Electrochemical Assays and Immunoassays of the Myeloperoxidase/SCN/HO System.

Strategies to detect and characterize myeloperoxidase (MPO) are needed, given that this "split personality" enzyme kills harmful microorganisms but also damages a host tissue. Here, we describe electrochemical approaches to measure MPO by using the pseudohalogenation (MPO/SCN/HO) and catalase-like (MPO/HO) cycles. Their kinetics were determined by monitoring the consumption of HO with a nitrogen-doped carbon nanotubes (N-CNT) electrode, which could detect 0.50 μM HO at -0.20 V. The unique design of internally calibrated electrochemical continuous enzyme assay (ICECEA) and electrode stability allowed use of one N-CNT electrode for over half a year to reliably determine MPO. The kinetic measurements showed that (a) SCN did not affect the affinity of MPO for HO, (b) catalase-like cycle was slower, and (c) MPO retained enzymatically active conformation after complexation with its antibody Ab both in a solution and on the surface of an antibody dipstick (d/Ab). The homogeneous assays could detect 5.2 μg L MPO (35 pM) via a faster cycle. The heterogeneous immunoassays with the capture of MPO on d/Ab could detect 60 μg L, which was suitable for the accurate detection of MPO in human saliva (101% recovery). Replacing a detection antibody of ELISA with ICECEA as a signal transducer for immunoassays offers a rapid method for the selective determination of enzymes; for example, time of MPO quantification was cut from 3-4 h (sandwich ELISA) to ∼20 min (ICECEA-dipstick).