groups L, (biotin may for example be used). 
     The mediator-linker-DNA probe is in this exemplary method treated with the sample suspected of containing the single-stranded genetic material which is the target for the DNA probe and the mediator-linker-DNA probe binds to any complementary sequence present. 
     An electrode G on whose surface is present an electrochemically active material R, that recognises the linker group L, for example avidin labelled with ferrocene, is then immersed in the reaction mixture. The current measured on applying a potential is reduced on the binding of the electrochemically active material R to the mediator-linker-DNA probe/target-DNA complex. The reduction of the current is again in proportion to the amount of the added single-stranded sequence with a target sequence complementary to the known sequence of the DNA-probe. 
     EXAMPLE 4 
     System Comprising Biotin Cytochrome-C and Peroxidase 
     In this particular embodiment, the electrode exhibits reversible electron transfer with biotin-labled cytochrome-C. The biotin-labled cytochrome-C acts as a mediator compound, but can also bind to avidin as described in general terms above. 
     The addition of biotin-labled nucleic acid (DNA or RNA) to the mixture sets up a competitive specific binding reaction between on the one hand a biotin Cytochrome-C complex with avidin and on the other hand an avidin complex with the biotin-labled DNA. 
     As the avidin (or streptavidin) concentration is fixed, an increase in the concentration of the avidin-biotin-nucleic acid complex will cause a corresponding increase in the concentration of the free biotin Cytochrome-C species, which will cause a related increase in the rate of transfer of charge from the enzyme to the electrode surface. 
     As an example a gold electrode surface modified with 4,4-bipyridyl, or like material, was used. In this exemplary method the biotin-labelled cytochrome C acts as a mediator to an enzyme, in the present example, horseradish peroxidase. 
     The enzyme transfers charge between the mediator, (biotin cytochrome C in this case), and the substrate (H 2  O 2 ), as shown by the catalytic current produced on addition of H 2  O 2 , as illustrated in FIGS. 4a and 4b. 
     Both the enzyme and the substrate are present in excess. The method operates by poising the potential of the cathode at a suitable, negative value, giving a resulting catalytic current which depends on the biotin-cytochrome C concentration. 
     Avidin or streptavidin is now added to the system. Some of the mediator complexes to the avidin, and the catalytic current consequently decreases. The addition of the biotin-containing DNA or RNA releases some of the biotin cytochrome C since the former competes for the avidin present. As a result the catalytic current increases in proportion to the added biotinylated DNA, as illustrated in FIG. 5. 
     Experimental conditions 
     A conventional three electrode system, was employed, wherein a 4,4-bipyridyldisulphide-coated gold electrode was used as the working electrode. Other components of the assay mixture were 0.05M phosphate buffer, pH 6.2; biotin-cytochrome C (SIGMA); 0.14-2.8 mg; H 2  O 2  300-80 mM; horseradish peroxidase 210 mg/ml; avidin 5-50 mg; biotin polyuridyl (BRL) 1-10 ul. 
     The cyclic voltammogram of biotin-cytochrome C (scan between -200 and +200 mV) does not change when the enzyme horseradish peroxidase is added to the cell, as can be seen from FIG. 4a. 
     The addition of substrate (H 2  O 2 ) results in a dramatic change due to the catalytic current, as shown in FIG. 4b. 
     The electrode is poised at a reduction potential as is shown generally in FIG. 5 the addition of substrate as shown at 5a, produces the catalytic current which depends on the biotin cytochrome C concentration. 
     In a second step, avidin is added to the assay system; the addition of the same amount of substrate results in reduced catalytic current because some of the biotin-cytochrome C is complexed, as shown in FIG. 5b. 
     In a third step as shown in FIG. 5c, biotin-DNA and RNA is added to the system; addition of the same amount of H 2  O 2  increased the catalytic current thus showing a dependence on the probe being present. 
     EXAMPLE 5 
     Activated Calf Biotin-Lablled Calf-Thymus DNA 
     This example employs an electrode surface which detects and measures in the presence of a mediator, which in this case is biotin cytochrome C covalently attached to DNA or RNA or cytochrome C covalently attached to DNA or RNA, an enzyme-catalysed reaction. 
     The catalytic current in the presence of excess enzyme or substrate is a measure of the biotin-cytochrome C-DNA(BCDNA), or cytochrome DNA(CDNA) present in the electrochemical cell. 
     Experimental conditions for preparation of activated BC Labelled calf thymus DNA 
     A cytochrome C bridge was used to attach the biotin to DNA, (Manning et al) Biotin-labelled cytochrome C (commercially available from SIGMA) 5 mg was added to 4 mg of activated calf thymus DNA (Pharmacia PL Biochemicals) in 0.1M triethanolamine (TEA) pH 7.8. 
