Glutaminyl Peptide Cyclotransferase (QPCT), also known as Glutaminyl Cyclase, is an enzyme responsible for the biosynthesis of pyroglutamyl peptides. QPCT is present in the pituitary and adrenal glands, where it is important for the generation of the Nterminal pyroglutamyl groups of peptide hormones such as neurotensin and thyrotropin-releasing hormone. Besides, Cathepsin B (CTSB) has been identified as an interactor of QPCT, thus a functional binding ELISA assay was conducted to detect the interaction of recombinant human QPCT and recombinant rat CTSB. Briefly, QPCT was diluted serially in PBS with 0.01% BSA (pH 7.4). Duplicate samples of 100 μl were then transferred to CTSB-coated microtiter wells and incubated for 1h at 37℃. Wells were washed with PBST and incubated for 1h with anti-QPCT pAb, then aspirated and washed 3 times. After incubation with HRP labelled secondary antibody for 1h at 37℃, wells were aspirated and washed 5 times. With the addition of substrate solution, wells were incubated 15-25 minutes at 37℃. Finally, add 50 µL stop solution to the wells and read at 450/630 nm immediately. The binding activity of recombinant human QPCT and recombinant rat CTSB was shown in Figure 1, the EC50 for this effect is 0.09 ug/mL.