Fig 1: G4 recognizes soluble forms of CD13, with higher affinity and selectivity than bestatin, and the membrane forms expressed on the surface of HUVEC cells. A Inhibitory effects of G4 and bestatin on the enzymatic activities of different aminopeptidases (rCD13, APA, and APB). Various amounts of G4 or bestatin were incubated with rCD13 (0.2 µg/mL in 60 mM Tris–HCl pH 7.4), APA (0.2 µg/mL in 60 mM Tris–HCl pH 7.4, 50 mM calcium chloride, 200 mM sodium chloride) or APB (0.2 µg/mL in 60 mM Tris–HCl pH 7.4, 100 mM potassium chloride) and the corresponding substrates at 0.1 mM (l-alanine-7-amido-4-methylcoumarin for rCD13, l-glutamic acid γ-(7-amido-4-methylcoumarin) for APA, and l-arginine-7-amido-4-methylcoumarin for APB). The mixtures were incubated for 30 min at 37 °C, and the cleavage of each substrate was monitored by measuring the fluorescence of methylcoumarin (λex 341 nm; λem 441 nm) using an Infinite 200 PRO plate reader (Tecan). Three independent experiments were performed for each enzyme. The results of one representative experiment for each enzyme are shown (mean ± SEM, duplicates). B Inhibitory effects of G4 and bestatin on soluble CD13 immunocaptured from human serum (sCD13) using microtiter plates coated with anti-CD13 mAb WM15. The microtiter plate was filled with a solution of mAb WM15 (5 μg/ml in DPBS) and incubated overnight at 4 °C. The plate was then washed with DPBS, blocked with 3% bovine serum albumin (BSA) in DPBS, and filled with human serum containing soluble CD13 (diluted 1:2 with DPBS containing 2% BSA). After washing with DPBS, mixtures of G4 or bestatin (0–100 µM in 60 mM Tris–HCl, pH 7.4) and l-alanine p-nitroanilide hydrochloride substrate (4.5 mM in 60 mM Tris–HCl, pH 7.4) were added and left to incubate at 37 °C for 2 h. Substrate cleavage by sCD13 dissociated from mAb WM15 was then monitored by measuring the absorbance at 405 nm in each well using a plate reader (Bio-Rad). The results of one experiment performed in triplicate are shown (mean ± SEM). C Effect of G4 on the binding of the anti-CD13 mAb WM15 to HUVEC. HUVEC were seeded at a density of 2 × 105 cells in 100 µL DPBS supplemented with 5% normal human serum (NHS) and incubated with a mixture of G4 peptide (0, 0.1, 1, and 10 µM) and mAb WM15 (0 or 0.13 nM) as indicated. As a negative control, we used the 11-mer peptide CgA429–439, which is a human chromogranin A fragment. Cells were incubated for 60 min at 4 °C, washed, and resuspended in 100 µL of DPBS containing 5% NHS and 1 µg/mL FITC-labeled goat anti-mouse IgG secondary antibody. After incubation for 60 min at 4 °C, the cells were washed and analyzed by flow cytometry. Data are expressed as the percentage of counts vs. fluorescence intensity units