Design of the immunogen and label for use in a fluoroimmunoassay for paracetamol

Highly specific, sensitive and rapid enzyme immunoassays for the measurement of acetaminophen in serum

Acetaminophen antibodies were purified using affinity chromatography and labelled with horseradish peroxidase (HRP). The antibody-HRP conjugate and a new acetaminophen derivative were used in the construction of two immunoassay methods facilitating the direct quantitative measurement of acetaminophen in serum: a 96-well microtitre plate and coated-tube ELISAs. A minimum detection limit of 0.2 μg mL(-1) and a dynamic range of 0.2 to 1 μg mL(-1) in serum were achieved using the 96-well microtitre plate ELISA. The tube assay was optimised for the measurement of the clinically critical acetaminophen concentration of 50 to 250 μg mL(-1) of serum. The quantitative and specific tests could be completed within less than an hour. Common drugs including aspirin showed less than 0.1% cross-reactivity.

Polyclonal catalytic antibodies

Polyclonal catalytic antibodies offer advantages in the evaluation of immunogens and in the ease of production of large quantities of antibodies. They comprise the entire immune response of an animal to an immunogen where monoclonals represent a subset. Polyclonal antibodies are consequently particularly suitable for evaluating catalytic antibody responses generated by different haptens or a group of structurally related haptens. The authors reported the first polyclonal catalytic antibodies in 1990. An unexpected finding is that polyclonal catalytic antibodies show single-site kinetic behaviour, i.e. whatever structural heterogeneity exists, the kinetic behaviour is homogeneous. Many groups worldwide have since published work in this area. Three groups are prominent. The authors' group, a group based in Austin, Texas, and led by Iverson, and a Shanghai group. The authors' group works with sheep antibodies and has published mechanistic studies and, more recently, specificity studies that revealed the catalysis of a beta-lactam. Most of this work over a 10-year period was performed by using a single bleed from a single sheep, which gives an indication of the ease of production and utility of such catalytic antibodies. Iverson's group works with rabbit antibodies and has published much work on the evaluation of catalytic antibodies generated by structurally related haptens. The Shanghai group has concentrated on the studies of polyclonal catalytic antibodies for electrocyclic reactions where any danger of contaminating enzymes is reduced. The use of immunisation to generate therapeutically useful catalytic antibodies (necessarily polyclonal) in a host animal is an attractive target. Several groups are working towards this. The authors themselves have published in this area and promising recent studies come from a group working in France on immunisation to protect against nerve agents and a Texas group (based in Houston) that has demonstrated active immunisation to generate antibodies that catalyse the hydrolysis of a carbamate insecticide.

Catalytic antibody activity elicited by active immunisation. Evidence for natural variation involving preferential stabilization of the transition state

1. The hydrolytic activity of IgG purified from (a) 13 samples of ovine antiserum collected from three animals during a two-year immunisation programme using a phosphate immunogen (comprising the amide conjugate bonded through the carboxy group of 4-nitrophenyl 4-carboxymethylphenyl hydrogen phosphate and amino groups of keyhole-limpet haemocyanin) and (b) a sample of ovine antiserum collected from another animal during an 18-week immunisation programme using an analogous sulphone immunogen (comprising the amide conjugate bonded through the amino group of 4-nitrobenzyl, 4-(4-aminobutoxy)benzyl sulphone and carboxyl groups of keyhole-limpet haemocyanin) were evaluated kinetically by using 4-nitrophenyl 4-(3-aza-2-oxoheptyl)phenyl carbonate and 4-nitrophenyl 4-(2-hydroxyethoxy)phenyl carbonate as substrates. 2. Catalytic activity was found in all 13 samples of anti-phosphate IgG but was absent in the sample of anti-sulphone IgG as well as in all samples of IgG isolated from the serum of non-immunised animals. These findings, taken together with the lack of catalytic activity of the anti-phosphate IgG towards the 2-nitrophenyl 4-(3-aza-2-oxoheptyl)phenyl carbonate, compel the view that the catalytic activity of the anti-phosphate IgG preparation is entirely antibody-mediated and is not due to contaminant hydrolytic enzymes. The fact that catalytic activity was found in all 13 samples of the anti-phosphate IgG provides the first evidence that it is possible, as a routine, to elicit a catalytic antibody response in a host animal via active immunisation. 3. The nature of the, albeit small, variation in the catalytic characteristics of the anti-phosphate IgG (increase in both kcat, the catalytic rate constant calculated as V/2[IgG] and kcat/Km, the apparent second-order rate constant for the overall catalysed conversion of substrate to products, with increase in Km suggests simultaneous improvement in transition state binding and deterioration in substrate binding as predicted from immunogen design and the postulated general mechanistic basis of antibody catalysis. 4. This interpretation is supported by the difference in the values of the dissociation constant Ki for the competitive inhibition by the transition-state analogue 4-methylphenyl 4-nitrophenyl hydrogen phosphate of reactions catalysed by two representative anti-phosphate IgG samples: for the catalysis with Km = 4.5 microM, Ki = 9 nM and for that with Km = 1.3 microM, Ki = 80 nM.(ABSTRACT TRUNCATED AT 400 WORDS)

