Quantitative measurement of bitagged recombinant proteins using an immunometric assay: application to an anti-substance P recombinant antibody

We have developed two different immunometric assays to directly quantify both the total and the active fractions of a recombinant antibody (single chain fragment variable, or ScFv) as obtained in a crude extract from an Escherichia coli expression system. For total determination, the assay is based on the simultaneous recognition of two different peptide Tag sequences (Ha-Tag and Myc-Tag) at each of the N- and C-terminal extremities of the recombinant protein. A monoclonal antibody (mAb 12CA5, directed against Ha-Tag), coated on microtiter plates, is used for capture, and the mAb 9E10 (directed against Myc-Tag), labeled with acetylcholinesterase (AChE, EC 3.1.1.7), acts as tracer. In parallel, for the determination of the active fraction, the capture is performed using microtiter plates coated with the antigen, while solid-phase-immobilized ScFv is measured using the same 9E10 tracer mAb. A synthetic peptide in which the two Tag sequences were joined was used as a standard, thus avoiding the laborious purification of a recombinant protein as reference. The method was applied to the direct measurement, in periplasmic extracts, of the total and active fractions of an ScFv produced at different induction temperatures.