     1.2 ml of 6% formaldehyde was added in the same buffer for cross-linking, and the mixture was incubated at 37° for an hour. The formaldehyde was removed by extensive dialysis. 
     Sodium chloride was added to the sample to give a final concentration of 1M (to dissociate any cytochrome C which was not covalently bound). 
     The solution was fractionated on an FPLC gel filtration column using Sepharose 6B. The fractions were detected at 254 nm. The extinction coefficient of 25 cm 2  /mg for DNA at 260 nm, and 7.7 cm 2  /mg for biotin-cytochrome C at 410 nm were used to calculate the nucleotide to cytochrome C ratio and was found to be approximately 20:1 in the product. 
     The BCDNA or CDNA acts in the electrochemical cell as a mediator to horseradish peroxidase. In the presence of substrate the catalytic current is a measure of the amount of BCDNA or CDNA present or the amount of hybrid present. 
     The electrode is poised at a reducing potential. The catalytic current indicates the presence of the biotin cytochrome C DNA (or RNA) or cytochrome C DNA (or RNA). 
     A standard three electrode system employing a 4,4 bipyridyldisulphide-coated gold working electrode was used in 0.05M phosphate buffer at pH 6.2. The mediator is biotin-cytochrome-DNA (BCDNA) in this example, although it is envisaged that any other electrochemical label attached through a cytochrome bridge which mediates between horseradish peroxidase and an electrode could be used. 
     If cytochrome-C is used, its concentration should be not more than 300 uM because of the danger of bleaching the cytochrome-C at higher concentrations. 
     EXAMPLE 6 
     Nick Translation of Biotin Lablled &#39;Phage Lamda DNA 
     In this exemplary method, use is made of an electrode surface which can detect or measure, in the presence of a mediator, an enzyme complex-catalysed reaction. In this example the enzyme complex consists of biotin peroxidase-avidin in a proportion such that it can bind additional biotin or a biotin containing material such as biotin-labelled DNA or RNA. 
     A steady-state current is obtained using the biotin peroxidase-avidin complex in the presence of H 2  O 2  and a mediator such as cytochrome C, a ferrocene compound or, in the examples shown herein [Ru(NH 3 ) 5  py] 2+ . The electrode is poised at a negative potential. The substrate, H 2  O 2  is present in high excess. The current increases when biotin labelled DNA or RNA is added to the cell. The biotin DNA may be either in solution or is attached to a membrane. 
     At a limiting concentration of the biotin peroxidase-avidin complex and excess mediator and substrate, if the electrode poised at a suitable potential, (in this particular instance negative with respect to a saturated calomel electrode) a steady state current is obtained which is proportional to the enzyme concentration. 
     From this point the experiment presented here by way of example was performed according to two different experimental protocols, that is either by the addition of; 
     (a) Biotin DAN or Biotin RNA added in solution 
     (b) Immobilised biotin DNA or biotin RNA, 
     In both instances the nucleic acid reacts with the enzyme complex, and gives a current increase which is proportional to, and acts as a measure of, the added Biotin DNA or RNA. 
     Experimental conditions for the Nick translation of biotin-labelled Phage Lamda DNA 
     Sonicated phage Lamda DNA was nick translated in the presence of biotin-11-dUTP, according to the BRL technical information protocol using their nick translation reagent system, the reagents employed being as follows; 
     (1) 10 ul solution of the mixture of 0.2 mM dATP, dCTP, dGTP in 500 mM Tris pH7.8 50 mM MgCl 2 , 100 mM 2-mercaptoethanol, and 100 mg/ml nuclease-free BSA. 
     (2) 4 ug of Lamda phage DNA in 0.1 mM EDTA, 10 mM Tris pH7.5 and 100 mM sodium chloride. 
     (3) 10 ul of biotin-11-dUTP in 0.4 mM in 100 mM Tris pH 7.5. 
     The volume was made up to 90 ul and the reaction mixture was carefully mixed. 
     (4) 10 ul DNA polymerase was added to the mixture, 0.4 units per ul, 40 pg per ml DNAase I in 50 mM Tris pH7.5, 5 mM magnesium acetate, 1 mM 2-mercaptoethanol, 0.1 mM phenylmethyl-sulfonylfluoride, 50% glycerol, and 100 ug per ml nuclease free BSA. 
