Enzymes as versatile labels and signal amplifiers for monitoring immunochemical reactions

Paper-based colorimetric enzyme linked immunosorbent assay fabricated by laser induced forward transfer

We report the Laser Induced Forward Transfer (LIFT) of antibodies from a liquid donor film onto paper receivers for application as point-of-care diagnostic sensors. To minimise the loss of functionality of the active biomolecules during transfer, a dynamic release layer was employed to shield the biomaterial from direct exposure to the pulsed laser source. Cellulose paper was chosen as the ideal receiver because of its inherent bio-compatibility, liquid transport properties, wide availability and low cost, all of which make it an efficient and suitable platform for point-of-care diagnostic sensors. Both enzyme-tagged and untagged IgG antibodies were LIFT-printed and their functionality was confirmed via a colorimetric enzyme-linked immunosorbent assay. Localisation of the printed antibodies was exhibited, which can allow the creation of complex 2-d patterns such as QR codes or letters for use in a final working device. Finally, a calibration curve was determined that related the intensity of the colour obtained to the concentration of active antibodies to enable quantitative assessment of the device performance. The motivation for this work was to implement a laser-based procedure for manufacturing low-cost, point-of-care diagnostic devices on paper.

Development of automated paper-based devices for sequential multistep sandwich enzyme-linked immunosorbent assays using inkjet printing

To the best of our knowledge, this is the first report on paper-based devices for automating the sequential multistep procedures of a sandwich-type enzyme-linked immunosorbent assay (ELISA) that require only a single-step application of the sample solution. The device was based on a piece of nitrocellulose (NC) membrane with specially designed channels, where all the reagents are applied at different locations in order to control the fluid travel to the detection region. The inkjet printing method, a simple and low-cost process, was used to create the flow channel and device barrier patterns. The fabricated barrier was found to be an efficient boundary for the liquid along the printed design in the NC membrane, enabling direct control of the reagent flow time. ELISA results were obtained with a single-step sample application. The developed devices (so-called automated paper-based devices) provided a simple procedure for the sandwich ELISA, while reducing assay time and reagent consumption. Colorimetric results were measured using digital camera imaging with software processing. The capability of the method developed herein was successfully used to determine the levels of human chorionic gonadotropin (hCG) by ELISA.

Development and clinical application of sensitive enzyme immunoassay for macromolecular antigens--a review

Radioimmunoassay has been a powerful tool to measure haptens and antigens which are important for the investigation and diagnosis of diseases, especially endocrine disorders. However, the use of radioisotopes in radioimmunoassay suffers from serious disadvantages. Radioisotope-labeled reagents are unstable and hazardous to health. The disposal of radioactive wastes is not easy. Furthermore, the sensitivity of radioimmunoassay is limited by the detection limit of radioisotope that depends upon the half-life. The detection limit of the most widely used radioisotope, 125I, with a half-life of 60 days is 5 to 10 amol, when it is carrier-free. By contrast, the use of enzymes has obvious advantages. Some enzymes are very stable and cause no health hazards or waste disposal problems, provided that appropriate substrates are chosen. The detection limits of some enzymes are lower than that of 125I and will be further improved in the future. Therefore, enzyme immunoassay is potentially more sensitive than radioimmunoassay. This article reviews the development and clinical application of sensitive enzyme immunoassay for macromolecular antigens, which has been replacing radioimmunoassay.

Alkaline phosphatase conjugated protein A as a sensitive reagent to immunoscreen an expression cDNA plasmid library: isolation of cDNA to the calcium-binding protein of the chick embryonic chorioallantoic membrane

A highly efficient immunoscreening procedure has been developed to isolate cDNA clones to the calcium-binding protein (CaBP) of the chick embryonic chorioallantoic membrane (CAM). A library of total CAM cDNA was constructed using the expression plasmid vector, pUC 19. Bacterial clones containing plasmids with CaBP cDNA inserts were detected immunohistochemically based on their expression of hybrid CaBP protein sequences. For immunodetection, nitrocellulose bacterial colony replicas were treated with specific antibodies to the CaBP followed by incubation with Staphylococcus aureus Protein A conjugated with alkaline phosphatase (AP) which served as a secondary immunoreagent. Positive clones were then histochemically identified based on AP enzyme activity. The identity of the immunopositive clones was further verified by in vitro translation of mRNA selected by hybridization to the cloned cDNA. The AP-based immunoscreening procedure yields stable reaction products with relatively low background, and should find general application for isolating specific cDNA clones from expression cDNA libraries.

