Electrochemiluminescence assay for basic carboxypeptidases: inhibition of basic carboxypeptidases and activation of thrombin-activatable fibrinolysis inhibitor

Carboxypeptidases catalyze the removal of the C-terminal amino acid residues in peptides and proteins and exert important biological functions. Assays for carboxypeptidase activity that rely on change of absorbance generally suffer from low sensitivity and are difficult to adapt to high-throughput screening. We have developed a sensitive, robust assay for basic carboxypeptidase activity that makes use of electrochemiluminescent (ECL) detection of reaction product. In this assay, a peptide substrate contains the epitope for antibody (G2-10) binding which is masked by a C-terminal arginine. Carboxypeptidase activity exposes the epitope, allowing the binding of ruthenylated G2-10 which is then detected using ECL. High sensitivity allowed detection limits of 1–2 pM enzyme for carboxypeptidase B and activated thrombin-activatable fibrinolysis inhibitor (TAFIa). The inhibition of several basic carboxypeptidases by commercially available inhibitors was studied. This antibody-based method can be extended to other sensitive detection techniques such as amplified luminescent proximity homogeneous assay. The high sensitivity of the assay allowed the determination of the activatable levels of TAFI in human and other animal plasma in the presence of ε-aminocaproic acid, an active-site inhibitor that stabilizes TAFIa. A method to isolate in situ activated TAFIa from human serum in the presence of ε-aminocaproic acid was also developed.