     The mixture was incubated at 15° C. for 90 minutes. The reaction was stopped by adding 10 ul 300 mM EDTA. The purification of the biotin-labelled DNA was carried out according to the &#34;Spin column Procedure&#34; for Maniatis et al [Mantiati, Fritsor and Sambrook, Molecular Cloning a laboratory manual, 1982, Cold Spring Harbor Labs 466-467]. 
     A 1-ml disposable syringe was used, which was plugged with a small amount of glass wool. A 0.9 ml bed volume of Sephadex was equilibrated with 0.15M sodium chloride, and 0.015M sodium citrate at pH7.0 was packed in the syringe, and spun on a bench centrifuge, several times until the packed volume, 0.9 ml. 0.1 ml of buffer was then added to the syringe which was then spun at the same speed and for the same time. 
     Finally, the nick translation mixture was added in 0.1 ml and re-centrifuged at exactly the same speed and time and the 100 ul of effluent was collected in a de-capped Eppendorf tube, which contained the biotin-labelled phage Lambda DNA. The biotin-labelled phage Lamda DNA BRL, probed, nick translated, as above, and biotin cytochrome C-labelled activated calf thymus DNA were attached to the nitrocellulose membrane by heating at 80° C. for 2 hours in a vacuum oven 
     EXAMPLE 7 
     Electrochemical Detection of Biotin-Lablled Lamda DNA 
     This experiment was performed in two ways, with DNA in solution and the DNA bound to a nitrocellulose membrane. 
     In both cases the assay system comprised a gold electrode in a 0.7 ml volume cell 
     The reagents employed for this example were; 
     0.05 M phosphate buffer pH 6.2 
     Mediator 
     (a) 0.15 mM ferrocene monocarboxylic acid 
     (b) 0.16 mM [Ru(NH 3 ) 5  ]Pyr 2+   
     Biotin-peroxidase-avidin enzyme complex concentration 60 nM-0.6 nM as stated. 
     8.8 mM H 2  O 2  as substrate. 
     The formation of enzyme complex was accomplished as follows: Vecta Stain ABC (Seralab) was used under their recommended conditions; 50 ul of biotin-peroxidase 5 mg/ml was incubated with 50 ul of avidin 1 mg/ml in 1 ml assay buffer containing 0.1% Tween 20 for 30 minutes. The further dilutions were made in the assay buffer containing 0.1% Tween 20. 
     EXAMPLE 7a 
     USE OF FERROCENE MONOCARBOXYLIC ACID 
     When ferrocene monocarboxylic acid was used as a mediator, the catalytic current was linear under the condition from 60 nM to 6 nM enzyme concentration giving a catalytic current in the range of 0.8-0.7 mA. 
     EXAMPLE 7b 
     USE OF RUTHENIUM COMPLEX AS A MEDIATOR 
     When [pyRu(NH 3 ) 5  ] 2+  was used as mediator, the catalytic current was approximately tenfold larger than in the previous case, therefore it was the choice in the three subsiduary experiments described by way of example below. 
     EXAMPLE 7c 
     ENZYME CONCENTRATION 6.0 nMOLAR 
     In this example the enyzme concentration was 6 nM. In the presence of H 2  O 2  a steady state catalytic current was observed, as shown in FIG. 8a. 
     When biotin labelled phage Lamda DNA (200 ng) was added to the electrochemical cell the steady state catalytic current decreased. This is illustrated in FIG. 8b. 
     Further injection of H 2  O 2  still results in a catalytic current. The result shows that the biotin-DNA complexes some of the biotin avidin enzyme complex, but still there is a high excess of the enzyme complex present which did not bind to the biotin DNA. 
     EXAMPLE 7d 
     ENZYME CONCENTRATION 0.6 nMOLAR 
     This example follows the lines of the previous example the only change being in the enzyme complex concentration, which is 0.6 nM in this case. 
     The catalytic current, as shown in FIG. 7b, characteristic of this enzyme complex concentration drastically changes, as shown in FIG. 7c, after addition of biotin DNA (200 ug). With further H 2  O 2  injection the current remains at a decreased level as illustrated in FIG. 7D. 
     EXAMPLE 7e 
     DNA IMMOBILIZED ON MEMBRANE 
     A catalytic current is obtained with H 2  O 2  substantially as described above. The biotin-labelled DNA immobilised on a nitrocellulose membrane, was added to the cell in the range of 0.5 ug-0.2 ug of DNA. After 10 minutes incubation with the biotin DNA membrane a further H 2  O 2  injection did not induce any catalytic current, as shown by FIG. 7e, thus indicating that the enzyme complex binds to the biotin DNA present on the membrane, and charge transfer is therefore inhibited.