Siglec-10 binds to the enzymatic groove of VAP-1 and acts as a substrate. (A) Examples of competitive stainings with Siglec-10–Ig chimera and anti–VAP-1 antibody Jg-2.10. CHO-VAP-1 transfectants were stained with the anti–VAP-1 antibody either in the presence of Siglec-10–Ig chimera or CD44-Ig (negative control) and analyzed by fluorescence-activated cell sorter. Percentages of cells positively stained with anti–VAP-1 mAb or a negative class-matched control antibody are shown. (B) Heart sections of KOTG mice were first incubated either with a control (CD44-Ig) or Siglec-10–Ig chimera and stained thereafter with anti–VAP-1 mAb (Jg-2.10). Some brightly positive vessels in the control section and fewer and less bright ones in the section pretreated with Siglec-10–Ig chimera are pointed out by arrows. Scale bar represents 50 μm. (C) Enzymatic activity of VAP-1 was measured as H₂O₂ (pmol) produced in 1 hour in the presence of Siglec-10 transfectants or mock-transfected control cells. The results are presented as relative SSAO activity ± SEM from 3 separate experiments, each having duplicate wells. A representative experiment showing the amount of H₂O₂ produced at different time points during a 1-hour measurement is presented in the upper left corner. (D) Binding of Siglec-10 transfectants to CHO cells expressing enzymatically active VAP-1, enzymatically inactive VAP-1, or mock controls. The results are mean fluorescence intensities ± SEM from 7 separate experiments, each having duplicate wells. (E) Binding of Siglec-10 peptide (CATLSWVLQNRVLSSCK-biotin) and mutated Siglec-10 peptide (CATLSWVLQNAVLSSCK-biotin) to recombinant VAP-1 (400 ng/well). The results are mean of relative binding ± SEM from 3 separate experiments with triplicate wells *P < .05. **P < .01. ***P < .001.