Comparative characterization of recombinant ZZ protein-alkaline phosphatase and its application in enzyme immunoassays

Genetic fusion of mussel foot protein to ZZ protein improves target detection in solid-phase immunoassays

In the process of a solid-phase immunoassay, the stability and binding orientation between the antibody and the solid matrix can substantially influence the results. ZZ protein is a modified peptide of the B domain of Staphylococcus aureus protein A, which can bind to the Fc fragment of an antibody. It is often used for oriented immobilization of antibodies during solid-phase immunoassay. However, the conjugate is often not retained during the process, for example during washing steps. The resulting low stability detracts from reproducibility and sensitivity. Mfp-5 protein comes from mussel, is one of the components of mussel foot silk protein, and has good adhesion and biocompatibility. In this paper, the fusion protein of ZZ and Mfp-5 was constructed and expressed in Escherichia coli. In this method, the ZZ domain was firmly attached to the solid-phase support by Mfp-5, the directional fixation of IgG was realized by binding the ZZ protein to an Fc fragment, and then a Fab fragment was bound to the antigen to realize the solid-phase immunoassay. In addition, a protein adsorption assay confirmed that the adhesion of ZZ-Mfp-5 was significantly higher than that of ZZ protein, and the presence of Mfp-5 improved the ability of ZZ protein to capture antibodies. In conclusion, compared with the passively immobilized ZZ protein, the ZZ-Mfp-5 protein had stronger immobilization and antibody capture, a 10-fold increase in sensitivity and wider linear range, and better stability of detection. This may be an attractive strategy for solid-phase immunoassays or biosensing assays.

Kinetics study on recombinant alkaline phosphatase and correlation with the generated fluorescent signal

Alkaline phosphatase (AP) (EC 3.1.3.1) is one of the most commonly used enzymes in immunoassays. In VIDAS® assays (bioMérieux, Marcy l'Etoile, France), AP catalyzes the hydrolysis of 4-methylumbelliferyl phosphate (4-MUP) in 4-methylumbelliferone (4-MU) producing a fluorescent signal. This work introduces an original method of characterization of the kinetic parameters K_m, V_max, and K_cat of AP embedded in VIDAS® assays. Assessment of such constants allows us to predict the fluorescent signal generated for given amounts of enzyme and its associated substrate; in the particular case of VIDAS®, it has been estimated that 0.06 nmol/L of AP produces 3144 Relative Fluorescent Values (RFV).

ABBREVIATIONS

4-MUP, 4-Methylumbelliferyl phosphate; 4-MU, 4-Methylumbelliferone; RFV, Relative Fluorescent Values; RFU, Relative Fluorescent Units; QDs, Quantum Dots; LoD, Limit of Detection.

Development of a nanobody-alkaline phosphatase fusion protein and its application in a highly sensitive direct competitive fluorescence enzyme immunoassay for detection of ochratoxin A in cereal

A rapid and sensitive direct competitive fluorescence enzyme immunoassay (dc-FEIA) for ochratoxin A (OTA) based on a nanobody (Nb)-alkaline phosphatase (AP) fusion protein was developed. The VHH (variable domain of heavy chain antibody) gene of Nb28 was subcloned into the expression vector pecan45 containing the AP double-mutant gene. The Nb28-AP construct was transformed into Escherichia coli BL21(DE3)plysS, and soluble expression in bacteria was confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot. Both the Nb properties and AP enzymatic activity were validated by colorimetric and fluorometric analysis. The 50% inhibitory concentration and the detection limit of the dc-FEIA were 0.13 and 0.04 ng/mL, respectively, with a linear range of 0.06-0.43 ng/mL. This assay was compared with LC-MS/MS, and the results indicated the reliability of Nb-AP fusion protein-based dc-FEIA for monitoring OTA contamination in cereal.

Well-oriented ZZ-PS-tag with high Fc-binding onto polystyrene surface for controlled immobilization of capture antibodies

The site specificity and bioactivity retention of antibodies immobilized on a solid substrate are crucial requirements for solid phase immunoassays. A fusion protein between an immunoglobulin G (IgG)-binding protein (ZZ protein) and a polystyrene-binding peptide (PS-tag) was constructed, and then used to develop a simple method for the oriented immobilization of the ZZ protein onto a PS support by the specific attachment of the PS-tag onto a hydrophilic PS. The orientation of intact IgG was achieved via the interaction of the ZZ protein and the constant fragment (Fc), thereby displayed the Fab fragment for binding antigen. The interaction between rabbit IgG anti-horseradish peroxidase (anti-HRP) and its binding partner HRP was analyzed. Results showed that the oriented ZZ-PS-tag yielded an IgG-binding activity that is fivefold higher than that produced by the passive immobilization of the ZZ protein. The advantage of the proposed immunoassay strategy was demonstrated through an enzyme-linked immunosorbent assay, in which monoclonal mouse anti-goat IgG and HRP-conjugated rabbit F(ab')2 anti-goat IgG were used to detect goat IgG. The ZZ-PS-tag presented a tenfold higher sensitivity and a wider linear range than did the passively immobilized ZZ protein. The proposed approach may be an attractive strategy for a broad range of applications involving the oriented immobilization of intact IgGs onto PS supports, in which only one type of phi-PS (ZZ-PS-tag) surface is used.