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Blood, 15 February 2006, Vol. 107, No. 4, pp. 1397-1404.
Prepublished online as a Blood First Edition Paper on October 13, 2005; DOI 10.1182/blood-2005-08-3452.
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HEMOSTASIS, THROMBOSIS, AND VASCULAR BIOLOGY
Antiplasmin-cleaving enzyme is a soluble form of fibroblast activation protein
Kyung N. Lee,
Kenneth W. Jackson,
Victoria J. Christiansen,
Chung S. Lee,
Jin-Geun Chun, and
Patrick A. McKee
From the William K. Warren Medical Research Center and Department of Medicine, University of Oklahoma Health Sciences Center, Oklahoma City, OK.
Circulating antiplasmin-cleaving enzyme (APCE) has a role in fibrinolysis and appears structurally similar to fibroblast activation protein (FAP), a cell-surface proteinase that promotes invasiveness of certain epithelial cancers. To explore this potential relationship, we performed comparative structure/function analyses of the 2 enzymes. APCE from human plasma and recombinant FAP (rFAP) exhibited identical pH optima of 7.5, extinction coefficients ( ) of 20.2 and 20.5, common sequences of tryptic peptides, and cross-reactivity with FAP antibody. APCE and rFAP are homodimers with monomeric subunits of 97 and 93 kDa. Only homodimers appear to have enzymatic activity, with essentially identical kinetics toward Met- 2-antiplasmin (Met- 2AP) and peptide substrates. APCE and rFAP cleave both Pro3-Leu4 and Pro12-Asn13 bonds of Met- 2AP, but relative kcat/Km values for Pro12-Asn13 are about 16-fold higher than for Pro3-Leu4. APCE and rFAP demonstrate higher kcat/Km values toward a peptide modeled on P4-P4' sequence surrounding the Pro12-Asn13 primary cleavage site than for Z-Gly-Pro-AMC and Ala-Pro-AFC substrates. These data support APCE as a soluble derivative of FAP and Met- 2AP as its physiologic substrate. Conversion of Met- 2AP by membrane or soluble FAP to the more easily fibrin-incorporable form, Asn- 2AP, may increase plasmin inhibition within fibrin surrounding certain neoplasms and have an impact on growth and therapeutic susceptibility.

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