IDENTIFICATION OF A NEUTROPHIL ELASTASE CLEAVAGE SITE ON THE Act -CHAIN OF PRIMATE FIBRINOGEN
Human neutrophil elastase (HNE) cleaves the Aα21-22 bond of fibrinogen thus releasing the fibrinopeptide A (FPA)-containing fragment Aαl-21. Plasma Aal-21 levels reflect in vivo HNE activity and peptide levels are increased in cigarette smokers and patients with chronic lung disease. To further explore the HNE-fibrinogen interaction, we set out to develop an animal model. The digestion of purified baboon and marmoset fibrinogen by human thrombin, HNE and extracts of baboon and marmoset neutrophils was monitored with a specific radioimmunoassay for human FPA. Thrcmbin produced quantitative release (2 mol/mol fibrinogen) of FPA. In contrast, HNE and the neutrophil extracts did not release FPA, but rather, produced quantitative release of a larger, FPA-containing fragment. Immunochemically, this fragment was clearly distinguishable from FPA in that in vitro thrombin treatment increased its immunoreactivity 1,000-fold (thrombin increasable FPA or TIFPA). TIFPA release by the neutrophil extracts was blocked by α1-proteinase inhibitor, a specific HNE inhibitor (MeO-Suc-Ala2-Pro-ValCH2Cl) and an anti-HNE IgG, indicating that elastase was the responsible proteinase and that there was homology between the human and primate enzymes. The products of HNE and neutrophil extract proteolysis of the primate fibrinogens were then separated by high performance liquid chromatography and the TIFPA-containing fractions were subjected to amino acid sequence analysis. The FPA-containing fragments each consisted of 21 amino acids, had minor substitutions when compared with human A α] -21 [Baboon: Aα(3) Ser - Thr; Marmoset Aα(l) Ala - Thr, Aα(3) Ser - Thr, Aα(ll) Glu - Ala], and exhibited complete crossreactivity with the human peptide. Using the TIFPA assay, there was good recovery of primate or human Aαl-21 added to primate blood and the mean peptide level in 8 healthy marmosets was similar to that in man (0.5 nM and 0.4 nM, respectively). In conclusion, (1) the Aα;21 -22 bond of baboon and marmoset fibrinogen is a cleavage site for human and primate elastase, (2) baboon and marmoset Aal-21 can be measured with the assay for the human peptide, and (3) the primate serves as a useful model for the study of elastase-fibrinogen interactions.