Distinct effects of CD13/APN on CXCL8 and CXCL11. Synthetic CXCL11(3-73) was incubated with CD13 and 10 mM benzamidine at a 1:4 molar E/S ratio for 2 hours at 37°C and desalted on C4 ZipTips prior to ion-trap mass spectrometry (panel A). The deconvoluted spectra of the multiple charged ions (with 8 to 12 positive charges) are shown in the inset. The 3 molecular ions A, B, and C correspond to CXCL11(5-73), CXCL11(4-73), and CXCL11(6-73), respectively. Alternatively, CXCL11(3-73) (panel B), CXCL8(1-77) (panel C), or CXCL8(6-77) (panel D) were incubated with CD13 and 10 mM benzamidine (except for the conditions without benzamidine that are indicated on panel C) at a 1:25 molar enzyme-substrate ratio for 5 minutes to 24 hours at 37°C and desalted on PVDF membranes prior to Edman degradation. The percentages of the remaining unprocessed chemokines (■) and the chemokines missing 1 (▩), 2 (▧), 3 (white bars with black diamonds), 4 (dotted bars), or 5 (□) NH₂-terminal amino acids that were calculated from the molar yields for the 10 NH₂-terminal amino acids are indicated in the histograms in function of the incubation time.