A polyclonal antibody preparation with Michaelian catalytic properties

1. 4-Nitrophenyl 4'-(3-aza-2-oxoheptyl)phenyl carbonate (I), an amide conjugate (XI) involving the carboxy group of 4-nitrophenyl 4'-carboxymethylphenyl phosphate and an amino group of keyhole-limpet haemocyanin, and a fluorescein derivative (XVII) were synthesized. 2. The conjugate (XI) was used as an immunogen with which to raise polyclonal antibodies in multigeneration cross-bred sheep; the fluorescent derivative (XVII) was used for the initial assessment of the antisera via binding assays monitored by fluorescence polarization; the carbonate ester (I) was used as a chromogenic substrate for the investigation of catalytic activity. 3. The IgG from the antiserum of sheep no. 270 was isolated by Na2SO4 precipitation and chromatography on Protein G-Sepharose. 4. This preparation of IgG catalysed the hydrolysis of the carbonate ester (I); the catalysis at pH 8.0 and 25 degrees C obeyed Michaelis-Menten kinetics with at least 25 turnovers, Km = 3.34 microM, and lower limits for kcat. of 0.029 s-1 and for kcat./Km of 8.77 x 10(3) M-1.S-1, on the unlikely assumption that the concentration of catalytic antibody is provided by twice the total IgG concentration (two sites per molecule); probable estimates of the fraction of the total IgG that is anti-haptenic IgG and of the fraction of this that is catalytically active suggest that the values of kcat./Km are actually very much larger than these lower limits. 5. The failure of the antibody preparation to catalyse the hydrolysis of the isomeric 2-nitrophenyl carbonate (II), which differs from compound (I) only in the position of the nitro substituent in the leaving group, compels the view that catalytic activity is due to antibody rather than contaminant enzyme; this conclusion is supported by (a) the failure of the following to discriminate effectively between the isomeric substrates (I) and (II): pig liver carboxylesterase, rabbit liver carboxylesterase (collectively EC 3.1.1.1), whole serum from a non-immunized sheep and whole serum from a sheep immunized with a derivative of 3-O-methylnoradrenaline and (b) the lack of catalytic activity in IgG preparations from sheep immunized with sulphoxide or sulphone analogues of immunogen (XI). 6. The various parameters used for the comparison of the kinetic characteristics of hydrolytic catalytic antibodies are discussed. 7. The characteristics of hydrolysis of compound (I) catalysed by the present polyclonal antibody preparation are shown to be substantially better in most respects than those of analogous reactions of two other carbonate esters catalysed by monoclonal antibodies.

Development of a radioimmunoassay for idazoxan hydrochloride

To facilitate the measurement of idazoxan in clinical trials an immunoassay capable of detecting low ng ml-1 concentrations was required. A stable derivative was prepared which, after linking to keyhole limpet hemocyanin (KLH), was subsequently immunized into sheep. Early assay development was carried out with both fluorescent (1-FITC) and iodinated (I-125) labels. The assay methodology was the same in both cases, using magnetizable solid-phase particles to which the antibody was linked. The sensitivity of the assay was such that a large sample volume was required, which in turn, led to problems of increased protein interference. The use of pepsin to digest the protein was used effectively after several blocking agents were unsuccessfully used. The limit of detection was in the region of 3 ng ml-1. Cross-reactivity studies showed that the antibody was specific for idazoxan. Intra- and inter-assay precision was 7 and 12%, respectively. Correlation with the analytical GC-MSD method was in the order of 0.90.

Development of a specific polarisation fluoroimmunoassay for paracetamol in serum

A specific polarisation fluoroimmunoassay for the measurement of paracetamol levels in serum has been developed for use in emergency toxicological screening. It is based on the use of a fluorescein-labelled analogue and a sheep antiserum and exploits the rapid dissociation kinetics of the hapten-antibody complex to enable the label and antiserum to be combined as a single reagent. Paracetamol levels are determined by adding 5 microL of serum to 1.5 mL of the single reagent, incubation at ambient temperature for a few minutes and measurement of fluorescence polarisation. In addition to its speed and simplicity, the assay is both accurate and precise and the results obtained correlate closely with those from the commonly used chemical and enzymatic techniques.