Amperometric assay for collagenase. Amplification by the use of glucose oxidase conjugated to insoluble collagen

A sensitive and rapid amperometric assay for collagenase has been developed. The substrate for the assay is glucose oxidase covalently linked to insoluble collagen with dimethylsuberimidate. The collagenase cleaves the insoluble collagen--glucose oxidase conjugate into smaller, soluble fragments that have glucose oxidase activity. That activity is proportional to the collagenase activity hydrolyzing the insoluble conjugate. In the absence of collagenase, no glucose oxidase activity is found in the soluble phase. Glucose oxidase activity was assayed by measuring amperometrically the rate at which hydrogen peroxide is produced. The kinetics follow that proposed for a soluble enzyme acting on an insoluble substrate.

A sandwich enzyme immunoassay of rabbit immunoglobulin G with an enzyme labeling method and a new solid support

A method was devised for enzyme labeling goat antibody to rabbit immunoglobulin G with beta-D-galactosidase (EC 3.2.1.23) using a heterobifunctional cross-linker, N-(gamma-maleimidobutyryloxy)-succinimide. Labeling of the purified antibody was by a continuous 2-step process and including a chromatographic purification procedure could be completed within one day. The partially purified anti-rabbit IgG was coated on Amino-Dylark cylinders, a new solid support, using glutaraldehyde as the coupling reagent. With enzyme-labeled antibody and the solid-phase anti-rabbit IgG, a sandwich enzyme immunoassay for rabbit IgG was developed with a lower limit of detection at 3.5 pM (0.1 ng/tube). The specificity of the assay was excellent and all 4 types of IgG tested showed 0.0001% or less cross-reactivity with rabbit IgG.

Antibody-induced conformational restriction as basis for new separation-free enzyme immunoassay

Under acid denaturing conditions, hologlucose oxidase labeled with 2,4-dinitrophenyl (DNP) was dissociated into flavin adenine dinucleotide (FAD) and DNP-labeled apoglucose oxidase (DNP-AG). Both lacked catalytic activity. The activity was restored by combining FAD and DNP-AG at about pH 7. If, on the other hand, anti-DNP serum was preincubated with the DNP-AG prior to the addition of FAD, activity was not restored. Furthermore, added DNP-aminocaproic acid counteracted the effects of the antibody in inhibiting the recombining of DNP-AG and FAD to form active enzyme. The anti-DNP serum probably prevented the DNP-AG from combining with FAD to form an active holoenzyme by restricting the mobility of the polypeptide chain of DNP-AG from folding into a catalytically active conformation. Based on such an antibody-induced conformational restriction of the DNP-AG, we developed a separation-free (homogeneous) enzyme immunoassay called AICREIA.

Enzyme modulator mediated immunoassay (EMMIA)

1. Enzyme modulator mediated immunoassay (EMMIA) is a separation-free (homogeneous) enzyme amplified immunoassay. 2. The assay is based on the ability of an analyte labeled enzyme modulator to modify the activity of an indicator enzyme and on ability of an anti-analyte antibody to abrogate the modifying action of the analyte-labeled enzyme modulator upon its binding to anti-analyte antibody. 3. The principle of EMMIA is elaborated in detail. The stages in the development of an EMMIA are described. 4. Criteria in selecting enzymes and modulators suitable for EMMIA are discussed. 5. Examples of EMMIA's using different kinds of modulators for different classes of analytes are presented.